1 Introduction.. 3

2 Overview of Streams and Methods.. 7

3 Technolithes and building material engineering.. 10

4 Mineral composition of technolithes.. 14

4.1        Mineral composition of heatproof technolithes. 14

5 Gannister.. 16

6 Glass.. 19

6.1        Mineral phases. 19

6.1.1 Products of devitrification. 21

6.1.2 Surface defects. 24

6.1.3 Streaks. 25

6.1.4 Cells. 25

6.1.5 Occlusion and devitrification. 26

7 Chamotte. 29

7.1        Mineral composition of used chamotte. 29

7.1.1 Chamotte and linings of blast furnaces. 30

8 Basic heatproof matters.. 32

8.1        Magnezite heatproof matters. 32

8.1.1 Molten magnezite. 33

8.1.2 Chrommagnesite. 34

8.1.3 Dolomite. 34

8.1.4 Microscopy of used basic heatproof matters. 34

9 Mullite and corundum matters - matters with high aluminium oxide content.. 37

9.1        Cast and molten heatproof matters. 38

9.1.1 Mullite-corundum products of the „Corhart Standard“-type. 38

9.1.2 Corundum products of Corvisit and Monofrax-type and corundum zirconic Bakor and Corhart ZAC-type. 39

9.1.3 Molten corundum.. 40

10 Molten rocks - Petrurgy.. 41

10.1       Mineral composition of molten rocks. 41

11 Mineral components of a high-temperature slag.. 43

11.1       Basic blast furnace slags. 45

11.2       Acidic blast furnace slags. 46

12 Siliciumcarbide and grafite matters.. 47

12.1       Carborundum products. 47

12.1.1 The mineral composition of the siliciumcarbide matters. 47

12.2       Graphite products. 48

13 Enamels.. 49

14 Cement clinkers.. 50

14.1       Portland cement clinker.. 50

14.1.1 Identification of clinker minerals using etching of polished sections and polished thin sections. 54

14.2       Aluminous cement.. 57

14.2.1 Methods of identification. 59

14.3       Mineral composition of air mortars and plasters. 60

14.4       Mineral composition of hydraulic mortars, plasters and concretes. 62

15 Concrete deterioration.. 65

16 The ceramic products.. 68

16.1       Brickware and stoneware. 68

16.2       Porcelain.. 70

16.2.1 Glazes. 72

16.3       Steatite ceramics. 72

16.4       Cordierite ceramics. 73

16.5       Porcelains not containing glass - oxide ceramics. 74

16.6       The spinel ceramics. 75

17 Summary.. 78

18 Bibliography.. 79




Miroslava GREGEROVÁ,

Ústav geologických věd, Přírodovědecká fakulta MU v Brně, Kotlářská 2,  61137 Brno,


Key words: Technolithology, technolithe, microstructure, microstructure, building material engineering

1 Introduction

Mineralogical and petrographical systems have been recently facing a serious schism. Official institutions try to exclude minerals and rocks affected by human activity while experts attempt to define methods for practical issues in the branch. The scenario relies on rather obsolete anthropocentric point of view without concerning the fact that man has been an integral part of nature and has been even more and more affecting all natural systems. If we accept this thesis also for the mineral/rock system, there is no more disputes. There are three main streams in mineralogy and petrology:

lithology - natural minerals/rocks with development unaffected by human activity;

biolithology - minerals/rocks developed by life activities of organisms including man;

technolithology - minerals/rocks (technolithes) that have developed artificially or intentionally in tight connection to human activity.

Technolithology is an interdisciplinary geo-technical scientific discipline. Position of technolithology in the earth material cycle scheme (technolithes, natural minerals/rocks) may be demonstrated using simple illustration adapted from Cohen (1980)-see Figure 2. The illustration shows that mineralogy and petrography play significant role in the material engineering branch (building engineering, respectively). Applied mineralogy and petrography include mineralogical and petrographical study of materials with respect to their potential application (raw materials). On the other hand, technolithology focuses on research of synthetic products resulting from technological transformation of natural materials. The study comprises microstructure, mineral/amorphous phases, chemical composition, and physical properties. It examines both quantity/quality changes during the technological processes and stability/degradation under specific load conditions.



Hornbogen (1983) was one of the first who recognized significance of microstructures/microstructures for rapidly developing material engineering branch. His assumptions have been based on historical aspects. He summarized practical properties of natural materials (for example, rocks, copper, meteoritic iron), well known procedures and technologies (for example, bronze, steel, raw iron, cast iron, ceramics, glass, concrete), applications of scientific procedures in up-to-date technologies (trace ingredients of Al, Ti, Mg, metal alloys, ferrites and others), and special studies resulting in development of Ni superalloys, Si3N4 ceramics, Al2O3-ZrO2, components, semiconductors/superconductors.

Figure 3 shows categories of geochemical, mineralogical, and petrographical research in technolithology including terminology used in material engineering.

In the technolithe mineral group, it is possible to determine mineral classess that correspond to those of natural rock minerals. As the technolithe composition always depends on composition of original blends and technological processes used, the mineral associations have been usually registered based on individual technolithe groups (see Table 1).


Figure 1 Position of technolithology and earth materials' cycle (adapted from Cohen, 1980)



Figure 2 Objects of study (geochemistry, mineralogy, and petrography) applied in material engineering (Szymañski 1997). Legend: structrone-basic microstructure polyhedron, cluster-embryonal crystal is a embryonic cluster of crystals that is not identifiable using roentgenography, domain-major microstructure element with poor X-ray diffraction, crystallite-microfragment of crystal patrice with distinct X-ray diffraction, aggregate-compact, non-porous particle consisting of crystallite, agglomerate-porous particle consisting of aggregates or crystallites, braun-non-regular particle of minerals or phases.


Table 1 Overview of common fireproof technolithes including composition and application temperature (Gregerová, 2000).


Fireproof technolithes

Mineral composition of original raw material

Mineral composition of technolithe

Main oxides


Lime silica

quartzite, chert + up to 5% CaO

cristobalite, tridymite, wollastonite

over 90% SiO2          below 5% CaO   

up to 1710oC

Quartz glass

vein quartz

amorphous SiO2

over 99% SiO2

up to 1710oC

Silica chamote

kaolin, claystone, clay, silicarenite

mullite, cristobalite        tridymite, quartz

10-30% Al2O3

up to 1670oC

Fire clay

fireproof clays, claystones


35-45% Al2O3

up to 1750oC

Peraluminic chamote

fireproof clays, claystones, Al hydroxides and oxides, Al2SiO5 minerals

mullite,         cristobalite       corundum

45-60% Al2O3                          60-75% Al2O3                         over 75 %         Al2O3

up to 1840oC                    up to 1900oC                    up to    1950oC

Fireproof mullite technolithes

Al hydroxides and oxides, sillimanite, andalusite, kyanite

mullite, cristobalite, corundum

60-85% Al2O3

up to 1960oC

Fireproof corundum technolithes

Al hydroxides and oxides

corundum, mullite

over 60-85% Al2O3

up to 1960oC

Spinel magnesite (magnesiochromite)

cemented magnesite magnesite -chromite + Fe2O3, SiO2, Cr2O3

periclase, periclas+spinel, spinel

over 80% MgO            30-70% MgO             10-30% Cr2O3

up to 1700oC

Dolomite with fixed lime


calcium silicites, periclase, CaO

cca. 40% CaO

up to 1900oC

Dolomite with free lime (cements)

dolomite, limestone, diatomite, magnesite, hematite fireproof clay

periclase, wollastonite, cristobalite, dicalcium/tricalcium silicite

cca 35% MgO admixture of other oxides 15-25%

up to 1780oC do 1900-2000oC




over 80% Cr2O3

up to 2000oC



silicon carbide

up to 80% C

up to 2200oC

Figure 3  Procedure for technolithe study


2 Overview of Streams and Methods

In 1981, the International Council on Applied Mineralogy (ICAM) was established. The council was initiated by a group of world's top mineralogists whose research had concerned industry-based mineral eCPLoitation.  The council was formed at the International Mineralogical Association (IMA) conference held in Johannesburg. At congress held in 1984 in Los Angeles, the ICAM was finally defined as an independent organization coordinating its activities through IMA. ICEM's objectives include organizing of international interdisciplinary activities and conferences. The conferences enable sharing information between experts from various branches somehow connected to mineral application (mineralogists, chemists, engineers, metallurgists, ceramics producers, health protection experts, and medicine).


Applied mineralogy comprises study of morphology, inner microstructure, and physical/chemical properties of minerals that enter the technogenesis process.  Results of the study make it possible to modify the technological process and control its progress. Qualified evaluation of final technolithe contributes to innovation and/or new technology development. It focuses on search for new raw materials in the solid industry waste field (for example, light ash, power-producing hydrated sulphate of lime) and production of synthetic minerals (for example, jewelry production, abrasives, and so on).

Applied mineralogy is very useful for anthrophogennous mineral study, biolithology, and biomaterials/bio-mineralization (medical applications mainly).


Technolithology (technical material petrography) is an interdisciplinary branch forming a bridge between material engineering and geology. The technolithes are studied by geology (particularly mineralogy, petrology, and geochemistry branches). It has been proved that by using common analytic methods on technolithes, a whole new range of scientific research opens to the community (material engineering including research of cement clinker, lime/hydrated lime, concrete, mortars and plasters, and ceramics). Basic research streams have been defined in some fields that might require geology and its methods to solve specific problems and first issues have been successfully addressed (Rovnaníková  et al.  1999, Gregerová 2000, 2000b, 2000a, 2000c, Gregerová  et al. 2000, Gregerová, Pospíšil 2000). Pattern-based relationships between stable, metastable, and secondary minerals in building mortars and plasters materials have been revealed using microscopic methods, RTG, DTA, and electron microscopy (Gregerová 2000a, 2000c, Gregerová, Pospíšil 2000a, Gregerová, Rovnaníková 2002,  Witzany et al. 2002).

It is obvious that petrography with its experts has become significant, too. Petrography is able to analyze both final products of various technological processes and the actual development processes/technologies. The products can be both mono-mineral and poly-mineral materials with variable chemical composition. Their compositions (microstructures and microstructures) reflect different kinetics of their genesis.

For rock melting process study that has been performed since the end of the 19th century, Huang (1962) used the term petrology while Ginsberg (1961) preferred the term petrurgy. Intense development of microscopic methods and their application to ory minerals resulted in a separate branch-metallography. Berry and Mason (1959) stated that natural minerals included industry-applicable minerals and industry-non-applicable minerals. Industry-applicable minerals further included general minerals, rare minerals, and strategic minerals (the ones required in electronics/weaponry development).

Requirements for ceramic product shape/composition variability resulted in definition of a separate branch entitled ceramography (Ryshkevitsch 1960). With growing technology development, new technical streams of research have been defined including material engineering. First laboratories of this type have been established in the sixties in Philips company (Eindhoven), General Electric company (Schenectadu), and Bell Telephone company (Murray Hill) (Kohl, 1977).

In following period, the technical mineralogy branch has been experiencing both vital and dynamic development. The development has been tightly connected to boom of electronics as the materials used have been exposed to extreme conditions. It was believed that only natural crystals (such as diamond, corundum, quartz) could stand the conditions well. However, new technologies succeeded in synthesis of new monocrystals/polycrystals with much better characteristics. These include phototropic/photosensitive glass, optical fibers, magnetic memories, monocrystalic layers, monocrystals used in lasers, and so on.

As petrogenetic studies combine various experimental/theoretic approaches with inductive/deductive statements concerning rock genesis/development, they are applicable all across the industry. The reverse approach is obvious as well. Technological processes can be used to rock genesis detection. Up-to-date technologies produce wide range of inorganic materials and artificial stones. These have been described as "technical materials", "technical rocks" or "technical matters" and studied by applied petrography/mineralogy, technical petrography, technical mineralogy or petrography of technical matters (Hejtman, 1956, Gregerová, 1996, Szymañski, 1997, and others). 

In 2000, Miroslava Gregerová proposed the term technolithe for such materials. Also, she has defined the term technolithology for the interdisciplinary branch focused on inorganic materials created using various technologies (including waste). Industry processes of technolithe synthesis attempt to change natural minerals/rocks into more precious (concentrated) materials or produce completely new products featuring required technical parameters. Each production phase/technological process of the inorganic materials transformation features specific mineral phase transformation and genesis.

It has become obvious that the technolithology uses a wide range of geological/analytic methods to study technolithes (Gregerová, Sulovský 1994). The methods include: optical microscopy (polarized/reflected light), electron microscopy/microanalysis, emission spectroscopy, mass spectroscopy, IR-spectroscopy, nuclear magnetic resonance, X-ray diffraction, catodeluminiscence, differential thermal analysis, thermogravimetry, microhardness, and so on.

Figure 1 Shows recommended procedure of diagnostic features description.


3 Technolithes and building material engineering


Rapid development of up-to-date technology has confirmed that there is a wide range of method for preparation of substances including mono-mineral, poly-mineral, crystalline, vitrophyric, metalloidlic, metallic, and mixed products. Technolithe microstructures and mineral composition reflect original microstructures/compositions and formation kinetics as well.

Even changes caused by mere cleaning of the raw materials (or mixing) may affect resulting mineral composition/microstructure of final product. Further affects become important during preparation of semi-products with controllable characteristics (for example, drying/watering process). This results in nomenclature confusion issues and complicates integration of standard/applied mineralogy and petrography. In fact, there have been attempts to systemize the technical products based on their appearance, composition, and microstructure (Beljankin et al. 1952), but sometimes it is not easy to find universal key criteria for such nomenclature. 

Mineral composition/microstructures of technolithes reflect temperature of their formation. The temperature has often been higher than formation temperature of adequate natural rocks. Different physical conditions result in formation of different phases, crystallic microstructures, and modifications that are rare or even unknown in natural environment. In the hot/flame process, quartz transforms into tridymite and cristobalite, wollastonite turns to pseudowollastonite, nepheline is substituted by carneigieite, sillimanite changes to mullite, and so on. As far as chemical composition/microstructure is concerned, many minerals of natural rocks and technolithes may be similar with major differences in outer appearance reflecting different genesis.

Table 2 shows comparison and common features of natural rocks and technolithes (solid inorganic products).


Table 2 Analogy between petrographical, genetic, and microstructure types of rocks and technolithes (Gregerová 2000).

Petrographical, genetic, and microstructure rock types

Solid equivalents-technolithes

Magmatic rocks

Metallurgical scoria, melted cements, glass, melted corundum, spinel, synthetic minerals.

Specific magmatic microstructures

Glass, baking namels, flow microstructures in partial devitrificated (recrystallized) glass, spherolithes, laminated silica.

Metamorphic rocks

Silicate technolithes (silica), chamote, cement clinker, standard porcelain, chinese hyalite porcelain, faience, stoneware, fireproof products, special ceramic materials: cristobalite, soapstone, baryte, ferrites, oxide ceramics, cemented oxides, synthetic monocrystals, and so on.

Pneumatolytic rocks

Authigenic minerals in glass/brick particles of recuperators (in metallurgical furnaces, lancashire kilns, and continuous tank turbaces), special crystalline glazes.

Contact zone rocks

Zones in used silica, recrystallized zones of inwalls and continuous tank turbaces, zones of corroded metallurgical furnace inwalls and fireproof materials.


Entrapped slags in steel, knot in glass (sandstones).

Sedimentary rocks

Various concrete types, silicate-lime bricks, foundry sands, light ash, power-producing hydrated sulphate of lime, plaster, air mortars and plasterss, and so on.


Each inorganic product (technolithe) that has been solidified using the technogenesis is similar to natural rocks. Technolithes are solid crystals/aggregates formed by crystals/glass made in artificial way.  Their characteristics depend on substance compositions and microstructures.

Microstructure/microstructure terminology has been derived from generally used terms in individual rock groups.  Table 3 shows basic technolithe microstructures sorted by analogical magmatic rocks. 

Technolithe microstructure is affected by original raw material composition and mineral dressing processes used. It depends on elemental composition and coupling force type, and reflects all technogenesis stadiums from "raw" product characteristics, preliminary heating, and degree of sintering ("sintering in solid phase") of polycrystalline aggregate (of for example carbon and clinker-forming oxides), to meshed glass phase with isolated mineral crystals and fully crystallized matter. Sintering processes determine not only fabric particles of new formed microstructures, but can be used to derive physical characteristics of new formed technolithe as well. If the technolithe includes glass phase, all its characteristics are derived from amorphous isotropic matters. In technolithes including space-oriented crystals with confining crystalline microstructure, the resulting product characteristics are isotropic or almost isotropic.

Temperature most affects the technolithe phase composition and microstructure. Because of different temperatures, different technolithes are formed from the same raw materials (for example, semivitreous porcelaine, keramzite, faience, engineering ceramics, stoneware). Table 4 includes firing limit temperatures for sample ceramic product group.

In standard petrography, chemical/mineral composition, rock microstructure/microstructure, and physical characteristics are available for rock genesis determination.

The technological process of technolithe formation is available rather than chemical and mineral composition, and their microstructure and microstructure. This technological process of formation can be controlled and affected. The intermediate products can be used within some technolithes [original raw material - firing of cement clinkers (intermediate product) - concrete (finished product)]. The third advantageous aspect in technolithe study is that we know the exact original composition of raw material entering the technogenesis process, and the original composition can be set up.


Table 3 Technolithe microstructures derived from analogical magmatic rocks.



Size classess  (crystal sizes)

Technolithe samples




Window glass



Granularity c)-d)

Glass-crystalline technolithes (scoria)


Porphyritic with vitrophyric groundmass (phenocrysts visible)

a) coarse-grained  1.0-5.0mm environment-grained 0.1-1.0mm

Stoneware, abrasive tools, fireproof materials, cement clinkers



b) fine-grained <0.1mm

Oxide ceramics, porcelain, faience, cemented metals, cemented carbides


Aphanitic (crystals not visible)

c) microcrystalline (microlites)

Glass-crystalline technolithes, cement clinkers



d) cryptocrystalline  (crystallites)

Polychromatic glass

Holocrystalline (perfect crystallized)

Phaneritic  (crystals visible)

a) coarse-grained  1.0-5.0mm

Fireproof materials



b) environment-grained 0.1-1.0mm

Fireproof materials, engineering ceramics



c) fine-grained        <0.1mm

Oxide ceramics, cemented carbides, nitrides


Aphanitic (crystals not visible)

d) microcrystalline      (microlites)




e) cryptocrystalline  (crystallites)


Special - by particle layout

Confining granular, fluidal (flow, schlieren), orbicular, spherulitic, etc.

From coarse-grained to microcrystalline

Optical fibers, engineering ceramics, scoria, keramzite, alloys

Special - by particle shape

Isometric-anisometric granular - lamellar -fibrous skeleton, dendritic, and so on.

From coarse-grained to microcrystalline

Engineering ceramics, oxide ceramics, scoria, keramzite

By technogenesis

Relict Authigenic 

From coarse-grained to microcrystalline

Portland, alumina, and magnesium clinkers, special technolithes


Table 4 Firing limit temperatures for selected technolithe group (recent ceramic products by Konta 1982).


Sintering temperature (no plastic deformations caused by glass phase visible)

Argil and potter's clays


Stoneware clays (stoneware clays include illite and mixed illite-montmorillonite microstructures)


Temperature of clay and claystone sintering


China sanitary-ware and kitchenware


Hard porcelain




Figure 4 Shows the technogenesis of original raw material composed from minerals in finished product. The higher number of different minerals in raw material means complex resulted product formation (for example, microstructures of oxide ceramics-engineering ceramics).


Figure 5 Melting scheme of two partially mixed melts and microstructures of resulting alloys by Szymańsky (1997).Simplified scheme of continuous microstructure transformation in example of two partially mixed components (they are transformed in raw material sintering process, as seen, for example, in optical microscope). Legend: Points between A-M and N-B have mostly single-phase composition and correspond to composition b in a and a in b being perfectly mixed (slightly different from pure components). Microstructure in points between M and C (or D and N); the first individuals of less present component are separated (b in a components and visa verse). More separated components (b in a,  b+a, a in b) are formed in C-K and K-D area.


Microstructure formations of metallurgical alloys are considered as a quite simple (see Figure 5). The more complex microstructures are formed in multi-component technolithes.



Chemical compositions of technolithes are expressed using main oxides. This concept is shown in Table 1 where you can see common fireproof technolithe overview including mineral compositions of original mixtures, main oxides, and thermal intervals of their wear-away. 

Figure 6 Technolithe positions in SiO2-(MgO+CaO)-(Fe2O3+Al2O3) system.


Triangle diagrams can be used for representation of main oxides in individual technolithes as well. SiO2-(MgO+CaO)-(Fe2O3+Al2O3) diagram in Figure 6 shows visual example of common cement technolithe areas.

Sophisticated photo-documentation is integral part of technolithe studies.






4 Mineral composition of technolithes

In the group of technolithes1, it is possible to differ the same mineral classess and subclassess as in the case of natural rocks. Therefore composition of technolithes always depends on composition of initial material mixtures and technological processes, mineral associations of technical materials are put along according to individual technolithic groups.


4.1       Mineral composition of heatproof technolithes

A survey of common heatproof technolithes, mineral and rock composition of necessary materials, the mineral composition of the final product, the chemical composition and the temperature stability of technolithes are synoptically summarized in Table 5.






Table 5 Survey of common heatproof matters, their composition and temperature of application.


Species of the heatproof matters 
Original materias
Mineral components of the product
Chemical contant of the main materials 
Quartz rocks, cherts + up to  5% of CaO
cristobalit, tridymit, wollastonite
over 90% of SiO2 under 5% of CaO 
up to 1710oC
Silica glass
Vein quartz 
amorphous SiO2
over 99% of SiO2
up to 1710oC
Silica chamotte
kaolin, clay stone, clay, quartz send
mullite, cristobalite tridymite, quartz
10-30% of Al2O3
up tp 1670oC
Clay (normal) chamotte 
Heatproof clays, claystones
35-45% of  Al2O3
up to 1750oC
High clayey chamotte
Heatproof clays, claystones, hydroxides and Al-oxides, Al2SiO5-minerals
mullite, cristobalite, corundum
45-60% of Al2O3 60-75% of Al2O3 over 75 % of Al2O3
up to 1840oC do 1900oC do 1950oC
Not molton (molton) mullite 
Al-hydroxides and oxides, sillimanite, andalusite, kyanite
mullite, cristobalite, corundum
60-85% of Al2O3
up to 1960oC
Not molton (molton) corundum 
Al-hydroxides and oxides
corundum, mullite
over 85% of Al2O3
up to 1960oC
Common spinel magnesite (chrommagnesite) 
sintered magnesite magnesite -chromite + Fe2O3, SiO2, Cr2O3
periclase periclase, spinel
over 80% of MgO  30-70% of MgO  10-30% of Cr2O3
up to 1700oC
Dolomite with fixed CaO
calcium silicates, periclase, CaO
about 40% of CaO
up to 1900oC
Dolomite with  free CaO (cements)
dolomite, limestone, diatomite, magnesite, hematite, heatproof clay
periclase, wollastonite, cristobalite, di-tri-calcium silicates
about 35% of MgO 15-25% of admixture 
up to 1780oC up to 1900-2000oC
over 80% of Cr2O3
up to 2000oC
Carborundum (Si-carbide) normal
carbide of silicon
up to 80% of C
up to 2200oC
Carborundum high valuable (recrystallized)
heatproof clay organic cement
Mullite, cristobalite
over 80% of C
up to 1700oC
Carbonaceous ceramic
grafit, heatproof clays
carbon, mullite, cristobalite
30-80% of C
up to 1700-1900oC
Carbonaceous recrystallized
80-100% of C
up to 2500oC in a reducing environment 
Carbonaceous coccoid
carbon (microscopic)
over 90% of C
up to 2500oC in the atmosphere


5 Gannister

The mineral composition of gannister is in fact given by types of initial materials, their mechanic processing, a sort and amount of admixtures and a way of firing.

Figure 7 Modification transitions of SiO2 in the row quartz - tridymite - cristobalite - glass in dependance on a temperature (Fenner 1913).


 Prevailing minerals are polymorphous forms of SiO2: quartz, cristobalite and tridymite (see Fig. 7). Their optical properties are summarized in the encyclopedy of menerals. Therefore we aim at the microstructure of gannister and identification of individual phases here. In thin sections of gannister, at a low magnification, one can observe coarse fragments and fine-grained matrix, which are connected by gradual interfaces (reflecting the granulometry of the used material). In a common fired gannister, the coarse fragments illustratively document a type of used quartzite. The microscopic characteristics of quartzite stay in fact not changed, although the mineral composition changes. Unchanged relict quartz (of primary genesis) occurs  in a fired sample. The quartz transition proceeds from the surfaces of grains along the syatem of irregular cracks. The cracks form at a reversible transition of α-quartz to β-modification at 573°C (see Fig. 7). This phenomena is well perceptible due to the fact that the transition first manifests at clasts with a large of specific suface. Very well perceptible transition only comes through at the temperature higher than 1250°C. Microscopically „amorphous“ cristobalite is a product of the transtition of quartz grains. It is so fine-grained that it causes opacification of those parts of quartz grains, which have already been transformed to cristobalite. The share of the transformed quartz in coarse fragments is usually significantly higher in quartzites with larger quartz crystals, in which cristobalite only forms thin linnings and fillings of thin crack and the original mosaic microstructure is preserved.  On the contrary, in quartzites with much lower share of quartz, wider cristobalite rims form with decreasing size of grains. The transition to cristobalite in the cement is practically finished, so only isolated coarser quartz grains „shine“ among the coarse fragments (at observation in CPL). The fineness of cristobalite documents its  formation directly from quartz, without a presence of melt. Tridymite is coarser-grained and forms by crystallization at the presence of the glass melt in matrix, i.e. fine-grained gannister matter, sealing the coarse-grained fragments of the original quartzite. Typical tridymite forms in gannister are compound triplet crystals, which appear lanceolate twin-like in longitudinal sections. A prismatic form is less frquent. It is more common in a recrystallized matter of used gannister, namely in the zone corresponding to the tridymite stability zone, i.e. up to 1470°C. Results from optical characteristics, in comparison to quartz, tridymite has somewhat lower birefringence. In a context with a smaller size of crystals and hence smaller thickness of their sections, its birefringence comes through as lower intereference colours - first rank gray. The lanceolate crystals of tridymite form a basic frame of the gannister matrix. The high temperature SiO2 modifications contant is variable in gannister. It usually fluctuates between 0 - 37 % of quartz, 14 - 53 % of cristobalite, 14 - 71 % of tridymite. The contant of glass melt and accessory minerals is about 9-20 %.

The share of accessory minerals and glass melt depends on the composition the initial quartzites. These minerals usually concentrate in fine-grained parts of gannister and along with fine quartz and admixtures, they form matrix. Accessory minerals, the amount of admixtures and cooling speed of the glass melt also play a role at formation of other authigenic minerals. A product of a  reaction of SiO2 + CaO - pseudowollastonite (Ca3Si3O9) is amost always present. Due to the presence of other oxides (particularly Al2O3, FeO, Fe2O3, MnO, TiO2 and oxides of alkali metals), other phases occur. E.g. monoclinic pyroxenes, frequently solid solutions of gannister, hedenbergite and johannsenite form. They are pleochroic in yellow-green up tp green shades (the intensity of pleochroism depends on the hedenbergite component content) or in shades of brown up to reddish pleochroic augite. At the particularly high Fe (12,5 % of Fe2O3) content, an opaque magnetite and a deep purple showing through hematite contants increase in matrix. Of other minerals, deep red monocalciumferrite (CaO.Fe2O3) and yellow-bown dicalciumferrite (2CaO.Fe2O3) may occur. The minerals have high birefringence, most of them aree conspiciously coloured and some of them are pleochroic. Thus, in matrix, at the observation in CPL, they are noticable as high birefringent particles despite their little sizes. But it is impossible to determinate the other optical constants of them. Along with yellowish or braunish coloured glass matter, they are carriers of gannister light yellowish up to ochroid brunish colouring. 

At the microscopic evaluation of gannister products, firs of all, contain, microstructure and distribution of tridymite and percental abundance and size of a untransformed quartz shoud be considered. The quantitative ratio of both the minerals is possible to identify planimetricly.

Microscopic characteristics of used gannister are somewhat different and they reflect conditions in places of its application. A prevalent part of gannister is used for linings of metallurgical furnaces, coking batteries and for vaults of glass furnaces. In all above listed cases, the point is an one-sided affect of a high temperature, slag, dust and volatile components. All of these factors come come through as changes of microstructure and a mineral composition. A formation of zones, macroscopically distinguishing from each other, is characteristic. E.g. at chamotte blocks of the Siemens-martin furnace one can observe 4 basical zones: primary, transient, black and gray. The differences in the chemical and mineral composition of  individual zones are listed in Table 6.


Table 6 Table demonstrating changes in the chemical and mineral composition of four basical zones of chamotte used in the Siemens-martin furnace.


Primary zone

Transient zone

Black zone

Gray zone



















































annealing loss





pseudomorphs after quartz, metacristobalite















devitrificated glass, pseudowollastonite





brown glass, orthosilicate and magnetite





The primary zone ordinarily stays preserved up to the of 1100°C. It is light ochroid, with an uneven grainy fracture, perceptible pores and visible white grains of the original gannister. The thickness of this zone depends on the total thickness of the block, i.e. it corresponds to the certain degree of deterioration and gradually gets thiner. This zone gradually passes into the transient zone.  Its thickness reaches approx. 6 cm and corresponds to a temperature of 1100-1350°C. It is macroscopically darker than the primary zone, there are distinct white grains in its fracture - coarser quartzite fragments. In a microscope, it is evident that in comparison to the primary zone, number of them decreases and the share of matrix increases. The quartz grains are strongly cracked and transformed to isotropic metacristobalite. They are lined with crystallic matrix with a confining microstructure, formed by approximately 25.10-6m sized acicular and lanceolate tridymite crystals. The contant of a glass phase and yellow-brown, short prismatic pyroxenes also increases. Besides them, wollastonite and magnetite also occur. Towards the center of the furnace, the transient zone passess into the tridymite zone (so-called black zone). Its thickness is about 5 cm and assumed temperature of crystallization 1450 - 1540°C. The name itself  says that it is macroscopically fresh layer, in which one can still observe isolated white quartzite grains. However, the fragments of quartz already do not occur in this zone. Tridymite prevails here. It forms either feltlike cross section after coarse quartz fragments and it participates at the composition of matrix at the same time. It reaches up to 500.10-6m (i.e. in comparison with tridymite in the transient zone, its size increases approx. forty times). It is almost parallelly distibuted in the direction of a temperature drop. Further, cristobalite occurs in this zone. Cristobalite has smooth lepidoblastic microstructure and it is isotropic. In matrix, the matter surronding coarse tridymite is yellow up to brownish and contains opaque magnetite, further monticellite and fayalite (formating as  products of the decomposition of pyroxenes, unstable at high temperatures). The last zone - the cristobalite one  is gray, in comparison to the previous one very thin (about 3 mm) and forms at a temperature of approx. 1540°C. It is even-grained, tridymite does not occur here. Original fragments of quartzites distinguish just in their finer microstructure. Cristobalite is lepidoblastic, often cumulated to rounded aggregates of 0.1 - 0.3 mm size, with a mosaic-like microstructure. Individuals of cristobalite reach up to 20.10-6m. One can observe intergrowths (symplectites), polysyntetic twins and pseudomorphs after tridymite. The intercrystal spaces are filled with a braunish glass melt, fayalite and magnetite microlithes. In the case of a high local warming-through of overloaded vaults of furnaces, the tridymite zone may be considerably reduced or absent. On the contrary, the cristobalite zone is totally absent in gannister from the bar screens of checker chamber of the Siemens-Martin furnaces. In average, temperature does not exceed 1300°C here. Except of automorphic crystals of tridymite, there was also pseudowollastonite identified in the black zone. Its contant decreases from the contact to the surface. The surface is formed by a microscopically pellucid glass layer with isolated crystals of magnetite, possibly of hematite. This glass melt dissolves and corrodes tridymite and contains prismatic pyroxene of the hedenbergete type in a contact with the tridymite zone. Gannister from coking batteries has also a similar distribution. The cristobalite zone is absent. The tridymite contains a considerable amount of carboneum matters and graphite. Pseudowollastonite concentrates in the transient zone.

Zones of in fact similar composition and extent as in the case of gannister from the Siemens-martin furnace form in gannister in the vaults the glass furnaces. However, they have significantly lower Fe-components content. Only anisotropic lepidoblastic cristobalite occurs in the cristobalite zone. In a finer form, up to 50.10-6m, it occurs in places of originally coarser fragments of quartzite. Coarser-grained cristobalite, up to 150.10-6m, is a product of a tridymite transition in matrix. Rarely, one can identify pseudomorphs after tridymite. This cristobalite is sealed with a yellowish up to yellow-brownish glass melt, which contains about 20. 10-6m sized dendrites of secondary cristobalite. According to the development of crystals, which is typical for cristobalite formed during the glass devitrification, it is apparently melt crystallization. About 7 -8  cm from the contact with a batch, the tridymite zone without free quartz, with coarser-grained tridymite in matrix and with an isotropic cristobalite in places of original coarser grains of quartzite. Further, high birefringent silicates are present in matrix.



6 Glass

An application of microscopic methods represents an essential part of a technochemical control of products in a glasswork.  Streaks, cells, products of devitrification and occlusion are microscopically watched. A glass type can be determined by a identifyind of refraction. A temper, its character and distribution can be evaluated by a birefringence measurement. A microscopic study is applicated at a material control, at a dust counting and at a study of  a corrosion of heatproof materials caused by a glass melt.

6.1        Mineral phases

From the listed mineral phases, there is α-cristobalite (SiO2) present in the technolithes. The mineral has a tetragonal symmetry and is stable at low temperatures. During heating to 198°C - 240°C, it transforms to a cubic β-cristobalite (metacristobalite) that is also stable at high temperatures. It forms from quartz at the temperature ranging 1000°C - 1470°C or from tridymite over 1470°C. Cristobalite formed directly from quartz is so fine-grained that it seams to be appearently amorphous - isotropic. It forms a lining around the quartz grains or it occurs in cracks fillings. If the fillings are formed by cristobalite, they are ordinarily poorely transparent, translucent and darker. If it is heated to a higher temperature than 1600°C (e.g. in a gannister gray zone in a vault of the  Siemens-Martin furnace or at lower temperatures in a work-zone of gannister in a vault of a tank furnace. On the contrary, in a crystallization process in glass, its typical shapes are dendritic aggregates of prismatic crystals, joining at a  900 angle to a central, axial crystal. A pyramidal up to clubbed ending of the individual needles is typical for cristobalite and frequently occurs.

The high-temperature cubic modification cristobalite has n = 1.486. An assemblage with quartz occurs in glass. It is flaked formed, in gannister and dendrites with rectangular, clubbed ended arms. 

Another SiO2 form is tridymite. Rhombic a-tridymite transforms to hexagonal β-tridymite at 117°C and to  γ-tridymite at 163°C. It forms from quartz at a presence of melt between 1200 - 1470°C. Over 1470°C it transforms to cristobalite. 

Unlike cristobalite, tridymite is usually coarser-grained and forms twinned intergrowths on (110). In a lengthwise sections, they are lanceolate or wrim-shaped. In a cross section, they occur as twins, easily recognizable in XPL according to a different extinguishning of the both individuals. The cross section can also be hexagonal. It sometimes crystallizes in a form of prismatic crystals. Described shapes are common in gannister, where they form matrix along with glass.  If it crystallizes from glass, it forms stellar spherulites with 600 angle of main arms forming diagonals of the hexagons. Lateral, long prismatic crystals are connected to the diagonals. Sometimes, they form hexagonal plates too. The plates are very thin in a cross section.


Mullite Al2[O|SiO4] occurs in chamotte, or in the contact ot chamotte and glass. It has a rhombic symmetry and forms long prismatic up to acicular crystals with almost square cross sections. A longitudinal cleavage on (010) is often perceptible. In chamotte or high clayey material, occlusions are often accompanied by corundum an nephelinite.

Corundum Al2O3 has a trigonal symmetry. It is often accompanied by mullite and nephelinite in occlusions. It forms xenomorphic grains, sometimes is columnar. Cross sections are triangular- or hexangular-shaped. It is not cleavable, it has just a cross division on {0001}. It is ordinarily pellucid.

Baddeleyite ZrO2 crystallizes in a monoclinic symmetry, forms plate-like or short prismatic pyramidally ended crystals. It may occur in a shape of rounded elongated grains, often is ovoid-like shaped. It has a perfect cleavage on {001}. In thin sections, it is colourless up to brown, rarely pleochroic between brown and yellow or green. A typically high refringence and birefringence, overlapped with a colour of the mineral itself, distinguishes it from other minerals.

Kaliofilite KAlSiO4 has a hexagonal symerty. In cross sections, it forms short prismatic crystals, imperfectly cleavable on {10ī1} and perfectly cleavable on {0001}. It contains up to 20 % of nepheline in a solid solution. Its hardness ranges 5.5 - 6 (cloce to orthoclase). It is colourless in thin sections, its specific gravity ranges 2.49 - 2.67. Values of refractions oscillate between: a = 1.527 - 1.533, g = 1.532 - 1.537, D = 0.005, Chm (-). The variability is connected with an existence of solid solutions. It is very often optically identified as nepheline. It has a negative relief and a low birefringence in cross sections.

Carneigieite Na2O.Al2O3.2 SiO2 represents a high-temperature polymorphous phase of nephelinite.

If it is cubic with a refraction n = 1,51, it forms from nephelinite at temperatures higher than 1248°C. In cooling below 687°C, its symmetry changes to a triclinic one. It forms xenomorphous grains, rod-like crystals and also needles. During very slow cooling, it transforms to nepheline.

Its polysyntetic twin lamellae are similar to albite that has higher refractions.  It is hard to differ from nepheline according to a lower refraction and a polysyntetic lamelling.

6.1.1 Products of devitrification

Often occuring outstanding features of the formation of occlusions by devitrification are geometrically symetric shapes. Some of the shapes are so typical for a morphology of the mineral that they enable to determination without detecting of the other optical parameters. However, on the other hand, every mineral species occurs in a series of morphological varieties and various microstructural configurations. They can occur as unoriented isolated crystals. Or as perfectly developed spherolites. The differences are connected with different physical conditions of crystallization. Therefore in some measure they enable to idetify passed processes. Reasons of the devitrification can be: lack of uniformity of the glass melt comming through, e.g. in the surroundings of occlusions and streaks, change of composition of glass or too low temperature in a certain part of the furnace or combination of several factors. In a processing of a „perfectly“ homogenous glass melt, viskozity decreases with rapid cooling. It is so significant that microlites of the phases are prevented from crystallization, even in spite of growing of the speed of crystallization to a certain maximum.  

Figure 8 The chart illustrates  crystallization ability of a hypothermic melt, which is given as a function of the undercooling, characterized by a behaviour of viscosity. Legend: VZK - the formation of crystallization nucleuses in the unit volume in the unit of time, at the given degree of undercooling; LKR - linear ability of crystallization.


A crystallization ability is in fact given by a spontaneous formation of crystallization nucleuses in a volume unit during a time unit at a given degree of undercooling.

The growing viscosity „slows down“ diffusion of ions, which is necessary for the growth of the released nucleuses and for releasing of other ones. From the Fig. 6, it is further obvious that maximum of LKR is flatter than maximum of VZK, does not correspond to it and  is usually removed towards a higher temperature level.  At a low viscosity, there are better condition for a faster growth of a lower number of crystallization nucleuses than for a higher number of them (in a zone of VZK maximum values) at a little higher viscosity already. A different LKR and VKZ maximum values are further a cause of various microstructures and various development of crystals that form at different temperatures. At the beginning of the crystallization - close below liquide temperature - at a low degree of VKZ and LKR, isolated minute crystals are released. They are rather hypauthomorphic and they are in equilibrium with the melt. Automorphic crystals of a maximum size develope with a rising temperature. Number of them rises very quickly and their size somewhat decreases at the same time. A columnar crystal habit is more frequent. The formation of spherulites proceeds over boths maximum values (LKR, VKZ). Number and size of spherulites falls with a crystallization temperature drop. Tiny stellar spherulites appear and hypautomorphic skeleton-like crystals are formed. In the end, after the devitrification, hypautomorphic skeleton-like crystals form. In the devitrification, formation of more mineral species proceeds, particularly if the composition the glass melt is close to an interface crystallization fields. Mineral assemblages form this way.

A range of composition of common lime glass lies in an area, where devitride or wollastonite, exceptionally cristobalite, crystallizes as a primary phase. A SiO2 cristallization in cristobalite, possibly tridymite form proceeds after CaO depletion of the glass. CaO is bound in above listed silicates. In glasses with the MgO content over approx. 3 %, diopside ordinarily forms instead of wollastonite and plagioclase forms instead of wollastonite in clay-rich glasses.

Devitride Na2Ca3.[Si3O8]2 forms by crystallization in Na-rich glassess. It occurs the most frequently in a flat glass and in some species of container flints. It crystallizes at approx. 725°C. It is stable up to  1045°C, wollastonite - Ca3[Si3O9] dissociates into a melt over this temperature. It frequently occurs on bottoms of flat glass melting tanks. Its crysral symmetry is rhombic, it ussualy forms acicular crystalls, mutually felt-like intergrowing. Fanlike aggregates and besom-like, penicillate and bunch-like reaching up to a few millimetres. These forms can be considered typical. The shape and microstructure manifests a fast crystallization from a very viscose melt. In thin sections, it is  colourless, pellucid, with an indistinct cleavage, α =1.564, β = 1.570, γ  =1.579, D = 0.015, Chm (+), Chz (+). Wollastonite is a similar mineral. Devitride can be distinguished from wollastonite according to brush- or fan-shaped bunches of needles, possibly, in the case of idividual crystals, on a basis of its lower refractions. 

b-wollastonite            Ca3[Si3O9] (parawollastonite) the most frequently crystallizes in a container flint that is somewhat CaO-richer than the flat glass. It can be found by a bottom in corners of  the melting tank, e.g. due to an exsolution of heavier limy component of stone. It is stable up to 1180°C and transforms to pseudowollastonite over this temperature.

It has a monoclinic symmetry and white colour. The crystals are acicular, if perfectly developed, elongated with lath-like up to wide lath-like cross sections. Laths, which are not flat terminated, are typical for  wollastonite. The temination of the laths is usually typical. In thin sections, a parallel „fringing“ comes through, as if the termination of the crystal would consist of several parallel individuals of a various lenght. Among them, one can find nonuniformities, which are parallel to the elongation and getting into the crystals. In fact, the nonuniformities are clevage cracks on {100}. b-wollastonite is perfectly cleavable on {100} and good clevable on {001} and {102}. It forms radial and fan-like aggregates. Rounded rims of the crystals manifest their reverse dissolution. It can be accompanied by devitrite or cristobalite, which crystallized directly on the large laths of wollastonite.

It has a positive relief (α =1.616 - 1.621, β = 1.623 - 1.633, γ  =1.631 - 1.635), D = 0.014 - 0.015, Chm (-) , Chz (+,-).

It differs from devitrite in shape, (if it is prismatic), in a character of zone and a higher refraction. From anorthite in an absence of twinned laths, from pseudowollastonite in a charaktere of zone. It is the most often confused with devitrite.

α - wollastonite (pseudowollastonite) CaO.SiO2 occurs less frquently, sometimes along with with b-wollastonite, into which it transforms below 1180°C. Alike b-wollastonite, it occurs in a container flint, where it can form at slow cooling and the high CaO content. From the β-modification it differs in a triclinic symmetry, it crystallizes rather in a shape of short prisms and hexagonal plates. Larger (n.10-6 m) hexagonal forms of b-wollastonite are pellucid in a central part and they become tarnished towards the rims. Sometimes, they consist of dendriform aggregates, similar to cristobalite. However, they differ from cristobalite in their significantly higher refractions and birefringence.

a-wollastonite has α =1.607 - 1.618, β = γ =1.649 - 1.663, D = 0.044, Chm (+) , Chz (-) .

a-wollastonite differs from b-wollastonite in its significantly higher birefringence, a character of zone and a shape of cross sections.

Diopside CaMg[Si2O6]

Diopside is a less frequent product of devitrification than wollastonite. It is ordinarily found in a coloured container flint with the MgO content over  approx. 3 %.

It forms isolated elongated prisms or it forms sherolites  . If they come out along streaks, they are considered to be an evidence of the nonuniformity of the glass.

It has a monoclinic symmetry, its optical and physical properties are summarized in the encyclopedy of minerals. Unlike wollastonite it has an inclined extinction, somewhat higher refreactions and birefringence and always positive length character. It is distinguishable from devitrite by its incclined extinction, positive relief and somewhat higher birefringence. 

Jeffersonite Ca (Mn, Zn, Fe, Mg) [Si2O6]

The mineral is a Zn-variety of schefferite (diopside + hedenbergite with Mn and Ca). Thus, it is certainly  analogical to diopside, where magnesium is isomorfously replaced by manganese, zinc and iron. It crystallizes from special glasses, particularly zincmagnesium ones.

From optical parameters, it is possible to state: α = 1.682, β = 1.690, γ = 1.710, D = 0.028, Chm (+). It has an inclined extinction, at the angle of 360 towards the elongation.

Cristobalite is usually the most common modification of SiO2 crystallizing at the devitrification process, even if it crystallizes outside the zone of stability, i.e. below 1470°C. At a longer heating below this temperature, cristobalite transforms to a low-temperature modification of SiO2 -  tridymite. It goes through a series of stages. The original rectangular dendritic forms, terminated with pointed columnar crystals which corresponds to a cubic system of high-temperature cristobalite transform into hexagonal stellar aggregates with lateral arms connected at an angle of 60°. These stars can sometimes remind aggregates of nepheline. In a series of transient stages, the final form is hexagonal lamellar tridymite. Sometimes, the both forms of the crystal occur together. The stellar aggregate of the transient form is edged with very fine dendritic cristobalite. Due to a contraction of volume at cooling induced by a reversible transformation of β-cristobalite into α-modification, a net of cracks forms in a place, where devitrification proceeded. A relateve rare form of cristobalite, crystallizing in boronsilicate glasses, misses a typical rectangular structure and consists of irregularl fragments. Its physical and optical parameters are listed in the encyclopedy of minerals, other facts in a passage regarding occlusions of heatproof materials.

The occurrence of tridymite indicates that the crystallization of SiO2 proceeded in a longer time interval. Cracks surrounding tridymite may also form.

Plagioclases are in detail described in a chapter about groups of minerals and in the encyclopedy of minerals. They crystallize in high-aluminous glasses.

Bariumdisilicate BaO. 2SiO2 has a rhombic symmetry. It crystallizes from barium glassess, where it represents a typical product of devitrification. In thin sections , it forms long prismatic elongated crystals, with lamellar or pseudohexagonal cross-sections (sometimes with a central cavity) or less developed crystals in a shape of iregular flakes. Refractions range α = 1.595 - 1.602, β = 1.610 - 1.617, γ = 1.613 - 1.632, D = 0.024, Chm (-) ,Chz (+) . It is  distinctly cleavaable on {001}, {010}and {100}.

Willemite Zn2[SiO4] may crystallize from glasses with the relatively low ZnO content. It forms small hexagonal prisms. Its optical and physical properties are listed in the encyclopedy of minerals.

Alamosite Pb[SiO3] has a monoclinic symmetry, it occurs in devitrification products of heavy lead glasses. In soft lead glassess, cristobalite occurs. It forms pellucid or white, fibrous up to long columnar crystals, which are often spherulitic.  It has a specific gravity 6.49, hardness 4.5 and a diamond lustre. It is perfectly cleavable on {010}. α = 1.947, β = 1.961, γ = 1.968, D = 0.021, Chm (-), Chz (-).In nature, it occurs in a zone of oxidation of lead deposits.

Nepheline is in detail characterized in the encyclopedy of minerals. It was detected among the occlusions of the heatproof materials. In glass, it ordinarily forms aggregates on the interface of natrium glass and chamotte and high-aluminous heatproof material. It is usually accompanied by mullite, possibly corundum. It differs from mullite in shape, lower refractions and a negative length character. 

Mullite is also listed in the encyclopedy of minerals and it is discussed in the passages above. It forms in the interface of glass and chamotte or in special glasses with the Al2O3 content of over 40%.

In opal and opalescent glasses, devitrification is induced on purpose. However, the size of crystals is usually smaller than a resolving of a polarization microscopeand. An electron microscopy is successfully used  for the study of them.

The microscopic determination of devitrification products serves not just for the identification of operating faults, but also for a study of equilibrium states of the glass using a method of crystallization in gradient ovens. It is possible to assess the liquide temperature, if a volatilization of some of the components does not come up and to find out a value of the crystallization pressure. At these mesurements, incised zones in a form of “overflowed drops” with a  different refraction sometimes occur on the surface of the glass before a precipitation the first crystallic phase. It is probably a detached liquid phase which dissapears during a following heating. The first precipetation of the crystals proceeds in these spots. 



6.1.2 Surface defects

A binocular microscope is used for watching of surface defects, cracks and streaks. Deposits on the internal sides of a container glass can be removed with a putty knife and watched in an immersion liquid. Sodium sulphate can form on a surface of glass to which alkali (e.g. Na2SO4) were partly brought or in which a reaction of fumes of  SO2 and alkali in an oxygenic atmosphere proceeded. It usually occurs in a melt, which temperature does not exceed approx. 1200°C.  Na2SO4 decomposes at a higher temperature. Sulphate incrustations form either by a condensation or by a straight reaction of SO3 and alkali on the surface of the glass melt.

Na2SO4 has a rhombic symmetry. It forms amorphous film of a bluish shade. Due to a movement of the glass melt during its formation, the film is usually not continuous and is separated into small isles. The film is partially miscible with the glass melt. Its shape is often eliptic up to teary. It is only perceptible in a form of acicular crystals after a recrystallization on the surface. α = 1.471, β = 1.477, γ  = 1.484, D = 0.013, Chm (+), it has a prallel extinction. Theformation of the sulphate may be caused by a crystallization of tridymite in the surface zone of glass. On the contrary in an reducing environment, in a glass enriched with alkali, its decomposition, leads to a crystallization of silicates. Due to the air humidity, leaching of alkali proceeds on the surface of the glass. If the alkali are not removed, alkali carbonates, particularly Na2CO3, filamentary crystals form on the surface. These are easily microscopically identifiable as birefringent dendritic formations. The glass surface is corroded, covered with dendritic „etchings“ after a removal of the carbonates. For its distinguishing from similar cristobalite we use an immersion liquid with the same refraction as the refraction of glass. In the case of dendrites, the surface relics disappear. In the case of the presence of cristobalite, the relief is conspicious. 

For a study of plied and flashed glass, material cross sections have been successfully used. The cross sections enable watching of e.g. a temper between both of the layers.

6.1.3 Streaks

The streaks usually represent locations of glass of different composition with a different specific gravity and a different comming through relief. At a microscopic study, Becke lines edging the streaks are typical. Glass streaks appear due to a series of faults, e.g. an insufficiently homogenized glass batch, a too coarse-grained material, a melting of a heatproof material of port arch and jamb wall, bringing devitrification products back to places with a higher temperature by a flow and changes of a composition of the melt.

The streaks with a lower refraction than surrounding glass have usually a connection with SiO2. Its source can be a sand of the glass batch or a corrosion of a gannister flying arch. The streaks with higher refraction form either due to a partial assimilation of a fireclay or an aluminous heatproof material or due to a contamination with wollastonite. The following Table 7 gives an outline of character of the contaminants.  

A suitable immersion liquid (i.e. with a close refraction) placed into a microcell should be used to study the streaks of fragments of an uneven glass. The glass should be immersed into the liquid.  The microcell should be made of a tiny vial cross section. A ring of a thickness of approx. 5 mm should be cut off cut from  the cross sectio. Finally, the ring should be sealed with an epoxide to a microscope slide.


Table 7 Relation of a change of refraction with a character of a contaminant.


0,1 % enriching

Refraction change




+ 0,0025


+ 0,0007


+ 0,0006


+ 0,0001


However, an affect of the change of the chemical composition on the refraction is not very significant. Therefore, a phase contrast is successfully used for searching of streaks that only a little differ from their surroundings. The streak that is otherwise indistinct displays a noticable shade, different from the surrounding glass. That’s why the refraction is determinable by the immersion method in the phase contrast. It is possible to to measure the refraction even more exactly in a polarization microscope.


6.1.4 Cells

The cells often occurs close the devitrification products. The phenomena can be explained by an affect of a decrease of solubility of gas in the glass melt.  The reason of a formation of cell is microscopically identifiable, particularly if the cell contains crystallic inclusions.  However, the microscopic watching destroys functions of the cell as a lens. Therefore, the glass should usually be a little grind close to the cell to create parallel faces (the grinding should be done bilaterally, in the case of a curved sample). The cell should be covered with an immersion liquid with the same refrafraction as the glass and driven through with a needle. Due to an underpressure in the cell, the liquid is usually sucked inside. The crystals should be determined in an environment of a homogenous immersion liquid. Bubbles caused by an oxidation of carbon contained in a form of cementite in pieces of iron can be filled with compounds of iron. According to a degree of oxidation, they are yellow-brown or green or can contain iron in a form of a metal or an oxide. In an instance of a decomposed sulphate that is only partially miscible with the glass melt, the cells can contain  Na2SO4. Sodium sulfate can ocuur either in a form of spherolites or in a form of fine cracked filling.


6.1.5 Occlusion and devitrification

The occlusions and the products of devitrification can be studied both in thin sections and, if we aim at an identification of phases and a microstructure, in grain preparations that enable us identifying of e.g. refraction. To prepare the grain preparation, glass with an occlusion should be bruised into coarse fragments. The ones which are suitable for another bruising, e.g. fragments containing crystals, should be selected in a binocular microscope. It is usually recommended to remove an excessive glass in a flame. Two hard glass canes should be seamed to the glass with the occlusion fragment. After a warming, the glass surrounding the occlusions should be pulled out by a fibre. One gets the occlusion, which is covered with a film of glass. The grain preparation contains only a minimum amount of glass then. If the occlusions or the devitrification products are perfectly transparent, the thin sections made of glass with occlusions can be thicker (up to 0,1 - 0,3 mm).  In thin-walled samples, one may replace the thin section by a thin fragment immersed into an immersion liquid with a refraction similar to the glass. Crystallic inhomogenity in the glass come from three sources. It can be a not completely melted stone, corrosion of a heatproof lining or it can possibly be of products of  crystallization of glass. To identify them, according to need, one can use a refraction, a character of zone and a birefringence. A very small size of crystal usually makes a determination of an optical character of a mineral impossible. In a  microscopic study, we are not bounded just by a composition of the material of occlusions, but we also observe their microstructure. We observe a shape and a fire polishing of rims of the occlusion, their size and development and distribution of crystals. In an interface of the occlusion and the glas, we aim at ationauthigenic formations - secondary crystallic phases and streaks and at manifestations of strain. These characteristic features usually can help us both at the identification of occlusions itself and at an explaining of a genesis of the occlusions. The basic reasons of the formation of occlusions from the glass batch are:

-          lack of easily soluble components of the glass batch;

-          low temperature during the melting of glass;

-          affect of a coarse-grained or polydispersive sand, dolomite and other substances;

-          insufficient mixing of material;

-          short time of the glass melting;

-          irregular starting of the glass batch.

Therefore, it is obvious that the most common occlusions of the glass batch are quartz grains, partly or completely transformed to a high-temperature modification SiO2. Further, these are minerals accmpanying quartz in sand, particularly accesory minerals (zircon and chromiumspinel). Due to its genesis, we sort here an allochthonous material brought by the glass batch, without being its part, e.g. iron too.

At the beginning, quartz occlusions (quartz grains in sand) keep their round shape with a smooth surface and thin irregular cracks. Number of the cracks increases with a rising temperature. The surface gradually becomes  uneven, the cracks are net-like. A form of SiO2 changes at the same time. surface of the grains becomes covered with a layer of cristobalite or tridymite. The layer becomes thicker and it fills the propagating cracks. Unlike in the case of quartz, the cracks are very fine. Their minute size is a reason of a seeming homogenous appearance of these linings and fillings. A less stable modification - cristobalite usually forms as the first one. Its formation is, apart from other things, suuported by present mineralizers (alkali). At the same time, in a direct formation from quartz, the formation of cristobalite requires a simpler recofiguration of a lattice than in the case of tridymite. Inside an original grain  of quartz, cristobalite can be without a distinct crystal form or it forms a typical lepidoblastic microstructure, occuring e.g. in a cristobalite zone of gannister. Only on a periphery of grains, where it forms from an equilibrium melt, it forms skeleton-like crystals with orthogonally joined lateral prisms. The prisms  are a typical form of cristobalite occuring in devitrification. Along with a gradual melting of sand the melt becomes enriched with SiO2 so much that the skeleton-like cristobalite also forms in isolation, outside the grains. However, it only forms close the grains. Later, the cristobalite filling the crack and rims can be partially or completely transformed to tridymite. Tridymite usually occurs in acicular or lamellar forms. In central parts of completely transformed grains of quartz, lanceolate-twinned tridymite was found, while the rims of the grains were made of skeleton-like cristobalite. The lanceolate-twinnned tridymite form occurs in gannister again.

Another sort of occlusions of the glass batch are clayey ones. Aggregations of clay usually come from an insufficiently washed sand and they form difficult soluble occlusions. They are almost „amorphous“, only translucent, with crystals of mullite of a size of n.10-6m order of magnitude. The mullite crystals are perceptible just in the highest magnification.  Occlusions of so-called heavy minerals (rutile, zircon, chromite) accompanying quartz in sand are quite rare and their genesis is the same. Dolomitic and calcitic occlusions usually containing periclase grains linned with diopside are also rare. 

Genesis of sulphate inclusions have been discussed in surface defects. Due to an partial miscibility with glass melt, the inclusions of sulphates make lenticular forms, sharp bounded with glass. Their microstructure is  irregularly cracked or filled with spherolites.  A contamination with a metal iron brought into the glass with batch, is always perceptible in a magnifier. However, a lead deposition in surroundings of an iron corpuscle is microcopically observable e.g. in lead glasses.

A heatproof material represents another source of occlusions. The heatproof occlusions are the most common ones. Their composition and microstructure depends on a series of factors. Particularly on a sort of the heatproof material, a placement of lining which they release from, on temperature and on an action time of the surrounding glass melt. 

Due to an action of alkali oxides, a melting down of a port arch proceeds. Drops, usually containing small gannistere fragments, form on a gannistere surface. Quartz is neither present on the surface of gannister nor occurs in the occlusions. The occlusions are formed predominantly by a lanceolate-twinnned tridymite. A dendritic cristobalite, usually linning tridimite, is sometimes present. One can differ the occlusions of port arch from the occlusions of batch on the basis of an absence of quartz. In an initial stage of reaction with the melt, mineral composition and microstructure of the chamotte occlusions correspond to the original material. They ordinarily form hardly transparent irregular grains, containing very fine crystallic n.10-6m sized crystals of mullite. Due to an affect of the glass melt, a accumulating crystallization proceeds and mullite forms. Mullite occurs as long prisms and needles up to 50.10-6m long, surrounded with the glass melt. Mullite crystallizes in all chamotte species. Therefore in acid chamotte, it is impossiple to e.g. draw a conclusion from its presence that the original heatproof material was highly aluminious. In the contact of occlusions, an external rim of aggregates of granular nepheline (KNa3[AlSiO4]4) forms by reaction with the alkalic melt.  Its formation is considered to be typical for the chamotte occlusions that were in a contact with the glass melt for a longer period. The nepheline optical properties are listed in the encyclopedy of minerals. Its refractions depend on a Na + / K+ ionts rate. Nepheline forms solid solutions. In the rims of the chamotte occlusions, nepheline occurs in a form of short prisms with hexagonal sections or it forms dendritic aggregates. In occlusions that were in a contact with the glass melt for a longer time, the aggregates can totally replace mullite and  b-corundum. In potassium glasses, nepheline can be replaced with kaliofilite K[AlSiO4]. A skeleton-like microstructure of both of the feldspathoids can be a cause of their confusing with cristobalite. They are differentiable on a basis of comparison of their refraction to the refraction of glass. Nepheline has higher refraction than a surrounding calciumsilicate glass. If chamotte reacts with melt at a tmeperature higher than 1245°C, a high-temperature polymorphous phases of nepheline called carnegieite forms. The mineral has a typical polysyntetic lamelling. The retransformation to nepheline is slow and both of the minerals can occur together. In high aluminous chamottes, a decomposition of mullite and a formation of authigenic forms proceeds by a chemical reaction with alkali oxides. Above a surface of the glass melt, mullite decomposes to corundum and a melt due to alkali metals. The process runs even at lower temperatures than the glass melting one. Then, corundum can be transported by the melt to the glass melt, where it stays preserved for a long time due to its high melting temperature, chemical resistance and relatively large size of individuals (up to 1 - 2 mm). Corundum forms perfectly developed hexagonal automorfic thin plates with a distinct relief. Its cross section is lath-like  Chz (+). Under the surface of the glass melt, the erosion of melting chamber proceeds and chamber occlusions appear due to the affect of streaming. Chamotte grains, ordinarily containing felt-like mullite aggregates in the central parts, are brought from the chamber occlusions to the glass melt. Mullite is usually long prismatic, the crystals have almost square cross sections. The crystal ends are round, fire-polished.  Sometimes, at a formation of a rim of β-corundum and a melt, exceptionally α-corundum can be also present. A bordering of β-corundum is similar to α-modificacation. Its relief is less distinct (refractions are lower). On the contrary its birefrefringence is considerably higher. In CaO-rich glasses, plagioclase represents a final product of the melting of chamotte in the glass melt. The transition of the occlusions to the melt can be either a gradual with a wide range of irregularly scattered products or, contrawise, completely direct. 

Due to a relativly coarse-grained microstructure and the content of chemically stable minerals, the electromelted occlusions dissolve very slowly in the melt. Their initial composition stays unchanged for a long time. Their material composition is different and it corresponds e.g. with flaking off, a penetration of the glass melt into cracks and cavities and with a washing up of resistant crystals from a less resistant glass matter. According to properties of the original materil, they mostly contain mullite, β- and α-corundum and baddeleyite. A resistance of  the mentioned minerals to corrosion caused by the glass matter also increases in the same direction. a-corundum and  baddeleyite stay preserved in the glass melt for the longest time. Unlike secondary (authigenic) corundum, which forms on the boundary between the chamotte occlusions and glass, primary corundum is mostly xenomorphic. The secondary corundum is corroded by the melt and it contains inclusions of baddeleyite. A distinct rim - streak with a higher refraction than surrounding glass forms around baddeleyite. It gets to the melt from a surface protecting coat. In a study of the genesis of the occlusions, it is useful, if necessary, to make a comparison with a microstructure and a mineral composition of the heatproof layers on the surface.



7 Chamotte

Chamotte is the most sigificantly used in metallurgy. Here, chamotte is exposed to the affect of high temperatures and is simultaneously corroded by slag, metal or fluids. The temperature gradient and diffusion of allochthonous oxides lead to the formation of distinct zones with a typical mineral and chemical composition. The authigenic phases correspond to a type of corroding environment and also to the composition of chamotte.

7.1        Mineral composition of used chamotte


Both microstructural components: coarse grains of an opening material and fine-grained matrix already become apparent macroscopically. They are the most conspicous in a fracture. The surface the fracture is suitable for an advance evaluating of chamotte. In a product of a  high quality, where the opening material in perfectly connected with matrix, the fracture comes through the most of grains, so the fracture surface is rather even. On the contrary, in a product with a looser bedding of the opening material, these grains stay untouched in the fractutre and they fall out. One can observe their prints in matrix. At the microscopical study of thin sections, one can observe differences among the components. The opening material is ordinarily transparent, yellow-brown up to gray-brown. The  mineral composition of the opening material is in fact the same as the mineral composition of grains of a fired shale or a fired clay. It is formed by mullite and a glass melt. However, mullite is submicroscopic. It is not possible to distinguish its individual crystals from a lower refractive glass melt. It is only possible to assesss a total refraction of the opening material, which is affected by the mullite/glass melt ratio and increases with a degree of mullitization. In the incident light, the mullitized opening material is more reflective than weakly mullitized matrix. In CPL, these grains are just indiscernible light without a characteristic extinction of the crystal individuals. One can also observe a birefringence of aggregate. There is an exception only in those grains of shale, which contain kaolinite bacilarites mullitized by firing in rough conditions. This pseudomorphs are perceptible, when watching in CPL, because the crystals of mullite keep a parallel direction on the direction of the flakes of kaolinite. Preferentially arranged aggregates, extincting almost simultaneously, form this way. In CPL, besides the orientated aggregates of mullite, one can distinguish irregular, anisotropic grains of quartz. Quartz is pellucid, its untransformed grains are cracked due to a reversible conversion of α-quartz to the β-modification. Fine-grained quartz and possibly also small crystals of accessory minerals, e.g. rutile or zircon are of the primary genesis and they come from the material.

Around the grains of the non-plasics, noticable cracks form. Matrix of chamotte is lighter and more porous than grains of non-plastics. They are parallel to margins of the grain due to the affect of strong shrinkage of the bond-clay. Mullite is weak-perceptible in matrix, it reaches 3-5.10-6m. Rarely, particularly in the margins of a thin section, one is also able to distinguish individual crystals. Hypautomorphous shape of mullite is only perceptible after its preparation using fluorhydric acid. Size and shape of the mullite crystals is affected by temperature and time of firing, way of cooling and viscosity of surrounding glass melt. Mullite arranged to felt like aggregates reaches a maximum size up to 50. 10-6m. At firing over 1200°C refraction of the chamotte glass melt decreases as a result of melting of fine quartz and oscillates between 1.52 - 1.54. In the case of a brown glass melt  containing iron, its refraction reaches values up to1.60 - 1.62.

 Acidic chamottes contain a largeger part of quartz both in a coarser-grained and a finer fraction. Grains of the coarser-grained fraction are considerably cracked. These grains are also unaltered, but considering their excellent adhering to the bond clay a thin glass layer around the quartz grain forms (the bond clay contains fine-grained quartz and also additives, therefore a large volume of glass melt forms there). Close to a quartz grain, the glass melt is pellucid. Due to  chemical composition changes and  drop of viscosity needles of mullite of 2-5. 10-6m length appear. The mullite needles cause tarnishing of glass melt.  Mullit is oriented in the direction of the gradient of diffusion of aluminous ions and viscosity, i.e. perpendicular to the margins of the quartz grain. This interlayer has a crutial affect on a strength and density  of the body. 

The high valuable chamotte has a suitable granulomertic composition and non-plastcs excellent mixed with a bond-clay, which is uniformly distributed. The iron content is very low and only in tiny fragments, so it deleteriously affects neither temperature resistance of products nor changes of volume. Pores are uniformly distibuted.

In a common quality chamotte, the considerable free quartz contant particularly perceptible. Its fragments reach up to 0.5 mm. Non-plastics is rather well mullitized, individual crystals reach a size of 10-15.10-6m, but mullite is completely indistinct. Only locally, it is possible to distinguish its individual hypautomorphic crystals. However their size does not exceed 5.10-6m. Pores are larger than in the high-valuable product and they are not uniformly distributed. 

7.1.1 Chamotte and linings of blast furnaces

Due to the affect of cooling, a considerable change of density does not come up in the major part of the lining. In spite of this fact, even macroscopically distinguishing zones form in chamotte. The primary zone is light ochroid and corresponds to an unused chamotte. The transient zone is darker, better mullitized and contains a fine-scattered corbon both in pores and in intergranulars of the non-plastics. The internal zone is usually densified by authigenic minerls.

Carbon releases the most intensively at a temperature between 450 - 550°C. In the final stage, the setting of carbon leads to a comlete corrosion of chamotte. The affect of alkali from charge manifests as a formation of authigenic minerals. A thin densified layer containing nepheline and kaliophyllite forms. Towards the transient zone, leucite and feldspars occur instead of them. Further, mullite and glass melt are present in the transien zone. Besides alkali minerals containing oxides, other minerals (pseudowollastonite, melilite and gehlenite) may be present in the surface zone. These minerals form as products of reactions with a lime slag or a charge limestone. Besides the formation of anorthite, the decomposition of mullite may lead to the  formation of corundum. At the chamotte corrosion of the lining of blast furnace, elongated polysynteticly twinned anorthite crystals reaching up to 0.5 mm are often identified. In a regenerated  chamotte from the Siemens-martin furnaces, 5 - 6 mm thick glass layer forms in the surface. The glass layer which contains acicular mullite crystals or corundum tables and alkali-calcium-ferric silicates.

Chamotte from ladles contains excellent developed mullite both in the non-plastics and in matrix. Its pores are filled with a glass melt and mullite needles.  The surface layer is hyaline, it usually reaches a thickness of 0.5 - 2 mm. In the contact with the transient zone, acicular mullite occurs. Towards the surface,  mullite is replaced with dendritic crystals of melilite, grains of spinel (MgAl2O4), hercynite (Fe2+Al2O4), galaxite   (Mn2+0.9Mg0.1Al1.9Fe3+0.1O4 ) or lath-like anortite (Ca[Al2Si2O8]).

In the surface of cast material a thin coating 0.2 - 2 mm thick forms. At the casting of a rimmed steel, only a hyaline layer forms in the surface. A killed steel much stronger attacks the chamotte. From the unaltered chamotte towards the corroded surface a recrystallization of mullite proceeds. The mullite crystals reach a size about 5.10-6m.  The chamotte then turns to a glass melt, which is about 0.15 mm thick. Besides the glass phase the glass melt contains confining needles of authomorphic mullite up to 10.10-6m long, with typical square-shaped sections. This layer passess to the surface layer (0.2 - 0.3 mm thick), which was occuring in the contact with the melted metal.  This layer contains rounded grains reaching about 5.10-6m. In a thin section, we observe that this relatively unbroken corundum layer is separated from the chamotte matter itself by a thin layer of the glass melt. The glas melt is a cause of easy jointing of the corrundum layer and the following formation of steel corundum enclosures. The glass layer forms as a result of a high temperature. It forms in the contact with the streaming steel at the simultaneous action of slag drops ripped by the metal. This layer is strong viscose. On its surface, there are identifiable corundum grains, which are products of  the oxidation process of aluminium. Aluminium is added as a desoxidative agent into the ladle. 

In the glass furnaces several  typical zones form during the  melting process of the surface of chamotte. The external primary zone, the transien zone and the internal white reactive zone. The internal reactive zone is formed by a thin greyish hyaline layer representing a transition to the glass melt of a normal composition. Transitions among the individual zones are quite sharp. In the transient zone, about 10 mm from a margin, strong cracked and relicts of surface-melted quartz grains appear. Besides them, one can observe the formation of mullite. In the internal zone, confining cofigurated acicular crystals of mullite and glass occur. This zone is intensively mullitized. The crystals of mullite reach several tenths of millimetre. Original quartz grains react with glass, which penetrates along the intergranulars into their centers. A pellucid glass forms in the places of original quartz. Pores are filled with glass matter. „Floating“ mullite needles occur in the glass matter. Towards the transient zone, porosity rises, but the share of closed pores increases  at the exclusion of open tiny canalicular cracks. The closing of pores proceeds as a result of diffusion of N2O into chamotte. A film of gray-brown glass appears, particularly in high aluminous chamotte. The film contains authigenic corundum, which forms by the decomposition of mullite due to the affect  of alkali metals. It forms authomorphic hexagonal plates. The corundum contant in the glass layer, in the glass matter and chamotte interface and also the mulite contant in the white zone, as well as thickness of these layers depend on the  Al2O3 contant in chamotte. In rotary brick furnaces, chamotte is used in colder parts of the furnace. The operating zone enriched particularly by forms of CaO on the surface. It is ordinarily formed by coarse gehlenite phenocrysts surrounded by fine matrix. The matrix is formed by pennate aggregates of pseudowollastonite, which prevail to fine-grained gehlenite. Monoclinic pyroxene may also be present. The gehlenite-pseudowollastonite zone passes to the transient zone, which is represented by a  densified layer of chamotte, with the high glass content. From the original chmotte, grains of the non-plastics are distinguishable in the glass matter, The grains are linned with  anorthite and pseudowollastonite. In a used chamotte from Czech cement plants, one comes across surface efflorescents formed particulary by chlorides (halite and sylvite). 

On the surface of used chamotte from a cement plant a 1 - 2 mm thick film forms. It is composed of often twinned anorthite plates and edged with fine-grained matrix composed of wollastonite and fine-grained anorthite.   Pseudowollastonite, gehlenite and pyroxene were identified in minute amounts. 


8 Basic heatproof matters

Besides common grain preparations, at a microscopic study of the basic heatproof matters, thin sections are used. The thin sections are used particularly in combination with etching tests. The methods are similar as at the study of cement clinkers.

8.1       Magnezite heatproof matters

If large grains and a fine-grained matrix are ditinguishable from each other, it is possible to study the magnezite heatproof matters using a small magnification. Coarse grains are formed by large periclase aggregates forming a dense, homogenous microstructure. Besides small periclase aggregates, the fine-grained matrix also contains a silicate cement. The silicite cement forms a film covering the grains of periclase as it locally accumulates. Microstructure, size and shape of the periclase grains, the volume of the silicate cement and degree of dispersion along with size and arrangement of pores depends on a series of factors. The factors are: composition the original material, firing temperature and way of cooling. Input and output parameters have a crutial affect on properties of the magnezite heatproof materials. Crystals of periclase, which forms at the forint of the magnezite clinker can be microscopically observed from firing temperature approx. 900°C. At 1000°C they reach a size about 5.10-6m and in the final product about 20 - 50.10-6m. They are isometric, usually rounded, rarely semi sharp angular or sharp angular. They are light-yellow to brown, cleveable on {001} is locally perceptible. Sintering and growth crystals are positively affected by some oxides, e.g. FeO. FeO does not form a melt, but it forms a greenish solid solution with MgO in a reducing environment. With rising share of FeO the refraction of periclase linearly rises from 1.736 to 2.31. Oxidation conditions of firing are more common. Here, Fe2O3 reacts with MgO approx. from a temperature about 1025°C at the formation of magnesioferrite (MgFe3+2O4). Magnesioferrite also forms the solid solution with periclase. However, this one is brownish. The miscibility of the components decreases with a decreasing temperature. At a proceeding cooling the solid solution of magnesioferrite decomposes and one can observe dark opaque inclusions in periclase. Only in the case of an extremely rapid cooling, magnesioferrite stays preserved in a form of solid solution in periclase and it gives periclase a brownish shade. Its refraction increases from 1.736 to 1.86 at the same time. However, in this case, magnezite looses its ability of a magnetic separation. At a rapid cooling, magnesioferrite crzstallizes at a form of small crystals, which are scattered in the periclase grains. At a slow cooling, in periclase, it forms lamellae, ordinarily parallel to the cleavage cracks on {001}. At still slower cooling, it crysallizes at a form of dendritic aggregates possibly up to spheric or irregular formations. Spinel magnesioferrite (pleonaste) has a cubic symmetry. In a thin section, it is usually black, sometimes also reddish. Its high refraction 2.39 is a condition of its high reflectivity. In the incident light it  distinctly differs from gray periclase.  Around the grains of periclase containing inclusions of magnesioferrite, there are films of silicate cement. Its composition depends particularly on the contant of CaO and SiO2. Most frequently, both the components are present in the concentration necessary for the formation of low temperature-melting monticellite (melting point 1498°C). In very exceptional CASE, free CaO that is not a „good component“ may be present. It is dangerous due to its rapid hydration. Another undesired mineral is dicalciumsilicate, particularly in a connection with the polymorphous transition between β and γ modification (at 675°C). Besides common monticellite, forsterit (melting point 1890°C) occurs in cement. It increases the compression strength of cement. At the abundance of CaO, relatively low temperature melting merwinite (melting point 1575°C) may be present. Technical aluminium oxide, which enters spinel (MgO.Al2O3) together with  MgO, is artificially added into produts with low SiO2-content.  Spinel increases a rapid temperature changes resistance and  a fire load kapacity of the product. Physical and optical properties of these minerals are listed in the encyclopedy of minerals. For comparison, microscopic characteristics and optical data of the most common cement minerals are also listed.  


All the mineral phases of the intergranular matter, are xenomorphic. Therefore at approx. the same refraction of forsterite and monticellite, one can use a higher birefringence of forsterite for the identification in an microscope first of all. On the contrary, merwinite differes from both of these minerals in its higher refractions. One can distinguish it from forsterite by lower birefringence, which is similar to monticellite. At the identification of merwinite, it is sometimes possible to use its polysyntetic lamelling. The identification of periclase and magnesioferrite according to their shapes, colours and isotopy of periclases and according to dendrites, opacity and high reflectivity of magnesioferrite, is quite easy. At the study in the incident light in polished sections, the cement represents the lowest reflecting part of the matter, which is apparent from the following order of reflectivity of minerals: magnesioferrite > periclase > β-dicalciumsilicate = merwinite > forsterite = monticellite. From the given survey, i tis obvious that is practically impossible to differ monticellite from forsterite in the etched polished section. Thez both mostly occur in the cement and there is a direct relation between their ratio and their fire loading capacities. Therefore, it is more convenient to combine the study in the incident and penetrating (in a polished thin section) light or to make an etching. Etching by e.g. 0,1 % HNO3 in ethyl alcohol for 5 - 15sec., 1% H2SO4 in 95 % ethyl alcohol, or concentrated HCl (for a time of 2 - 3 sec.) is recommended. Given agents etch only monticellite. For the colourizing of monticellite, using of concentrated HF for 2 - 3 sec., or 10 % HF (5 - 60 sec.) is recommended; forsterite does not become colourized. In the consequence of etching, monticellite is darker than intact forsterite in a polished section. Non of the listed agents etches magnesioferrite. Therefore it stays the most reflective component. Periclase is weak etched only by concentrated HCl and weak colourizes by concentrated HF. The other agents do not have anz affect on it. At the microscopic description of magnezite, one aims not just at the form of periclase and magnesioferrite but also at their amount, distribution and character of the silicate cement. The silicate cement contant ordinarily reaches 5 - 10 % vol. In products of a lower duality, it reaches up to 20 % vol.

8.1.1 Molten magnezite

Here, the prevalent mineral is excellent crystallic periclase, reaching a size of up to several mm, with distinct and regular cleavage cracks. During a melting, analogically as in corundum, a migration of lower-thawing admixtures to cooler, margin parts of the block proceeds. In the central part of the block, periclase is isometric, of a size about 1-5 mm and it usually contains rounded inclusions of forsterite, monticellite and glass. The inclusions reach 10 - 80.10-6m. Present iron forms solid solution with periclase. The solid solution is in a form of gray-green colourred magnesiowüstite (MgO.FeO).  Periclase is elongated, reaching a length of up to 30 mm and a width about 5 mm in the bottom part of the block. It is green and its elongation is oriented perpendicularly to the rims of the block, so it is a process of thermotaxis. Products made of molten magnezite distinguish on a raised resistance to rapid changes of temperature. The products contain coarse grains of periclase with distinct cleavage cracks and a fine-grained matter of a similar composition or matrix with periclase in rounded aggregates.

8.1.2 Chrommagnesite

Besides minerals present in molten magnesite, chromite occures here. Considering its opacity the study of pollished thin sections or polished sections is convenient. Chromite forms coarse grains, ordinarily sharp angular, which are opaque or just weak red-brown showing through. As the consequence of its high reflectivity they are whitish, similarly as higher mentioned magnesioferrite. Grains of chromite are penetrated by a net of distinct irregular crackes. The system of the cracks devides the grain into small fragments. In the periphery and along the cracks of translucent grains, there is a distinct non-transparent thin lining. At a heating, FeO begins to split from chromite already from 500 - 650°C. With chromium, FeO forms another spinel - (Fe3+,Cr)2O3, so-called ferroferrichromite. It crystallizes directly in the grains of chromite in a form of needles with higher reflectivity than surrounding chromite. The largest amount of ferroferrichromite forms at 1400°C. With a rising temperature, its amount again decreases and it forms a solid solution with chromite. However, in chrommagnezite of common composition, formation of ferroferrichromite does not ordinarily come up, because the released FeO is saturated by surrounding abundant periclase. FeO forms spinel magnesioferrite with it. Presence of this compound manifests itself as becomming dark of the periclase grains in a vicinity of chromite. In matters where contrawise chromite prevails, a displacement reaction and chromite enriching for MgO, which enters the solid solution of (Fe, Mg)O.Cr2O3-type, proceeds. In periclase, magnesioferrite again simultaneously forms. In a thin section, due to splitting of chromite, FeO gradually becomes translucent and and lighter. Its refraction simultaneously decrease to 2.08 ± 0.02. The chromite contant decreases at the same time. In matteres containing maximum 30 % vol. of chromite, the total depletion of chromite by the surrounding periclase may come up at a sufficient fineness and after a time long enough. In matters with the chromite over 30 % vol. contant, part of chromite always stays reduced. One can observe the reaction of chromite and periclase at so-called thermic etching of a polished section at 600°C for 8 - 10 minutes in an oxidative atmosphere. On the surface of chromite the film of fine anisotropic high refractive needles of hematite forms. Hematite  crystallizes in a connection with the oxidation of FeO, fixed in chromite. In chrommagnezite, periclase forms clusters of rounded grains with magnesioferrite, similarily as in the magnesite products. It forms a fine-grainded matrix surrounding coarse chromite, or directly in the contact, where it is up to opaque. In vicinity of chromite, periclase is dark-brown. The silicate cement emerges in a form of thin linings around periclase and chromite. Locally it forms clusters.  First of all, the siliceous cement forms from a serpentine minerals. The gaunge reacted with fine parts of magnesite clinker. The silicate cement forms forsterite and monticellite. In CPL, it represents the only anisotropic phase. In incident light, it belongs to the darkest, the least reflective polished parts of the section.

8.1.3 Dolomite

For the stabilization of free CaO, an adequate amount of SiO2 is necessary. C2S, stbillized in the form of the β-modification by the affect of apatite admixture, forms. Besides periclase and free CaO, non-stabilized dolomite contains products of reaction of these oxides and natural (hydraulic) contaminants and ash from fuel. A small share of alite or belite and brownmillerite forms this way.

8.1.4 Microscopy of used basic heatproof matters

A predominant part of the basic heatproof matters is used in steelmaking furnaces. The corrosive and erosive affect of metal, dust and slags containing high percentage of CaO, FeO and SiO2 and lower percentage of MnO, Al2O3 and P2O5 amount joins the affect of high temperatures. Temperature fluctuations and often a change of the degree of the oxidation/reducing affect of the atmosphere change are the other important factors.

Chrommagnesite from the vault of the Siemens-Martin furnace. In the section of a springer block, one can distinguish three zones: primary, transient and contact one. All of them are macroscopicly perceptible.

Thickness  of the primary zone depends on the total deterioration of the block. This zone reaches from the cold end up to approx.a place, corresponding to a temperature of approx. 650°C. It is brownish and has the shape of unused chrommagnesite.

The transient zone of 50 - 120 mm  thickness is dark-brown, denser, more compact and somewhat more fine-grained. 

The contact zone is gray, in vicinity of the melting metal, ite surface is black, considerably porous and fine-grained. It reaches the thickness of 26-60 mm.

In a microscope, the primary zone does not in fact differ from unused chrommagnesite. Towards the transient zone, in chromite grains, hematite needles may gradually occur. Their number increases towards the contact, they gradually get larger and their shape is irregular. Due to formation of magnesioferrite, periclase surrounding chromite gets darker. The contant of silicate cement stays does not, forsterite prevails there. In the transient zone, towards the contact, the content of silicates continuously increases and their composition gradually changes to monticellite, possibly also merwinite. These minerals prevail in the higher part of the transient zone. The distribution of cement is homogenous. However, its recrystallization proceeds and the forming  crystals of silicates are more than ten times larger in comparison to the primary zone. In this part, the volume of cement may oscillate between 25 - 30 %. In grains of periclase, in the contact with chromite, a distinct darkening appears and refraction increases. At the same time, corrosion of chromite caused by melt appears. The corrosion starts in margins and the chromite grains gradually become decomposed. Microstructure does not change. Towaras the contact, a thermotaxis  is perceptible in periclase. The crystals of periclase are dstinctly elongated and oriented in the direction of a temperature gradient.  The contact zone is sharp separated from the transient zone. Black colur manifests the infiltration of iron oxide. Periclase is opaque, strongly recrystallized and saturated with magnesioferrite. One can also observe elongated forms of periclase with a parallel orientation. Alike in a pure periclase, also chromite is continuously replaced by strong reflective opaque spinelide of (Mg, Fe)O.(Fe, Al, Cr)2O3 composition, which forms the entire matter towards the contact. The share of silicate cement disltincly decreases. However, it is homogenously distributed and first of all ferromonticellite (α = 1,648, γ = 1,672) is represented in it. Directly in the contact, there are iron oxides present, predominantly magnetite and there is the high share of several milimetres sized pores.

Due to the affect of the oxidation of iron, a dense layer called „plated unfired magnesite“ forms in the place of the contact of the melt and magnesite. The affect of the migration of MgO into this layer of oxides leads to formation of magnesioferrite together with hematite. Considering the fact that the diffusion of Fe2O3 into magnesite is slow, it come through only in some parts of the contact (so-called reaction surface).

At the deterioration of magnesite on walls of the Siemens-Martin furnaces, four zones are macroscopicly distinguishable. The brown primary zone, which is analogous to unused magnesite. The transient zone reaches a thickness of 50 mm. Its light colour is caused by getting translucent and hereby lightening of periclase aggregates, which are brown only in their margins and yellowish in the central parts. The silicate cement contant is oscillates 16 % vol. and forsterite prevails there. This zone passess into the light gray one, which is about 40 - 60 mm thick. Periclase is light-gray here (refraction = 1.745). The silicate cement contant somewhat increases. Uniformly distributed  monticellite prevails in the cement. The following black zone sharp contrasts to the previous one. This part corresponds to the highest temperatures. Due the affect of the solid solution with magnesioferrite, in the black zone, periclase is again brown and has a high refraction (n = 1.790). In the silicate cement, which volume is about 33 %, forsterite again prevails. On the surface of the block, there is a layer of a spraying matter, composed of coarse angular fragments of magnezite clinker containing rounded grains of periclase. The  colour of pericalase varies from light-brown up to black, with corresponding refractions (from n = 1.790 to n = 1.870). The silicate cement is formed by olivine. Considering the refraction α = 1.643, γ = 1.678 a D = 0.035 the forsterite component prevails to the fayalite one in it. The volume of silicate cement is larger in comparison to fragments of periclase. In the colder parts of the block, monticellite crystallizes rather than forsterite. During the deterioration, diffusion of CaO and SiO2 from the slag into the internal parts of the block and corrosion of MgO proceeds. MgO migrates in a reverse direction - into the zones of the highest temperatures.

Formation of so-called magnesite „vault“ of the Siemens-Martin furnaces is accompanied by the reactions of the magnesite clinker and approx. 10-25 % vol. Siemens-Martin slag. The recrystallization of periclase grains that get larger and their colours change (from yellow-brown to dark red-brown) at the simultaneous increase of refraction to the value n = 1.81 proceeds. This refraction corresponds to 9 % by weight Fe2O3, present in the form of magnesioferrite. The cementing matter is composed of first of all ferromonticellite with the admixture of Mn (it has higher refraction than monticellite, D is approx. by 0.010 higher). Along with it, dicalciumsilicate is present in the form of needles or lamellae, often surrounded by monticellite. On some C2S crystals, polysyntetic twinning with lamellae is developed. The lamellae are perpendicular to the vertical direction and they usually extinct  < 16°. Due to the affect of ferrous, magnesium and manganous silicates and whitlockite (tricalciumphosphate 3CaO.P2O5), C2S is mostly stabilized in a the β-modification in the solid solution. Otherwise, due to the transition of the β →γ-modification a decomposition may proceed. During the service of the furnace, other changes and zone formation  proceeds in the „soil“. In the transient zone, which is lighter, periclase is present and the share of magnesioferrite increases. The dark contact (work) zone already contains periclase. Here, periclase is almost non-transparent up to opaque in the connection with abundant wűstite (FeO) and magnesioferrite, which forms up to 50 % of the volume. The periclase refraction increases up to the value of 2.17 and size of its grains reaches about 0.5 mm, their typical rouded shape continuously disappears. A compact matter containing lamellae of ferrite forms. The lamellae are oriented along cleavage cracks. In the compact dark phase, silicates are scattered. Predominantly β-C2S, merwinite and brownmillerite. During the metalurgical process, the soil gets into the contact with a metal and a local Fe2O3 reduction to FeO proceeds (in magnesioferrite and periclase). FeO with periclase also forms a solid solution but its colour is lighter. The lightening disappears with depth.

On the surfaces of magnesite sinks, there is a glass layer, where only long prismatic forsterite occurs of the crystallic phases. The glass refraction is 1.62. The surface layer passess into a brown-black layer of 2 mm thickness , which is again composed of relative large periclase aggregates. Periclase is brown, often up to opaque. The aggregates are bond to each other with crystallic forsterite. This zone passess into the transient one, which is gray,  8 - 13 mm thick and has the same composition, but the forsterite cement is finer-grained and periclase is light or yellowish here.

Chrommagnezite from cementation rotary furnaces changes due to an interaction of chrommagnesite and calciumsilicates contained in the portland cement clinker. Futhermore, alkali and sulphates released from  maretial and fuel also take part in these reactions. Furthermore, the affect of the cooling of the heatproof lining and the porosity of the protective layer of melt significantly manifests. At the air cooling, in the clinker zone, 4 - 5 zones  form in chrommagnesite. In cooler parts, there are usually three zones. The most significant change occurs in chromite and in the siliceous cement, periclase usually represents the most resistant phase. In the case of chromite, one can observe the formation of hematite and the following formation of magnesioferrite by the reaction of hematite and periclase. Reactions of released Cr2O3 and alkali are running at formation chromate. Chromates form mixed crystals with alkaline sulphates and they concentrate first of all in the second zone, which borderes with the cool one. In chrommagnesite, due to the affect of CaO from clinker, the original forsterite cement changes to a monticellite one, particulary in the „work“ zone. Afterwards, monticellite gradually replaces forsterite also in cooler zones. In the lining, which is cooled by water, the movement of alkali and sulphates into the external zones is significantly prevented. The alkali and sulphates concentrate into the zone bordering with the layer of melt. In this contact zone, due to the reaction of a clinker and chrommagnesite, pyroxene crystallization proceeds. Diopside (melting temperature 1390°C) and  hedenbergite (965°C) form here. In the contact zone, monticellite forms too. The formation of low-meliting pyroxenes leads to a significant densifying in the „work“ zone of the block. In more distant, cooler layers, monticellite is present together with forsterite. In the direction of a temperature decrease the share of forsterite increases.

9 Mullite and corundum matters - matters with high aluminium oxide content

Technolithes with high aluminium oxide content has been fired at higher temperatures and contain higher share of mullite in comparison to chamotte. Mullite forms not just by a microstructural flowing in clay materials, but also by a solid-phase reaction of remaining SiO2 from kaolinite and added aluminium oxide. Thus, the mineral composition and development of mullite is strongly affected by a way of enriching with aluminium oxide-rich component. Fineness of both the reacting components, their reactivity, mutual contact, homogeneity of the mixture and temperature and firing time are crucial. In products, where aluminium oxide is irregularly scattered and it is concentrated e.g. in scatterd grains, mullite is present only in matrix in a form of tiny, unperceptibly developed crystals. The mullite crystals are surrounded by glass matter as well as in chamotte. In the scattered non-pastics, fine isometric corrundum approx. 0.1 mm sized and only isolated, long columnar crystals of mullite occur. The compensation of composition and a direction of microstructure is very slow and it is ordinarily connected with volume changes of the product. Alike in corundum matters connected by clay or kaoline, the matrix is chamotte-like and the resorption of the grains of corundum is not perceptible in the unused products. Only at a longer use, corrundum becomes partly or completely altered to mullite by resorption. If such products cotain except of corundum fired shale non-plastics, intensification of mullitization and increase of the Al2O3 contant proceeds after a long-time of application. However, this reaction is accompanied by a decrease of porosity. Therefore at a microscopical evaluation one particularly aims at the homogenity of the mineral composition.

In matters based on natural materials with high aluminium oxide contant, e.g. sillimanite or kyanite, homogenity is usually more perfect. This is a result of a less distinct difference between mullitization of non-plastics and matrix. In kyanite matters, autigenic mullite is usually confining arranged. On the contrary, in the sillimanite matters, it rather forms undirectional aggregates in pseudomorphs after fibrous sillimanite. Considering anisotropy of its properties, this type of microstructure is less suitable. Another microstructure occurs in dense products made of natural material. Mullite is rather columnar than acicular. In some isolated aggregates, its microstructure reminds the original material, probably from the sillimanite group. In another type of high aluminous matters, it secondarily forms by a reaction of SiO2 of clay material  and added technical aluminium oxide. The perfect dispersion of Al2O3 and sufficient firing temperature are crutial. A connection of non-plastics and cement is called mullitization. The mullite material with with the content of Al2O3  over 52 %  becomes homogenous. The firing temperature must exceed 1500°C. In used high aluminous matters, the zones, particulary noticeably sepatated ones, form again.  In the operating zone of these matters  used in electric furnace vault or in regenerators of the SM furnaces, mullite is ordinarily already decomposed to corundum. Sometimes it is present in a form of automorphous hexagonal plates, usually accompanied by spnelids: greenish hercynite (FeO. Al2O3) or magnesium or manganese spinel, brownish glass and sometimes also magnetite. Corundum may be completely replaced by spinel. Towards the unaltered part, there is the transient zone behind the operating zone. The transient zone contains excellent developed mullite and glass. In the mullite matters used in a transformer, formation of a 0.5 - 1 mm thick black reaction layer containing coarse mullite, dendrites of magnetite and dark yellow glass proceeds.

9.1       Cast and molten heatproof matters

The casting process runs in zigzag kilns at temperatures of approx. 2000°C. Commoly used materials are: bauxite, diaspore, kyanite, andalusite, corundum, a technical aluminous clinker with an addition of clays, kaoline, quartz send, zircon, baddeleyite and other components. A result are high aluminous heatproof mullite, mullite-corundum, corundum, zircon-mullite and periclase materials. The production of this type of heatproof material started in the USA in 1926 - 27. According to the Al2O3 / SiO2 / ZrO2 ratio, molten products with different phase composition were obtained. From pure mullite (Corhart-type)  to aluminous b-Al2O3 based (100% Monofrax H) corundum and in combination with a-Al2O3 (38% - Monofrax MH and 80% - Monofrax K) and glass from a spinel one to aluminium-silicate-zircon ones (the Corhart-type -ZAC: 45% corundum, 27% baddeleyite, 3% mullite and 25% glass). Accessory minerals are represented by magnetite and rutile in these products.

9.1.1 Mullite-corundum products of the „Corhart Standard“-type

Mullite and corundum are essential minerals, ilmenite, rutile and glass are accessory ones. Mullite crystals are atomorphic, long prismatic. Their  perceptible square sections are typical. The crystals reach size of 0.2 - 3 mm, locally they are up to 15 mm long and 0.05 - 1 mm wide. Their část size varies. They increase from rims to the centres. The ratio of mineral phases (and hereby also chemical composition) changes in the same direction. Mullite has a distincet cross jointing, longitudinal cleavage cracks are conspicous. In the sections parallel to elongation of the crystals, light white pleochroism is perceptible (solid solutions with Fe and Ti are present in mullite). Mullite forms small shef-like, omnidirectional arranged aggregates or an „ophitic“ microstructure of the individual prisms. Corundum is lamellar up to thin lamellar, reaches an average size of 0.3 - 5 mm. On surfaces of large cavities - shrinks  up to 25 mm sized, approx. hexagonal, flat plates form. In matter of the products itself, there are orinarily the corundum crystals smaller than crystals of mullite. Corundum differs from mullite in distinct cross cracks and its positive relief. At the identification, somewhat lower corundum birefringence (0.009), in comparison to mullite (0.012), is a certain guidline. The intercrystal space is filled with brownish up to pellucid glass matter (approx. 10-15% vol.). The glass matter is penetrated with acicular up to lamellar ilmenite with a typical fringed termination of crystals. The crystals of ilmenite reach a size of 10 - 200.10-6 m. Close to ilmenite, the brown glass matter is lighter, for colouring iron oxides were taken away from the glass. Besides ilmenite, fibrous rutile-sagenite occurs in the glass. The fibres reach 5 - 20.10-6m, they often cross each other or form star-shaped spherolithes. Refraction of glass oscillates beteen 1.53 - 1.55. To assess the mineral composition, polished sections can be successfully used. The  intensity of refraction rises with rising refractions in the order: glass, mullite, corundum, sagenite and ilmenite. Ilmenite is light white in the incident light. The mineral composition differs in margins and central parts of the casts. In the marginal parts, 55 - 70 % vol. of mullite,  4 - 15 % of corundum, 12 - 15 % of ilmenite + sagenite and about 15 % of glass occur. However, in the central part of the stone, the mullite contain drops to 45 - 52 % and corundum contain rises to approx. 8 - 25 % vol. Simultaneously, the shares of ilmenite and sagenite increase to approx. 15 - 20 % vol. and glass up to 23 % vol. The glass contain shortens a durability of the product and shall be as low as possible (below 10 % vol.). Thus, at a microscopical evaluation an assesssment of mineral pheses and a quantitative share glass including evaluation of size of crystals and microstructure is very important.

After application (tank furnace), three zones can be distinguished in the corundum products. In the zone that is the most distant from the cast itself and is closer to the untransformed matter, an approx. 10 - 20 mm thick layer of dark gray matter appears. In this layer one can observe a graual decrease of pleochroism of mullite. Another zone, dark blue, 1 - 2 mm thick, is almost transparent (a distinct accumulation of ore minerals appears here). Due to the affect of RO concentration, strong reactive glass contain increases there. The reactive glass decomposes mullite at the formation of corundum. Last zone, which is the closest one to the cast is 1 - 2 mm thick, porous and is light gray. It contains corundum and brownish glass matter, ordinarily rutile and sometimes also nepheline. A white film forms on a surface of this layer. The film contains strong corroded rounded corundum grains, surrounded by glass and nepheline that forms at a reaction of corrundum and alkali from glass matter.

9.1.2 Corundum products of Corvisit and Monofrax-type and corundum zirconic Bakor and Corhart ZAC-type

In corundum products α- and β-corundum prevails, they sometimes contain Cr2O3 in a solid solution, sometimes corundum is accompanied by chromite. Microstructure is confining grained, the habitus of corundum is grainy or lamellar, locally, signs of a skeleton-like development are perceptible.  Size of crystals of α-corundum oscillates between 0.05 - 3 mm. If β-corundum is present in small amounts, it forms characteristic polysynteticly twinned excellent cleavable small plates on {0001}. In average, their are about  0.01 - 0.05 mm sized and they are ordinarily smaller than α-corundum. β-corundum occurs partly among crystals of α-corundum, partly forms small phenocrysts directly in skeleton-like α-corundum. The cotundum-types can be mutually distinguished according to refraction (higher in α-corundum), cleavage (excellent in β-corundum) and higher D (in β-corundum). In longitudinal sections of β-corundum, one can observe authomorphic hexagonal form and distinct parallel cleavage in cross sections. In matters with high alkali contant, β-corundum forms matrix with a confining structure that surrounds and isolates the individuals of α-corundum.  Products of Monofrax H-type contain only aggregates of plates of β-corundum.  Products of Monofrax K-type contain α-corundum in isometric grains approx. 0.02 - 0.05 mm sized with pink-viotet rims that gradually pass into pale pinkish matter of the grain. Small opaque chromite grains (5 - 40.10-6m sized) come out. In the incident light, dendrites of admixed Fe2O3 are distinguishale on them.

Corundum-zirconic products of „soviet“ Bakor or „american“ ZAC Corhart have a confining microstructure, formed by lamellar α-corundum fan aggregates or plates of corundum, which have ophitic structure. Corundum forms 0.05 - 1 mm sized, in average 0.01 - 0.15 mm wide crystals. At a small magnification it is non-transparent, dimmed with small isometric to elongated, rounded or teary crystals of baddeleyite. Baddeleyite forms partly phnocrysts, perpendicular to the surfaře of corundum; baddeleyite less frequently occurs in intergranular spaces of glass. The value of the baddeleyite contant between 20 - 40 % vol., the mineral is pale brown. At watching in CPL it has noticeably high intereference colours. The length of its crystals oscillates between 5 - 100.10-6m, width 5 - 40.10-6m. Very frequently, the grains connect to each other, form dendrites of variable size and combinations of crossing of their axes. Dendrites of baddeleyite appear on the surface of  hexagonal plates of α-corundum. Its crystal axes form an angle of 600, so this orientated growth can be considered epitaxial. The intergranular matter is vitrous, ordinarily pellucid, with refraction lower than 1.54 or it can be crystallic (minerals from the mellilite group). On the surface of the used products Corhart ZAC-tzpe a protective layer of baddeleyite crystals forms. The crzstals of baddeleyite are the most glass dissolution-resistant. At first, the corrosion appears in the intergranular matter that is being dissolveld as the first one. Therefore, „loosening“ of its microstructure proceeds and the volume of the intergranular matter increases. Afterwards, corundum is being dissolved, however, the reaction is significantly slowed by inclusions of baddeleyite. Nepheline is the final product of the reaction of corundum and alkali glass. Baddeleyite aggregates stay joined together into chains and dendrites.

9.1.3 Molten corundum

Size and shape of crystals display characteristic differences in different parts of blocks of molten corundum. In the centre of the block, its crystals are ordinarily isometric and the largest are accompanied by the smallest part of other admixtures. Contrawise, these amixtures concentrate on the surface and in the bottom part of the block. The grains of corundum reach a size of up to several milimetres in the centre of a white block. In margins, unidirectional growth of orientated crystals proceeds. The crystals have the largest volume of cavities. In the surface part of the block, corundum forms just connecting matter among numerous bubbles. At the same time, “natrium and calcium” β-corundum that are present in the marginal part sof the block concentrate here. In cast tank stones of the  Monofrax H-type, its appearance and shape are equivalent to β-corundum. In brown corundum, crystals of α-corundum are xenomorphic bounded, isometric. Due to the affect of the solution with Fe2O3 and MnO, they are pink and pleochroic (between blue-green and grayish) in thin sections and they contain inclusions of metals. In brown corundum, the inclusions are formed by opaque paricles of metal alloys or a compound of reduced TiO2, particularly TiO, TiN, TiC and Ti2O3. Brown and white crystals contain round or perfect spherical cavities 10 - 50.10-6m sized. Some of them are filled with gas, other are al level-shaped and are filled with glass. One can find irregular and hexagonal opaque idividuals of probaply Ti2O3 or grafite in them. Other hexagonal plates in the cavities are pellucid. Their shape shape and optical properties are typoval for β-corundum. The inrercrystal spaces are filled with gray up to brownish glass. It is pigmented with small, approx. 1 - 10-6m sized irregularly shaped particles. These are ordinarily represented by suboxides, carbides and titanium nitrides. The molten corundum further contains long prismatic acicular mullite, of a length reaching 50 - 100.10-6m. The individuals of mullite have charactericstic square and lozenge sections. In corundum with higher MgO-content spinel was found in glass matter. At the excess of CaO and corresponding amount of SiO2, not just β-corundum but also anorthite forms. Besides the above mentioned suboxides of titanium, rutile and dendritic spinelids can be present.


10     Molten rocks - Petrurgy

A large group of molten rocks with a chemical composition varying in a wide range between 38.7 - 56.5 % SiO2 is devided into eight basic groups. These are characterized on the basis of authigenic mineral phases and their succession of their crystallization. There are two main mineral species occuring in all of the groups - magnetite and monoclinic pyroxene. Besides them, there are other important mineral phase present: olivine, plagioclase and monoclinic amphibole. Another group is represented by accessory autigenic minerals: rhombic pyroxene, melilite and olivine (forsterite) and higher tempetature monoclinic pyroxene (clinoenstatite). However, of the above listed minerals prevail just those, for which crystallization the chemical composition of the melt (the material) is adequate.  This is always crucial, without reference to its original mineral composition. The chemical composition conditions a succession of crystallization of minerals and hereby morfological shape of individual minerals. We study first of all cross sections of the products. At a microscopic study of molten rocks we aim both at the identification of individual minerals, their development, configuration and contant and at a microstructure of matrial as a comlex. Listed factors play an important role in physical and technological properties of the products. First of all, they affect mechanic strenght and grindability, which are usually crucial for an application of molten rocks.

10.1  Mineral composition of molten rocks

Magnetite Fe3O4, which forms in recrystallized melted basalt usually forms fine crystals bounded with crystallic faces, opaque in thin sections, with typical cross sections. It often occurs in centres of pyroxenic spherolites. It forms just their centres of crystallization. Magnetite forms submicroscopic pigment of glass matrix. It occurs in all species and zones of products made of molten rockcs.  

Rhombic pyroxenes ordinarily occur along with magnetite in all layers of the products. Here, rhombic pyroxenes are represented by enstatite (Mg2[Si2O6]) and ferrosilite (Fe2+2[Si2O6]).They form fine, acicular, skeleton-like or dendritic crystals. They most frequently occur as spherolites composed of needles, diverging from the central crystal of magnetite or they are just star shaped. The framework of the sperolites is formed by three bundles of paralelly oriented acicular crystals that are mutually perpendicular. Side needle grow on them. They are mostly yellow up to brown. Parallel extinction, Chz (+) and some lower birefringence in comparison to monoclinic pyroxenes are their characteristic features. With an increasing refraction of ferrosilite component and values of D gradually growing from α = 1.650, β = 1.652, γ = 1.659, D = 0.009 in enstatite up to α = 1.77, β = 1.785, γ = 1.79, D = 0,020 in ferrosilite. Rhombic pyroxenes concentrate particularly in those parts of the produsts, where rapid cooling of melt was running, because they crystallize at a lower temperature than monoclinic pyroxenes. Therefore, they occur in marginal layers of tiles made of molted basalt and in external (rarely internal) zone of centrifugally cast pipes.

In the central parts of castings, due to a higher crystallization temperature, we come across monoclinic pyroxenes, which cooling speed is slower - a higher temperature is being kept for a longer time. Their shape is columnar, we almost always observe crystals with a sign of a skeleton-like growth, rarely hourglass microstructure. They miss a terminal ending. An external form is considered to be a proof of crystallization at temperatures of gradual cooling. According to an angle of extinction, one can assume that augite (γ/Z = 45-54°) is the most frequently present, less frequent is diopside (γ/Z = 39°).

Olivine occurs in a form of relic crystals - i.e. remains of not melted porfyric phenocryst of basaltic material and also in a form of autigenic crystals. Due to a fire polishing, the relict crystals are rounded, strong coroded by the melt. There are cleveage cracks perceptible in larger crystals. The cracks are sometimes filled with fine-grained magnetite. Due to a gravitational differentiation, they concentrate on the reverse side of tiles and in external zone of pipes. They usually reach 0.035 - 0.43 mm. In relict olivines, rims formed by autigenic acicular crystals appear. These are parallel intergrowths, the crystals have an identical optical oriontation. For an autigenic olivine, acicular skeleton-like crystals are typical. The crystals a crystallize at a lower temperature and thus at a higher viscosity of the melt. Long prismatic crystals usually have a central cavity, so their  longitudinal sections are H-shaped. Another form of autigenic olivine represent short columnar crystals, formed at a higher temperature. One can observe them in the external zone of the pipes. In a surroundings of the olivine crystals, there a very fine magnetite pigmentation occurs there. The pigmentation commes through as a brown opacity.

A glass melt occurs in all sorts of castings. Particularly, where the melt cooled fast, i.e. in the corners of tiles and in the external zone of pipes.  The glass melt is often pigmented with magnetite and it forms layers, parallel to the external surfaces of pipes and bounded with layers of crystallic phases.  It is braunish and in  the surroundings of large crystals sometimes lighter due to the fall of the contant of colourising Fe ions. 

Alike in the devitrification of glass, the visual appearance of the glass melt is a result of temperature during individual phases of crystallization. In the beginning of the crystallization process, at a low number of formating crystal nucleuses and a low crystallization speed, confinning oriented and somewhat irregularly shaped crystals sporadicly appear in the glass melt.  At pyroxenes, incipient spherolites aleready appear in this stage. Later, the crystallic phase increases, a coarser-grained up tp hyaloophitic  microstructure with automorphic crystals forms. A following temperature drop leads to a development of finer-grained crystals. They are hypautomorphic and their shape changes from a long columnar to acicular up to skeleton-like. At a temperatre drop, a fine-grained ophitic microstructure transforms to a microstructure characterized by crystal aggregates, particularly spherolites and dendrites. Mutual relations of the crystals  reflects a succession of crystallization of the individual phases. The minerals grow around and through each other, and close autigenic magnetite (pyroxenes and olivine). The affect of the temperature of crystalliyzation on a morfology and microstructure of the crystals may be documented at an example of a tile of molten basalt. Spherolites of rhombic pyroxene always closing crystals of magnetite in their centres occur in the surface layer of the tiles. The size the spherolites increases up to approx. 60 - 90.10-6m (0.2 mm from the surface) towards the central parts of the tiles. Aggregates of 5 - 15 spherolites are mutually separated with approx. 10 - 17. 10-6m thick layer of glass melt pigmented with magnetite. The poresity of the surface layer reaches 35 - 60.10-6m. The size of pores increases up to 85 - 290.10-6m in the center. Simultaneously, corroded olivine relicts of 115.10-6m size and secondary acicular and skeleton-like authigenic olivines with an hourglass microstructure occur. Monoclinic pyroxene spherolites (of 85-105.10-6m size) separated from each other with a dark glass melt and pigmented with magnetite document a higher temperature of crystallization. In the bottom side of a tile, in the contact with a metal mould, the spherolites of pyroxenes again decrease, similarly as in the direction from the centre to the surface.  Due to gravitation, there is the higher relict olivine content in the bottom zone, the authigenic olivine almost misses. The pores are largest in this zone, they reach up to 600.10-6m. They formed at the contact zone of the melt and a wet mould.

In the cast pipes, three similar zones form. In comparison to the tiles, they have more distinct differences in their microstructures, in a sort of crystallizing phases and in a degree of crystallization. The internal zone contains star-shaped spherolites of rhombic and monoclinic pyroxene with a central crystal of magnetite. The needles of pyroxenes are mostly monodispersive, approx. 16.10-6m large. The spherolites are surrounded by a glass melt pigmented with magnetite. The internal layer further contains pores of 50 - 80.10-6m size and remains of primary, strong corroded olivine. Their size increases from 50.10-6m in the surface up to 80.10-6m in the depth of 0,8 mm from the internal face. The percentage of coarser-grained olivine relicts (50 - 160.10-6m) is higher in the central zone, which is 5 mm distant from the internal surface. Skeleton-like up to dendritic authigenic olivine appears there and, the contant of monoclinic pyroxene increases at the exclusion of rhombic pyroxene. At the same time, size of crystals increases. The crystals reach the maximum size in one third from the internal face, where the highest temperature existed. A coarser-grained microstructure corresponds to it as well. The pores reach 30 - 80.10-6m. The external zone corresponds to a lower temperature. It can be ditinguished by the presence spherolites of rhombic pyroxene close the border of the central zone. The part of the glass melt increases towards the external surface of the pipe. Except of the relict olivine of 130 - 160.10-6m size (in the border of the central zone), authigenic columnar olivines of 250.10-6m size appear here. Pores are small, about of  30.10-6m size. A distinct linear structure, formed by strips of the glass melt, oriented parallel to the surface, separated with strips of tiny spherolites is percptible.

At a microscopical evaluation of products made of molten basalt, is necessary to keep on mind their grinding hardness that particularly dependes on their microstructure. The finer-grained and the evener the material is, the higher grinding hardness can one assume. Other factors are orientation, size and morphology of pyroxene crystals, which are carriers of the grinding hardness. Small star-shaped spherolites composed of the same long pyroxene needles are more suitable than the confining oriented acicular crystals of various length or the skeleton-like crystals.

A number, genesis and size of olivines are important. Large grains of primary olivine decrease the grinding hardness. Authigenic dendritic crystals are considered to be less deleterious. Size and number of pores also play an important role. The pores of a larger average size than a size of grains of abrasive material are more deleterious, because they enable a breaking out of rims. A degree of crystallization and distribution of the glass matter are also quite important. A grinding hardness rises with a decreasing contant of the glass melt and at its uniform distribution. These factors are relevant.


11 Mineral components of a high-temperature slag

Slags have usually hemicrystalic microstructure. They are composed partly of crystals, party of a glass phase. Some of the minerals belong among common minerals of basalts (e.g. melilite, olivine, oxides of iron). The microscopic evaluation of the high-temperature slags enables their use as a cement admixture, a concrete aggregate or as roadstones. A result of the microscopic study is already affected by a place of collecting the sample, which manifests a speed of cooling, degree and character of crystallization of mineral phases. The fastest solidified slag is called granular; it is followed by a slag collected with a spoon at a tap, further a slag from a runner and from a slag field. The study should be done using grain preparations, thin sections and polished sections. The high temperature slags are devided on the basis of the (CaO + MgO) / (Al2O3 + SiO2) ratio into groups of the basic ones, the slags with the ratio higher than 1 and the acidic ones with the ratio lower than 1. Slags from both the groups are addeed into cement, the acid ones are used as an aggregate and for the production of a slag cotton. The basic and the acidic components ratio in slags and an assumed mineral phase are summarized in Table 8. For clarity, there are technical abbreviations of the individual minerals used in the Table 8: C corresponds to CaO, S to SiO2, A to Al2O3, F to FeO and M to MgO.


  Table 8  Mineral and chemical composition of slags


B/K ratio











Basic slags




Cinders and acidic slags




Brown coal ashes and cinders




S  SiO2


CS wolla -stonite



C3S2 Calc. justite




C2S belite









C3AS3 calc. sarko- lite

C2AS gehle-nite







MS klino enstatite







M2S forsterite






CMS2 diopside


C2MS2 äkerma- nite



CMS montice- llite






FS only in mixed crystals







F2S fayalite






CFS2 heden- bergite


C2FS2 ferro äkerma-nite



CFS  ferromon- ticellite



xMnO. ySiO2



Mn[SiO3] rodonite






 Mn2[SiO4] tefroite


Fe3O4 magnetite












MA spinel

Mixed crystals





justites   melilites





KAS4 leucite











A quick preparing is an advantage of  the grain preparations; refractions of the individual slag minerals can be identified and the quantitave content of glass can be estimated. For a faster identification, using a sieve, it is possible to devide the ground slag into several fractions ranging  200-40.10-6 m. The most convenient for the microscopical study is the fraction ranging  60-90.10-6 m. The mineral phases are not constant in the whole volume of the slag and their composition may vary even within individual fractions. Therefore, an informative study of every fraction is recommended. At the estimation of the  quantitative content of the glass melt, immersion liquids with n = 1.68 - 1.69, in which glass has a lower refraction (the highest refraction have the Mg-slags, where it reaches 1.675). The glass fragments differ signifantly from the crystallic phase - the fragments are isotropic. Using the immersion liquid having refraction 1.655-1.658 is possible to differ the lower-refractive γ-C2S from the higher-refractive β-C2S.

For identification of cubic oldhamite (CaS) one uses a sugar of lead in a diluted acetic acid solution. The oldhamite grains become more suspicious and, due to the affect of the solution, become darker and opaque. It is also possible to use this agent to identify CaS in a polished section. 2% pickle alum solution is used for the qualitative evaluation of hydration power of slag grains, around which calcium-aluminium-suphatehydrate crystals form. For identifying of the slag grains is recommended to use oleic acid [CH3.(CH2)7.CH:CH.(CH2)7.COOH]. Fibrous crystals of calcium oleate form around the grains of clinker in a short time, slag does not react with the agent. In thin sections, except of the identification of mineral phases, it is possible to study a microstructure and a degree of crystallization. A granulated and cellular slag gets harder by cooking in balsam or gets saturates with the balsam in vacuum. To make pores more conspicious, it is possible to tinge the balsam with an agent called in Czech „sudan“  (a light red powder , insoluble in water). It is possible to successfully use polished sections and thin sections. In the polished sections, the granulated slag is imbeded into the epoxide resins. As well as in cement clinkers, etching is recommended in the slags.  A twin lamelling of the grains of C2S becomes conspicious by using of water. Similarly, it is possible to use 0,25 % HNO3 in alcohol solution affecting for 3 seconds, to differ C2S, CaS and C3MS2 (merwinite). 10 % MgSO4 water solution heated up for 50°C represents a specific  C2S agent, the twin lamelling of C2S β-modification becomes perceptible after one second.  1% HNO3 in alcohol solution clearly distinguishes the glass melt from the crystallic phase during 30 seconds.

11.1  Basic blast furnace slags 

The most basic slags consist of dicalciumsilicate C2S (already mentioned in above, particularly in the passage regarding the Portland clinker cement). Its percentage representation depends on its chemical composition and a cooling speed. C2S forms round shaped grains having distinct positive relief and relatively high birefringence. Listed properties distinguish it from a lower-refractive glass and melilite (that has a very low birefringence). Due to FeO affect, refractions and birefringence of C2S rise in the solid solution. E.g. at the content of 50 % vol. of fayalite (2FeO.C2S), α = 1.690, β = 1.730 a γ = 1.738, D = 0.048 the birefringence rises. On polished surfaces, if examining in the incident light, the twin lamelling, typical for the β-modification C2S, is perceptible. At the higher 2CaO . SiO2 content and slow cooling, γ-C2S in a form of aggregates with significantly lower refractions than the β-modification may also occur. In a grain preparation, the distinguishing is the fastest using a refraction the immersion liquid with refraction ranging 1.655 - 1.658. All the refractions of γ-C2S and β-C2S are lower than the refractions of the above mentioned immersion liquid. The transition of modifications is accompanied by a distinct volume change. The volume increases for 10 %, and is a cause of a the decomposition of slag.  Therefore, the identification of the modifications of C2S is very important. At the higher MgO content the dicalciumsilicate may be gradually replaced with merwinite (3CaO.MgO.2SiO2), monticellite (CaO.MgO.SiO2) up to forsterite (2MgO.SiO2).

From MnO-rich slags, magnetite releases as the first mineral, it is followed by olivines, (forsterite….M2S, fayalite…F2S, tefroite…2MnO.SiO2, along with monticellite….CMS and ferromonticellite …CFS), further manganese- ich pyroxenes and in the end melilite, possibly perovskite at a proper  TiO2 or (CaO.TiO2) content.

Monticellite (CMS) is more significantly represented particulary in cases, when dolomote is used for the neutralization of acidic ore deposits. This way is e.g. undesirable formation of C2S prevented.

Sometimes, slags contain crystals of spinel (MA) and also leucite (KAS4).

Along with the above mentioned metalloids, metals also occurs in slags. 

Solubility of sulphides in the slag melt is the higher, the more basic the slag is. Then, the sulphides release in a form of submicroscopic drops and crystals, firs of all CaS, FeS and MgS. Afterwards, at a suitable composition of the melt, spinel (MgO.Al2O3) and afterwards melilite may release. In some slags one may observe the formation of paralel „layers“ of microstructures of a variable mineral composition.

There is almost always the 10 - 50 % contant of the glass melt with a refraction 1.645 - 1.665 present in the slag. The contant of the glass melt can be assesssed quantitativly using a planimetric analysis. The glass melt is usually pigmented with very fine sulphide particles. These are mostly represented by CaS - oldhamite, which has a high  crystallization ability and therefore it also occurs in slags composed of glass only. It is light up to dark brown, its specific gravity reaches 2.58, hardness 4, it has a metallic lustre and it is well cleavable on {001}. Its symmetry is cubic, it is optically isotropic and it also sometimes formes dendrites. Its refraction is high  n = 2.137. Dana (1990) states the empirical formula Ca0,5Mg0,25Fe2+0,15Mn2+0,1S. However, a series of solide solutions may exist in the slags (Table 8).

11.2  Acidic blast furnace slags 

There is the higher iron content in the acidic blast furnaces slags than in the basic ones. It usually reaches 85 - 95 %. A refraction of glass is lower, between 1.64 - 1.66. A prevailing miral in melilite there. Melilite represents an isomorphic mixture of gehlenite and äkermanite.

In slags, melilite (Ca,Na)2(Al,Mg,Fe++)(Si,Al)2O7 forms tables on {001} and short columnand xenomorphic grains. In thin sections, its cross sections are square or oblong. A zonal or skeleton-like and so-called tack like microstructures are typical. The zonality, perceptible both in PPL and CPL is a result of the different chemical composition  of individual zones of growth. The tack microstructure is perceptible particularly in cross sesctions. The tack-forms have a glass character and they penetrate in a form of thin wrims into the crystals in a direction from rims to the center. Irregularly oriented inclusions in a form of grains are mostly composed of C2S, less often of  merwinite. C2S differs from melilite in higher birefringence and a positive relief.

Merwinite,  alike C2S, has a distinct positive relief, but in the contrast to C2S quite low birefrinence.

Melilite may close sulphides  and oxides of metalls, which are grainy or dendritic. At the study of the process of crystallization of melilite, tiny spherolites are the first identifiable form. They are gradually replaced with plumule or fan like aggregates that represent the transition to skeleton-like development of crystals. At first, the crystals are very small, forming a fine-grained microstructure. Later, large crystals gradually form. The large crystals have long or short oblong cross sections, often hourglass-shaped, which is typical for melilite. Gehlenite, äkermanite and mellilite are described in detail in the encyclopedy of minerals. In the following part, there are given facts regarding their occurence in slags.

Refractions, birefringence and a change of optical character in a row of solid solutions are shown in the diagram in Fig. 5. Fe-äkermanite has refractiond: α = 1.658, γ = 1.670, D = 0.012, Chm (-), Fe-gehlenite: α = 1.661, γ = 1.666, D = 0.005, Chm (-). From the diagram, it is obvious that crystals containing more than approx. 50 % of gehlenite and more than 50 % of äkermanite are up to isotropic, γ = α. Anomalous intereference colours - grey-blue up to levender blue are typical. 

For distinguishing, one can use zonality , tack and hourglass microstructure, shape and anomalous intereference colours.

In acidic slags, accssory minerals are represented by sulphides and less often by oxides of so-called RO-phase, in fact  FeO, MnO and MgO. They often occur in a shape of dendrites. In thin sections, they are opaque and distinct reflective in polished surfaces. In a mineral assemblage of low Al2O3-content acidic slags, pseudowollastonite and wollastonite appear along with minerals of melilite group. Pseudowollastonite forms lamellar or elongated crystals with high biregfringence (α = 1.607 - 1.617, γ = 1.649 - 1.663, D = 0.044), wollastonite is acicular and sometimes distinct pleochroic (in shades of yellow, brown and green colour). The pleochroism exhibits by an effect of a solid solution with ferrosilite (Fe2+2[Si2O6]). Its presence manifests, first of all, in a fall of birefringence D = 0.014 - 0.015. In slags with the higher MgO content, wollastonite may be replaced with diopside (CaMg[Si2O6]) or even  klinoenstatite (Mg2[Si2O6]). On the contrary, at rather high Al2O3 contant, anortite (Ca[Al2Si2O8]) crystallizes in the slag. It appears only in very slow cooled, perfectly crystallized slags. It forms typical polysyntetic twinned crystals. Sometimes, one can observe an indication of a zonal microstructure. It forms low-birefringent lamellar crystals with low refractions (see the encyclopedy of minerals). In slow cooled slags with higher MgO and Al2O3-content, spinel (MgO.Al2O3) also appears.

In the microscopic study of slags, an assesssment of glass and crystallic phases ratio is first of all important. Further   the identification of γ-C2S and metallic sulphides is also necessary.  The unstable slag distinguishes by a high contant of inclusions (with high intereference colours) in melilite. Melilite is opaque. On the contrary, in voluminal stable slag, there are pellucid, perfectly developed crystals without inclusions. C2S configuration is also very important. If it is regularly scattered, it is less active than acumulations of aggregates. In a slag, iron and manganese sulphides ordinarily form at the FeO contant over 3 % and S contant over 1 %. They have a shape of small cubes or they form irregular opaque grains. Due to the affect of water, they decompose to H2S and hydroxides, which form brown, red-brown up to orange translucent pseudomorphous forms afterwards. During gradual cooling, one can microscopicly assesss and predict the mineral composition of rapidly cooled, mostly glass slags. At slow cooling, if a slag contains a significant part of small grains of γ-C2S, its decomposition into  β-modification will probably come up due to the affect of the transformation of γ-modification. On the contrary, if a brown glass melt and tiny particles of the RO phase prevail in the slag, melilite appears at slow cooling.


12 Siliciumcarbide and grafite matters

12.1  Carborundum products

Carborundum products contain grains of angular siliciumcarbide. The grains are bluish less frequently colourless or opaque. In the incident light, they display an outstanding reflectivity, which distinguishes them from surrounding gray cryptocrystallic matrix. If cement is represented by clay, there is matrix perceptible among siliciumcarbide grains. Matrix is formed by glass with small mullite needles. In products cemented with fine SiC, besides carborundum, there is only glass without mullite perceptible. With increasing firing temperature the number of fine SiC grains decreases and the amount of glass increases. A whitish porous film composed of glass and cristobalite may form on the surfaces of fired products.


12.1.1 The mineral composition of the siliciumcarbide matters

The mineral composition of the siliciumcarbide matters is a function of temperature in a section of the Archeson furnace and diffusion processes and recrystallization. The diffusion runs at the synthesis of silicon carbide. In the zones of crystallic and amorphous products, in green SiC, a hexagonal modification prevails. This modification contant in the silixicone zone and further towards marginal parts decreases. On the contrary, the contant of cubic modfication SiC increases. The cubic SiC is the most abundant in the consolidated zone and towards the margins its contant also decreases. In black SiC, a hexagonal modification prevails only in the crystallic zone (in the core) and already from here decreases towards the marginal layers. So the hexagonal SiC contant in the amorphous layer is lower than 50 % vol. On the contrary, the cubic modification contant is significantly lower here than in green SiC. In the consolidated zone, it reaches 50 % vol., and further to the magin its contant decreases. In the core and also in the amorphous zone, besides the forms of automorphic SiC, we come across pseudomorphs of SiC afer carbonaceous material. In the amorphous zone, SiC contains small inclusions and possibly thin layers of graphite. In the amorphous zone, in green SiC, this pseudomorphs occurs less often and the crystals of SiC are relatively larger than in black SiC. In the amorphous zone, green SiC contains small inclusions of metal alloys. Black SiC is accompanied by needles of colourless aluminium suboxide. In the amorphous zone, it occur togethr with colourless to yellow calcium carbide. Along with SiC, it is possible to identify graphite and the metal alloys (aluminium, iron etc.) in the both zones.

Silicon carbide SiC is hexagonal or cubic. If it is automorphic, it forms hexagonal plates imperfect cleaveable on {0001}. Specific gravity 3.22, hardness 9.5, metal lustre. Considering its high hardness it is hard to cut and grind. In a thin section, it is yellowish, greenish, blue, less frequently colourless or gray. Coloured crystals are pleochroic. It has an emerging relief and a strong reflectivity in polished sections. Refractions α = 2.654, γ = 2.697, D = 0.043, Chm (+).           

Cubic SiC most frequently forms pseudomorphs after a carbonaceous material - petrolcoke, anthracite or saw dust. In the incident light, it differs from the hexagonal modification in its shape and lower reflectivity In a thin section, in its lower relief and its lower refraction.

Calcium carbide CaC2 crystallizes in a rhombic symmetry. During grinding it reacts with water, therefore the polished and thin sections must be prepared in a waterless environment. It is xenomorphic, grainy or it forms pseudocubic crystals with polysyntetic twin lamelling. Due to the affect of chromophores, it is yellow, reddish, gray-green or colourless. It belongs among excellent cleavable minerals. It is cleavable on (001), (010) and (100), β ≥ 1.75, D = 0.050, Chm (+) .

Aluminium suboxide Al2O is hexagonal and it forms prismatic to acicular crystals (nε = 2.13, nω = 2.19).

In thin sections, carbonaceous material is opaque, having up to metal lustre, medium reflectivity and indistinct relieph in the incident light.

Graphite is hexagonal. It forms aggregates of flaky crystals, which are steel gray, gray-black to black, of metalloidlic lustre. In a polished section, it displays a nacre gleam, low relieph and weak reflectivity. It is excellent cleavable on {0001}.

12.2  Graphite products

Graphite products contain grains of light chamotte surrounded with black matrix. Graphite is concentrated in the matrix. In non-plastics grains, there is sometimes carbon diffusion into their marginal parts perceptible.


13 Enamels

For a study of the reaction enamels with metal, polished sections imbedded into epoxide resins should be used. Afterwards, the sample should be ground either perpendicularly to its surface face or at a certain angle, where the  interface of the metal and the enamel is better perceptible. At the grinding, a relief should not form in the interface of  materials with different hardness and abrasiveness. In all stages of grinding and polishing, the abrasive material shall move the way, so as that enamel would be pressed into the metal. The surface of the enamel is possible to observe either directly in the incident light or in an electron microscope. At the application of the electron microscopy, microstructural details are being emphasized and ihibits the effects of inside reflections and the effect of differences in surface reflexion.  

In the material control (in a stereoscopic microscope) we aim first of all at matters, which could induct the formation  dark or colour spots in a burnt out enamel. Their detailed identification should be done in a polarization microscope. Microscopically, one may observe reactions during the firing of enamel and also some defects. A corrosion of metals caused by the formation of enamel during the firing, which leads to roughing of surface is well perceptible. An improvement of cohesion occurs this way. An iron oxide in the dissolution enamel, which appears at an excessive firing, when a moving ahead oxygen oxidizes iron, was observed  microscopically. A colouring of enamel and rising of refractions occur. The most of the defects may have various causes. Therefore the identification of their genesis using microscopic methods is difficult. E.g. so-called „copper heads“ are  caused by a locally risen concentration of iron oxides in a basic enamel. In polished sections, it is possible to identify stong reflecting acicular hematite, irregular magnetite and dark gray FeO in  the interlayer between metal and enamel. An opacification of the layer of enamel is the most frequently caused by fine particles of  reduced Fe, Co or Ni. The reduction proceeds due tothe action of hydrogen, which forms by reaction of enamel, water and metal and is detectable in cells of enamels. At the study the cells, one shoud aim at their amount and location, which shoud be watched in cross sections and according to size. Small cells form a more opaque film than the large ones. If they are in contact with a metal, they can serve as „reservoirs“ of hydrogen. The hydrogen releases at the firing of enamel and steel sheet. Thus, in that case, no scale-like forms were observed. The scales are thought to form at the breaking away of the enamel caused by the released hydrogen. An excessive oxidation of the steel sheet leads to an elimination of the „layer of cells“ and together with it to the occurrence of the scale-forms in the beginning of the enamel firing. However, at high moisture and at temperatures oscillating about 0°C, excessive sized cells - over 75.10-6m - lower the resistance against the formation of cracks. Surface finishing of the bottom metal before the enamel lining is also microscopically observable. One may study the surface roughing in the incident light, either in cross cuts or directly on the surface in a small magnification, approx. 30 x. The microscope should be focused on microstructure details on the most elevated surface spots, while depressions should be outside the focusing. A sheet, in which focused relief elevations take approx. half of the visual field is considered to be suitable.  Surface, which is possible to focus at a time, is considered unsuitable. However, it is necessary to emphasize that the surface roughing is not the only factor affecting cohesion, it is only one of the conditions. Similarly, it is possible to assesss a state of the surface after a send jetting and cross sections etching in a sheet, which was in advance nickel-plated. This method enables a conservation of details in outlines during cutting and polishing. A number of depressions in the length of 1cm is considered as decisive. In the given space, there should be at least 500 of them.


14 Cement clinkers

14.1  Portland cement clinker

Approx. five main mineral phases can be differed in the portland cement clinker (see Table 1).


Table  1 Main mineral phases of portland cement clinker.

Technical name of the mineral

English terminology


Common abbreviation









Dark interspace matter




Celite - light interspace matter



C4AF or C6AxFy

Free lime





From secondary mineral phases periclase (MgO), compounds of alcali metals and glass phase may occur in specific cases.


Tricalciumsilicate - C3S - alite, 3CaO.SiO2, forms pseudohexagonal, oblong and polygonal short prismatic and lamellar cross-sections in thin sections. The existence of up to five modifications (triclinic, monoclinic and trigonal) is being supposed. Their relations has not been unambiguously resolved so far. Alite belongs to the group of unstable minerals. In the case of decomposition of C3S to C2S and CaO, the pseudohexagonal shape of C3S disappears and fringe-rimmed grains surrounded by mixture of  products of decomposition appear. They successively pseudomorph alite along cleavage cracks from its surface to the grain centre. Refractions and partly the colour are affected by oxides of non-ferrous metals. Study of the forms of alite in industrial clinkers proved that a smaller part of alite is optically uniaxial and a larger part is biaxial. Biaxial alite is supposed to be formed by solid phase reactions. The uniaxial alite is formed by melt crystallization. This way of formation is supported also by higher contents  of FeO, MgO, MnO, TiO2 and Cr2O3 admixtures and almost everytime of tricalciumaluminate (from 4 to 6%). Formation of a zonal alite is being interpreted by the presence of tricalciumaluminate in a solide solution, too.

Hexagonal or polygonal shape of cross-section, pellucid appearance and a low birefringence are considered to be typical distinguishing signs of tricalciumsilicate in penetrating light.


Dicalciumsilicate - C2S - belite, 2CaO.SiO2, forms three stable modifications: . Conversion of the β modification into the γ one is connected with approx. a 10% volume increase. Manifestation of this increase is decomposition of clinker. It is possible to stabilize the β - C2S modification chemically using a solid solutions of  P2O5, V2O5, Al2O3, Cr2O3 or physically by β - C2S  „covering“ with a glass lining that can be reached by rapid cooling. A lot of oxides in a solid solution affect optical constants of all C2S - modifications. High - temperature  β modification, the most common in clinker, often displays a polysynthetic lamination. Optical orientations of β-C2S  and  γ-C2S - see Fig. 2 and Fig. 3. Lamellae configured to several variously orientated clusters give the evidence of  C2S  formation and of rapid cooling. Unidirectionally cofigured lamellae in somewhat wider strips are typical for a normally cooled clinker. When the cooling is particularly slow, a fine-grained microstructure is being formed. The microstructure is caused by exsolution of accompanying substances from solid solution with C2S during the conversion of the high-temperature modification into the low-temperature form.

Figure 9 Optical orientation of b-C2S in a 100 plane.


The exsoluted phase in the form of very small inclusions keeps usually streaming orientation of polysynthetic lamellae. C2S crystallizes in the form of round grains, particularly in the case of the rapid cooling. It may have a fringe-rimmed irregular shape in the slow-cooled clinker.  In the case of a coarsely ground limestone, C2S accumulates into relatively large aggregates - „nests“. Small C2S grains sometimes line rims of a hexagonal C3S. This is the manifestation of travertine corrosion, where a melt is subsaturated with carbon dioxide.


Figure 10Optical orientation of  γ -C2S on 010 plane.


C2S modifications are distinguishable according to a gray-yellow colouring, opacification, round shapes and above  all to a distinct relief in the penetrating polarized light. In comparison to C3S, some higher birefringence is distinguishable, when observing with an analyser and a polarizer (the interference colours are usually yellow of the 1st order). If comparing the studied dicalciumsilacate modifications to natural minerals, α-C2S is the most similar to nagelschmidtite, α´-2S  to bredigite,  β-C2S to larnite and merwinite, and γ-C2S to monticellite. In cement clinkers, all listed minerals are included under the single term belite (dicalcium silicate).


Tricalciumaluminate - C3A -  3CaO.Al2O3, forms so-called dark interspace matter of cement clinker. Its identification in thin sections and grain preparations is complicated, because its refraction is close to the glass phase and more or less to  C2S and C3S, too. In industrial clinkers, tricalciumaluminate is quite easy to identify according to hypautomorphic perpendicular cross-sections and its violent reaction with distilled water. It forms tiny, transparent, colourless crystals imperfectly cleavable on octahedron. Its  microstructure is of the perovskite-like type. In a solid solution of cement clinker, it may contain a lot of foreign substances.

A C3A polymorphism wasn’t unambiguously proved, even despite the fact that according to some researchers a so-called „prismatic“ phase of clinker may be equivalent to a metastable C3A modification stabilized by alkali metals. Rarely a cubic C5A3  (5CaO.3Al2O3) modification  may appear in Portland clinker.

If alkalis are not significantly represented in the original material or if the clinker cooling was very slow, tricalciumaluminate is cubic, isotropic, n = 1.710. If the content of Fe2O3  in solid solution reaches 10 %, affect of the increasing Fe2O3  in solid solution contain causes a rising refraction up to 1.736. The rising refraction value, along with a microchemical analysis are significant indicators of formation of solid solutions.

In a thin section, C3A is colourless, it forms isometric grains with rectangular or hexagonal cross-section (it reminds of C3S). Clinker cooled in a standard way is fine-grained, without the presence of oxides of alkali metals. If the clinker is being cooled very slowly, it crystallizes intensively and closes inclusions of other silicates, CaO and periclase.

Long prismatic (lath-like) anisotropic crystals, reaching 10 - 15 microns represent another C3A form. In penetrating light, their birefringence is weak and their extinction is parallel. It was verified that this prismatic phase is formed just in clinkers containing Na2O and K2O. Therefore it was stated that the prismatic phase is an alkali-stabilized C3A form. There is usually 5-13 % of prismatic phase present in a normally cooled clinker. refractions have not been measured yet successfully, the commonly stated mid-values are  n = 1.72, D = 0.005 - 0.015. The mineral is usually opticaly biaxial with a 2V = 52° mid-angle. Therefore, in penetrating light, it can be even more easily confused with C3S than the cubic, isotropic modification of C3A. In rapidly cooled clinker, the prismatic C3A may be totally absent, alike in the clinker with the aluminate module lower than 1.63.


Brownmillerite - C4AF or C6AxFy - celite = tetracalciumaluminiumferrite, 4CaO.Al2O3.Fe2O3. Brownmillerite is a part of the interspace matter and along with a ferric glass of the so-called light interspace matter. Its reflectivity and refractions are of the highest values among the commonly present clinker minerals. Calciumaluminiumferrites are of a rhombic symmetry and if they are surrounded with crystal faces, they form tiny prismatic crystals or dendritic (skeleton-like), salient, high birefringence formations. They are characteristically brown-yellow. They are distinctly or even significantly pleochroic in light brown, dark red-brown or yellow-green shades. Because they crystallize as the last phase, they are usually xenomorphic and they fill spaces among surrounding calciumsilicates and calciumaluminates. Prismatic crystals are rare. Their shape is affected  by a cooling speed and by the Al2O3 / Fe2O3 ratio. Having a low value of aluminate module and being cooled slowly, the crystal shape is short prismatic, the crystals have a significant pleochroism and they are relatively large.  If the Al2O3 / Fe2O3  ratio is high, the crystals are small and acicular. Typical dendritic formations are formed in a rapidly cooled clinker and components of this phase remain in a heavy reflecting ferric glass matter after rapid cooling.

However, refractions stated for brownmillerite [α = 1.96, β = 2.01, γ = 2.04, D = 0.080, Chm(-)] differ from „brownmillerites“ in common clinker-types. Lower refractions are usually detected, e.g.  α = 1.852 + 0.005, β = 1.961 + 0.005, γ = 2.001+0.002, D = 0.148-0.159. This is one of the  evidence of a solid solution with C3A or  of a rhombic C5A3 which is microstructurally close to C4AF. In these solid solutions, refractions may drop down to  following values: α = 1,714, γ = 1,854. On the contrary, a brownmillerite solid solution with a higher  2CaO.Fe2O3 content has higher refractions than pure C4AF. E.g. at approx. 10 % 2CaO.Fe2O3  content the refractions increase to α = 1.98 and  γ = 2.08. Such solid solutions are formed in high Fe3+-content clinkers with the Al2O3 / Fe2O3 ratio < 0,64. C4AF can further contain less than 1 % MgO that is manifested by the drop of refractions refractions, more intensive colouring and pleochroism. An average content of brownmillerite in clinker varies around 10 %, usually between 8 and 13 %.


A larger free calcium oxide amount occurs in industrial clinkers due to technological errors during a production process. The mineral is considered to be an undesirable admixture. The cause of its formation is a very high CaO content, imperfect limestone grinding, bad material homogenization, low furnace temperature or a slow cooling. So it is concerned the unreacted primary free calcium oxide (I) or in the other case, the free secondary calcium oxide (II) formed by decomposition of C3S (at the eutectic temperature) according to the following equation:


3CaO.SiO2 ↔ 2CaO.SiO2 + CaO.


Therefore, sometimes one can find CaO that forms aggregates of isometric orbicular grains. They occur in a form of inclusions in crystals of C3S and C3A. Tricalciumaluminates and tricalciumsilicates isolate free CaO (I) from a remaining melt during the clinker cooling process. In the other case, it forms very small particles surrounding C3S - free CaO (II) grains along with C2S. In this case, CaO (II) splits off the tricalciumsilicate during a slow cooling below 1250°C. Disintegration starts from the tricalciumsilaicate surface. Besides formation of free lime,  content of C2S increases  to the exclusion of C3S too. Free calcium oxide forms small round grains 1 - 5.10-6m sized.  They are colourless and isotropic. In thin sections, they have a distinct relief (n = 1.833 - 1.846).



Periclase - MgO occurs in clinker in a form of transparent, colourless, skeleton-like up to automorphic tiny crystals. The crystals are small, cube and octahedron-shaped, scattered in the interspace matter. Their size reaches 15 - 20.10-6m in a normally cooled clinker. In a rapidly cooled clinker, it is very fine-grained and its size reaches up to 3.10-6m in maximum. Periclase has usually a relatively high thermic expansivity. It takes up a smaller space than the space, which it was filling immediately after the crystallization. Thus, it is possible to state that it is „freely placed“ in the interspace matter. Small cracks are formed around the MgO crystals. They make the relief more conspicuous and they cause spalling out  of MgO crystals during grinding. Due to its high refraction  n = 1.734 to 1.737 and its isotropy, it can be easily confused with C3S or C3A in the thin sections. Due to its considerable hardness, it has a distinct salient relief, if preserved during the grinding and polishing. The periclase content is always lower then a total volume of MgO. A part of MgO (up to 1%) transforms to C4AF or to glass matter. Therefore in a slowly cooled clinker, the periclase content is higher than in a rapidly cooled one, where all MgO may be present in glass phase.


Glass phase is a part of the interspace matter. It is usually yellow-green or brown to black coloured. In a covered thin section, its identification is practically impossible. It is identifiable using etching in a polished section and in a polished thin section. Glass phase refraction is variable and depends on its chemical composition. It varies around n = 1.70 in a high Al2O3 / Fe2O3 (over 3.2) ratio clinker. The value of refraction is significantly higher (n = 1,85) in a low aluminate module clinker (e.g. 0.64). The total percentage share of glass phase depends on cooling speed and oscillates between 0 - 3% in a slowly cooled clinker and between 8 - 22% in a rapidly cooled clinker.


In Portland clinker, alkali metals compounds may be present in a form of solid solutions in previously mentioned silicates or they rarely form separate minerals. In common clinkers, the content of alkali usually does not exceed 2 % and has a significant affect on microstructure and mineral composition of Portland clinker. Oxides of alkali metals concentrate first of all in glass matter. They may form a solid solution with C3S along with present aluminates and increase its stability. If sulfate anions are present, separate minerals - sulfates may be formed. Isometric K2SO4 grains with a very low birefringence D = 0.004 and with α = 1.493, β = 1.494, γ = 1.497, Chm (+) are formed first of all. K2O.23CaO.12 SiO2 is formed, if a larger K2O amount is present. Its outstanding feature is a polysynthetic twinning, α = 1.695, γ = 1.703, Chm (-). A  K2O.8CaO.3Al2O3 compound is formed in a rapidly cooled clinker. Na2O reacts with SO3 more slowly and after K2O. A Na2O.8CaO.3Al2O3compound with the refractions α = 1.02, γ = 1.710, D = 0.008, Chm (-) is formed. It is similar to C3A. The alkali compounds are a part of the dark interspace crystallic matter. In case of their crystallization the content of free CaO increases.


14.1.1 Identification of clinker minerals using etching of polished sections and polished thin sections

For identification of clinker minerals, usage of a lot of agents is being recommended. Relevant action times are usually listed simultaneously. Reactions proceeding during the etching process depend on following factors: chemical composition, compactness, forming conditions of clinker and simultaneously on the concentration, temperature and action time of etching agents. Therefore, it is necessary to consider the recommended times of etching to be just informative. An optimal time of the etching process has to be verified experimentally for individual cases. Strongly burnt out clinkers from rotary furnaces usually demand a longer action time of etching agents than the softly burnt out, more porous ones. An etching agent should be dropped with a small pipette on a polished thin section (polished section) or the lustre surface should be immersed into the agent placed in a small flat bowl dish, possibly is being it should be exposed to the affect of  agent vapours. A relevant time later, the excess volume of the agent is being sucked out using a filter paper, the preparation is being washed with a waterless ethyl alcohol minimally twice. The excess volume of alcohol is also being sucked out.

 Success of observing of etching tests done on the polished surfaces depends on a quality of the polished surface and also on a perfection of degreasing. Due to hydration, the polished surface looses its luster if it was prepared long time before the etching tests. Repolishing with micropolite on velvet is then recommended. Storing of preparations in desiccator is suitable. A clinker microstructure is usually being evaluated along with individual mineral phases.  The percentage share of clinker minerals and the uniformity of their distribution, their size, development, shape, mutual relationships of crystals, size and number of pores etc are watched.

The characteristics often depend on the place of collecting of the studied sample. There are differences between  a surface and an interior part of a sample. Differences of mineral composition between the surface, aproximately 5 mm thick, layer and  the interior part of clinker  have often been observed. The exterior part is usually rich in C2S, the interior one contains more  C3S. The  C3S degradation in the surface layer has been explained in a connection with an affect of a non-equilibrium melt that reacted with C3S. This reaction leads to the formation of β-C2S. It is suitable to correlate the microscopically assessed modal composition of clinker with the results of calculations of equilibrium-state normative chemical analysis.

Differences of individual assessments are considered to be a criterion of equilibrium and they complexly reflect a complete manufacturing process: mineral composition, homogenity and material fineness, burning out temperature and cooling conditions. 

Periclase is the easiest identifiable mineral in a polished thin section. It usually occurs in a form of very small, angular grains with a positive relief. Further, round grains of free lime with a feeble negative relief having a white to greyish reflection are observable. The free lime soon becomes darker, due to a reaction with air humidity. The free lime usually occurs in clusters.

Water is the least intensive clinker etching agent. CaO and C3A (the dark interspace matter) are being etched for 1-3 seconds. C2S, C3S and glass matter with low content of iron become corroded after a longer (5 - 20 seconds) affect of an etching-agent. After the longer etching-period, C3S becomes darker than C2S.

After a short (2 - 5 seconds) etching with water, it is convenient to go over to etching with 1% HNO3 ethanol solution (for 2 - 5 seconds) or 1 % NH4Cl water solution (for 10 seconds) to make free CaO and C3A more conspicuous. The affect of the second agent is stronger than the commonly recommended first one‘s. Polygonal, first comming out, tiny alite crystals differ from later comming out round belite grains that keep a lighter reflexion shade in comparison to alite. They are being etched less intensively. Alite gets significatly darker after a long period of etching. In the interspace matter, C3A is dark, usually prismatic. Its shade is similar to the irregularly bordered glass matter with low content of iron, which significantly differs from the light reflecting phases - C4AF and a ferrite phase.

Instead of the above proposed way, one may use 0,25 - 1 % solution of icy acetic acid dissolved in ethylalcohol (2 - 60 seconds period of etching) for the etching first, or  1 % HNO3 solution in isoamylalcohol (2 - 15 seconds). CaO, MgO, C3S, C2S become more conspicuous and C3A becomes distinguishable from the light interspace matter. Alite is being etched more intensively than belite and thus it is less reflective - darker. Sequence of the effect of acetic acid on individual clinker minerals is following: CaO, C3S, MgO, C2S, C3A and C4AF. On the contrary, belite is darker, alite is gray-blue and C4AF stays light, a low ferrite matter is dark when using 0,5 N  HCl  (for 5 - 10 seconds).

10 % KOH water solution at 29-300C temperature (for 15 seconds) is being used to recognize the light interspace matter after usage of acid etching agents. So the Fe-rich acid agents resistent glass matter differs This is the way to distinguish Fe-rich glass matter, which is resistant to acid agents from C4AF in the rapidly cooled clinker. Due to KOH effect, the glass mater becomes darker, but brownmillerite stays light reflective. This is not the only way of identification. Except of the above mentioned sequence of the etching procedure, one can use other agents to identify individual phases (see Table 2). To make C3A more conspicuous in the interspace matter, polishing with 10 ml of a N-oxalic acid  in 50 ml of 95 % ethylalcohol solution (for 5 - 15 seconds) after a short-time etching with HNO3, is recommended. After this procedure, C3A reflection becomes red-brown, reflections of the other clinker minerals do not change. After etching with  CH3.COOH or HNO3, usage of a  10 % MgSO4 water solution is possible. So, alite, which is dark gray, is distinguishable from light gray belite this way. In a similar way, it is possible to make alite more conspicious by etching with ammonium sulphide (for 5 - 30 seconds). Dark gray to blue C3S is intensively etched, while belite stays light. Following etching with a 1 % HNO3 alcohol solution makes belite more conspicuous too. Before usage of polysulphide, etching with water is recommended to make free lime more conspicuous. This method is particularly useful to distinguish secondary, fine-grained CaO and C2S from remains of primary C3S (from which they were formed by decomposition under 1250°C).  Secondary C2S and CaO may rarely pseudomorph completely the original  C3S hexagonal cross-sections. It is possible to make C2S more conspicuous against C3S by a HF-vapours affect (for 30 seconds). Originally, belite (C2S) is yellow to orange and during the effect of HF its reflection shade becomes green to blue. In a grain preparation, according to Astrejeva (1959), C3A is identifiable by colouring the preparation, previously etched with 0,1 N HCl (10-15 seconds), with the patent blue (pale acid blue 3). After etching with HCl and colouring, the preparation is being washed with a waterless alcohol. The other clinker minerals do not become coloured, so it is possible to do a quantitative analysis too.

In the grain preparation, it is possible to assess the free lime volume by a microchemical reaction using the White’s Agent.  The agent consists of 1 g of phenol dissolved in 3 ml of nitrobenzene  with one drop of added distilled water.  It reacts with CaO within 10 minutes along with the formation of long columnar or acicular calcium phenolate crystals. In the CPL, the crystals display various interference colours, while they are almost indistinct in the PPL (due to their refraction similar to the White’s Agent’s one). In this solution, reliefs of fragments of clinker minerals are the most distinct ones. The White’s Solution reacts with Ca(OH)2 a longer time later (after approx. 20 minutes or more ).


Table 9 List of etching agents and their affect on clinker minerals.



Etching agent

Period of etching

Temperature of etching

The affect on succession of clinker minerals according to the intensity of etching

Acid etching agents

1% HNO3 amylalcohol

2-15 sec.


CaO silicates


1% HNO3 ethylalcohol

5 sec.     


CaO silicates aluminates


1% ice-cold acetic acid in ethylalcohol solution

2-5 sec.




10 ml of oxalic acid in  50 ml of 95% ethylalcohol solution

5-15 sec.




1% H2SO4 in alcohol solution



Aluminates Non-differentiated interspace glass matter


HF couples



Changes colour of C2S



5-10 sec.


CaO aluminates

Acidic water-based etching agents

Concentrated fluorhydric acid watter-based solution

2-3 sec.


C2S and ferrites


1:10 fluorhydric acid water solution

30-60 sec.


C2S and ferrites

Alkali water-based etching agents

1% borax solution

10 min.


CaO and C3S pentacalciumaluminate


1% borax solution

10 min.


CaO and C3S pentacalciumaluminate


8ml of 10% Na(OH) +2ml 10% Na2HPO4 solutions

1 min.


Everything except of dicalciumferrite and dicalciumsilicate

More etching agents following one after another

H2O,  then solution of 1% HNO3 in ethylalcohol

2 sec. 3 sec.


CaO, MgO, „dark“ crystallic and glass intergranular matters  silicates


H2O,  then solution of 1% HNO3 in ethylalcohol

3 sec. 3 sec.


CaO, MgO, „dark“ crystallic and glass intergranular matter  silicates


then 10% KOH water solution

15 sec.


Light intergranular matter

14.2    Aluminous cement

The aluminous cement was preared by Biede in France in 1908 for the first time. It is made of pulverized bauxite and limestone mixtute [1:1ratio]. There are two ways of its preparation. Either a firing to the sintering point [1350°C] - sintered aluminous cement or a high temperature melting (e.g. in electric zigzag kilns) - electrocomelted aluminous cement. The aluminous cement significantly differs from the Portland cement. The difference is that the aluminous cement contains calciumaluminates. On the contrary, the base of the Portland cement are calciumsilicates.  The calciumsilicates are undesirable in the aluminous cement.

The chemical composition of the common aluminous cement for construction purposes (under  50 % A12O3) may fluctuate in a certain range.  The content of  individual oxides usually fluctuates in ranges: SiO2 5 - 15 %, Al2O3 35 - 50 %, Fe2O3 5 - 15 %, TiO2 1,5 - 2,5 %, CaO 35 - 45 %, MgO 0,1 - 1,5 %. Its mineral composition has a direct relation to the content of oxides. Calcium aluminates prevail over silicates.

We differ:

- primary phases: monocalciumaluminate CA, a basical component, very active concerning hydration, up to  65 % ;

- secondary pheses: dodekacalciumheptaaluminate C12A7, very active; calciumdialuminate CA2, medium active; gehlenite C2AS, slightly active in a crystallic form, active in a form of a gehlenite glass that rather corresponds to melilites; C6A2F-C2F, listed mostly as brownmnillerite C4AF, active; dicalciumsilicate, C2S, little active;

- other phases: C2(F,M)S, possibly C6A4(F, M)S, pleochroite; CaS that forms in a reduction environment, CF2, possibly CF, calciumdiferrite or ferrite; FeO, Fe3O4, possibly another phases (e.g. CT).

To make the picture complete, it is necessary to add that mineral forms of iron oxides have not been completely surveyed yet. Their character depends on the environment where they formed.


Monocalciumaluminate - CA  - CaO . Al2O3 crystallizes in a monoclinic symmetry. If it forms in a solid phase reactions, it usually forms short prismatic crystals. These are cyclic coupled in cuts parallel to a basal face, the cross sections are pseudohexagonal. When it forms from an eutectic melt, it is fine-grained and usually accompanied by a simultaneously formed gehlenite. 

Figure 11 Optical orientation of monocalciumaluminate in the cut according to 010.


It is usually bounded with crystal faces, it is rarely xenomorphic. The crystal rims are sometimes fringed. It is pellucid, significantly cleavable in the elongation direction according to (110). Refractions: α = 1.641, β = 1.655,  γ = 1.661-1.663, D = 0.020. The optical orientation - see Fig. 4. It formes solid solutions with  monocalciumferrite CF in an oxidative atmosphere and the refractions rise to α = 1.66 - 1.70, γ = 1.68 - 1.72. In the aluminous cement, it is a reactive mineral, its contant fluctuates between  4% and 61 %, usually between 25 - 40 %.


Pentacalciumtrialuminate - C5A3  (now C12A7) occurs in two modifications. Their formation is affected by  oxidative or reduction atmosphere. It is an undesirable product, because it makes a cement hardening faster.  It usually occurs in smaller amounts than CA.

The α - modification is stable at a normal temperature, crystallizes in an isometric symmetry. It is usually itense green and forms round, xenomorphic, perfect isotropic grains without cleavage, n = 1,600 – 1,626, during a manufacturing of the aluminous cement in a reduction atmosphere. The green colour dissapears if warmed in air and the refraction is 1.608.

The a´- modification is unstable at a normal temperature, crystallizes in a rhombic symmetry. In the aluminous cement it forms in an oxidative atmosphere. It  forms acicular, long pillar-like and possibly lamellar crystals that sheaf-like or spherolitic agglomerate. Their pleochroism is characteristic - in the X-direction it is blue-green, in the Z-derection it is olive gray. Mg presence leads to a change of its pleochroism. The colour changes to light or even dark violet. Its refractions are  α = 1.617, γ = 1.634 -1.692, D = 0.017 - 0.065.


Monocalciumdialuminate  - CA2 (earlier  tricalciumpentaaluminate) - CaO.2Al2O3 occurs rarely and only in the aluminous cement. The crystals are long prismatic to acicular. It crystallizes in a monoclinic or a tetragonal symmetry. The mineral has a characteristic high birefringence D = 0,035. The refractions: α = 1.617, γ = 1.652, Chm (+). The cleavage has not been observed.


β - dicalciumsilicate is usually present in the intergranular matter among crystals of CA along with gehlenite and C12A7.  It forms very small grains. In thin sections, it is distinguishable according to its higher refraction that conditions a distinct relief and according to its higher birefringence. It is usually represented only marginally. Optical characteristics are given above.

Fig.  5 Chart of a dependance of optical characteristics on a chemical composition in an äkermanit-gehlenite isomorphic series.


Gehlenite - C2AS - 2CaO.Al2O3.SiO2 represents a common component of the aluminous cements. If the SiO2 content in the material is higher, there can be up to  40 % of gehlenite present in the final product. The mineral is not hydraulic and its presence is undesirable. CaO and Al2O3 that would otherwise form hydraulic calciumaluminates enter the mineral microstructure. It forms short prismatic crystals with square lateral and oblong longitudinal cross sections, possibly xenomorphic grains.  It is pellucid to ligh bluish, sometimes zonal and skeleton-like; α = 1.658, γ = 1.667 - 1.670, D = 0.009 -  0.012. Values of the optical constants vary according to a composition of solid solutios (see Fig.5) which gehlenite forms with a series of oxides.  A cleavage according to {001} is well distinct, the cleavage cracks are perpendicular to a crystal elongation. It differs from a similar calciumaluminate, which has a close refraction too, by significantly lower birefringence and an inverse orieotation of cleavage. Gehlenite is usually one of the basic components of the matrix or it forms intergrows with prevailing calciumaluminate crystals.


Perovskite - CaO.TiO2  - occurs in the aluminous cement  very often. It crystallizes in a rhombic symmetry. It has a high relief, n = 2.34 - 2.38, it is weak birefringent, scattered in a form of fine, often skeleton-like crystals in matrix.  Iron can be bound to brownmillerite and calciumferrite., magnetite Fe3O4 or wűstite FeO forms during an oxidative burning or in reduction conditions.  A smaller amount of iron deoxidates to a ferrosilicon in a zigzag kiln.  

14.2.1 Methods of identification


It is possible to determinate the mineral composition of the aluminous cement a similar way as the Portland cement, in grain preparations, polished thin sections and by etching tests too. The identification is easier in the case of melted samples that are coarser crystallic than the clinker ones. In both cases, CA, C12A7 and C2AS distinguishing according to a hight of birefringence and based on a development of crystals is the most convenient.  CA or C2AS forms a coarser grained phase called phenocrysts, the other minerals form fine grained intergranular matter according to a composition of the cement. C12A7 is typically green if observed in PPL. A CA presence test may be done using an immerse liquid with n = 1.652 - 1.657. It corresponds to mid-values of α and γ  of CA, if the liquid doesn’t contain CF in a solid state. If there is some mineral with birefringence higher than a value corresponding to CA present, it is suitable to do a test of presence of CA2. We use an imerse liquid with n = 1.630 - 1.640. A test of  presence of free CaO using  the White’s solution is necessary in the case of a clinker aluminous cement. In polished sections and thin sections, etching of CA at 20°C for an hour for an identification is recommanded. (C2AS is etch-resistant), for etching of C12A7 using of water at 100°C for 10-15 min. or 1% borax water-solution at 100°C for 30 seconds is recommended. For an identification of C2AS 2 % NH4Cl and 2 % Na2HPO4 . 12H2O water solution at 100°C for 3-5 minutes.


14.3  Mineral composition of air mortars and plasters

An air lime for mortars and plasters and plasters preparation should be prepared by calcium carbonate (calcite) burning. 

The calcium carbonate decomposition during its burning proceeds according to an equation:

During a burning of a pure limestone containing just calcium carbonate, 56 parts of calcium oxide and 44 parts of carbon dioxide form 100 parts of original material. 422 kcal of heat is necessary to decompose 1kg of CaCO3. The decomposition of magnesium carbonate proceeds according to a similar pattern: 310 kcal is necessary for a decomposition of 1 kg  of MgCO3 ( Králík, Fojtík 1972).

The air lime slakes at a contact with water and calcium hydroxide (slaked lime) forms according to an equation:

The slaking proceeds very rapidly along with a heat releasing.

According to a amount of water one can gain:

-          Dry hydrate (at small water amounts, so - called dry slaking);

-          plastic lime paste (with a 230 - 300% of a soft water excess).

According to a chemical composition we differ:

  - pure lime (with the content of CaO > 90% and MgO  < 7%)

  - common lime (with a CaO contant ranging 85 - 90% and MgO  < 7%)

  - dolomitic lime (with the content of CaO + MgO > 80%, the content of MgO should be  > 7%)


Table 10 General scheme of manufacturing and processing of lime.

Technological component

Chemical scheme

Microstructural scheme

Limestone, dolomitic limestone

CaCO3, possibly a mixture of CaMg(CO3)2, contaminants: content of  Fe2O3, Al2O3 and so on

Microstructure formed by crystals of calcite (sparite, micrite), possibly dolomite, clay minerals an so on.

Crushing, sorting

No changes

Mechanical disintegration and formation of a grainy (dispersion) mictorstructure

Burning at 1100-1300oC

Decarbonation by releasing of CO2 from the mixture

Disintegration of the original microstructure

Lump lime, ground lime

CaO, possibly MgO too, contaminants in compounds

Formation of a new microstructure, composed of grains (crystals) of CaO, possibly MgO and other phases, numerous pores

Water addition


Disintegration of the previous microstructure, transformation to a  plastic mixture

Calcium hydrate, lime paste with a water excess

Ca(OH)2, possibly Mg(OH)2 and H2O

Formation of new phases, mostly grains and crystals of portlandite(Ca(OH)2

Filler addition (aggreggates)

Variable chemical composition, does not take part in any chemical reaction

Inactive grains of variable size

Water addition


Formation of a plastic mixture

Mixing and using for a certain purposes

Carbonation of hydroxide Ca(OH)2 by air CO2

A colloid dry shrinkage and carbonation - micrite of  the plastic mixture 

Hardened solid product - mortars and plasters, plaster etc.

CaCO3 and a chemical filler of variable composition

New formed microstructure composed of grains of inactive filler and  CaCO3 micro crystals joining the grains of the filler together,  a presence  pores is characteristic


The problem of the formation of solid lime mortars and plasters or plaster includes three processes:

- An exsiccation of a lime suspension accompanied by a mortars and plasters shrinkage along with a formation of a compact mass;

- A formation of a solid calcium silicate phase by quartz dissolution in an alkaline environment of calcium hydroxide;

- Carbonation - includes a processes of a chemical conversion to calcium carbonate (see Table 5).

A friction process has the most significant affect on a strength of lime mortars and plasters. A mixture of a lime cement and a gauge water (along with soluble alkaline calcium silicates) fill pores among aggregates and the mortars and plasters. Calcium carbonate forms due to air carbon dioxide. Calcium carbonate microstructure corresponds to micrite. A speed of carbonation is the highest at a relative air humidity ranging 50-60%, it does not proceed in a dry environment at all. The process almost stops at a higher relative air humidity due to a difficult CO2 penetration through the porous system filled with water. An originally high alkalinity of fresh mortars and plasters (stated pH values 12,5 - 13,5) gradually drops to a value of pH approx. 8. It is possible to say that the lime mortars and plasters strength depends on a partial pressure of CO2 (its normal ratio in the air is about  0,03%), on an amount, species and degree of the lime slaking (the unslaked lime accelerates the hardening process, but „blocks“ the CO2 penetration to the center of mortars and plasters), on a porosity of mortars and plasters (given as a water coefficient, cement/aggregates ratio, a size of sand grains and their distribution curve) and is determined by a moisture and temperature of atmosphere too.

A microcrystallic carbonate (sparite) appears (first in a porous system of mortars and plasters and subsequently in cement) due to a partial dissolution and following recrystallization of micrite. A share of sparite increases with time. However its increse is not linear, but depends on a series of factors. 

A degradation (ageing process) of mortars and plasters runs at a presence of the air humidity (or as a result of capillary action of water, rain and snow affect, a placing of mortars and plasters in a construction in various cardinals points, at various hight levels, in places with a permanent moisture or with its cyclic changes). The process manifests as a slower dissolution of micrite and a graduate enlarging of calcite crystals - the sparite formation first of all.  A long-term study of mortars and plasters and plasters verified that these process runs faster. E.g. the process is faster in mortars and plasters and plasters prepared by slaking of an imperfectly burnt lime than in mortars and plasterss made of a perfectly burnt one. A crystallization pressure of authigenic calcite crystals (which are already microscopic according to a place of their formation) leads to a strenght decrease of mortars and plasterss and a follwing  damping off of the masonry face.

Physical and optical properties of calcite are listed in the encyclopedia of minerals. 


Gauging gypsum mortars and plasters belong to a group of air mortars and plasters. Calcium carbonate forms a base of these cements. The gypsum mortars and plasters represent rather widespread group of cements.

A strenght of hardened suspension made of gypsum with a prevailing contant of hemihydrate CaSO4.0,5 H2O is an important criterium for its application. The hardening process of gypsum is characterized by a reabsorbing of water and a recrystallization to dihydrate. 

0,18g of water is necessary to transform one gramme of hemihydrate to a solid phase, i.e. it gets transformed to gypsum. If there is more water in the suspension, it stays unreacted among dihydrate crystals in pores. The water evaporaters after the hardening and it leaves a continuous pore-net.

A strenght of hardened gypsum suspension depends on a misromicrostructure of the hardened product, on qualitative and quantitative parameters of the microstructure. A formation of this microstructure is a function of series of variables. These are partly internal ones, material, e.g. composition of gauging plaster, water coefficent, additives etc., partly external ones, to which belong e.g fineness of grinding, temperature of water, time and intensity of stirring.   

Dihydrate CaSO4.2 H2O occurs as a natural gypsum and on the other hand as a product of hydration of gypsum. The natural gypsum is rather compact material with a relative low porosity (its physical and optical properties are listed in the encyclopedia of minerals). An aggregate composed of small acicular crystals is a product formed by the process of hydration. The crystals are irregularly orientated, they can be feltlike and partly adherent. An average lenght of crystals reaches n.10-6m. A certain amount of dry hydrate is added into the gypsum in some cases. In this case, a micrite calcite occurs besides the acicular crystals of gypsum. The micrite calcite is translucent, brownish and sometimes makes an identification of the gypsum mortars and plasters more difficult. The gypsum recrystallization occurs with a proceeding process of ageing in suitable climatic conditions.  Size of individual crystals extends and their identification becomes easier.


14.4  Mineral composition of hydraulic mortars, plasters and concretes

Microscopic study of hydration and identification of products is extremely complicated by a small size of authigenic crystals. An electron microscopy is more convenient for studying them. For a petroligical microscope study, using of modelling of conditions and turning some away from a common routine is necessary to get largeger observable products.

For the study, we use identification in grain preparations, thin sections, polihed thin sections and polished sections again. Before grinding, a sample should be dried at  120-125°C for 6-15 hors. Afterwards, an adsorbed water shoud be removed, the sample should be consolidates by boiling in the Canada balsam (or in the epoxide resin) and ground without water. A waterless alcohol or glycerine, kerosine or oil shoul be used. A thin section thickness should range 15 - 30.10-6 m.

Using the thin sections one can determinate a percentage of non-hydrated grains and their size, type of porosity, size and distribution of pores, mutual relations of the original and authigenic cement phase, compactivity and formation of authigenic forms. 

In a normally set cement, one can with difficulties identify pellucide, porous, matrix with non-hydrated residuum of clinker minerals:  C2S, less often C3S and C4AF too and crystals of birefringent lamellar portlandite Ca(OH)2 . Ca(OH)2. In normal conditions of hydration, portlandite occurs in a form of small hexagonal tables. The porosity of gels extends with an increasing size of cement grains and water coefficient. A gel usually fills small canals and pores, which are smalled than  1.10-6m. It may occur in larger pores too and as well as Ca(OH)2, it can rim graines of cement clinker. The gel looses its homogenity and isotropy and recrystallizes. A crystallic, usually grainy phase occurs. However, the crystallization proceeds very slowly. In a polished thin section, one can differ pellets of the primary clinker and  Ca(OH)2 by tinging. Using of a solution of alcohol methylene blue with an addition of benzoic acid or an alcohol solution of naphtol green is the most common. Only the gel gets dyed.

Using polished thin sections and polished sections, one can identify a ratio of non-hydrated cement pellets too. The pellets are distinguishable according to their distinct positive relief for the first sight. It is suitable to make a repolishing before the application of chemical agents. A shammy leather and SnO2 should be used to decrease a spalling out. Before the polishing, the products of hydration should become more conspicious due to an affect of  the alcohol patent blue. The non-hydrated cement is colourless after the polishing. In the cement pellets, rims of tricalciumsilicate and dicalciumsilicate get emphasized by etching wiht HCl dissolved in alcohol (a drop of concentrated HCl per 10 ml of alcohol) during three seconds.

If using the grain preparations, before the observation itself, a separation of unreacted pellets from hydrated phases must be done. One can realize it by a soft knocking and pulverizing the sample in an agate bowl. The fine part should be separated with a sieve with an opening range 40-60.10-6m. One uses a n = 1,67 immersion liquid to differentiate a higher refractive unhydrated cement from a lower refractive hydrated products too. A Ca(OH)2 presence can be proved by the White‘s test at the temperature of  40°C (the reaction speeds up). It is possible to determinate a medium refraction value of the hydration products by the immersion method. The hydration products are too fine-grained to realize a measurement of orientation of the individual grains. The most distinct autigenic mineral of hardened mortar, plasters and concretes is calcite. Calcite forms by a reaction of air carbon dioxide with Ca(OH)2. Ca(OH)2 gradually releases during the hydration of clinker phases. On the first stage, a fine-grained micrite forms. Due to their high birefringence, its fine particles mask low birefringent products of hydration.

A study of the hydration process of cement clinker  should be done using a fine-dispersive powder with a maximum size of grains ranging 30-40.10-6m. The material should be wetted by water to form as large hydrated crystals as possible. The hydration process of the most common used clinker minerals is summed up in Table 3. A clinker of the Portland cement has been chosen as an example, because the Portland cement belongs among the most produced cements worldwide. Before its application itself, it is being special-purpose modified using various correction materials. According to them one comes across various cement types:

- high initial strength cements;

- sulphate-resistant cements;

- road cements;

- white cement, colour cements;

- expansive cements;

- plasticized Portland cements;

- strontium and barium cements etc.

These correction components (with an exception of gypsum) are not included in the following list.


Table 11 Hydration of individual components of the Portland cement.

C3S (3CaO.SiO2)

2C3S+6H®C3S2H3+3CH 2C3S+7H®C3S2H4+3CH


The reaction is not stoichiometric, because a hydro silicate gel of the C-S-H phase (RTG amorphous) with a fluctuating composition forms. The basic reaction of the cement hydration and the formed product of the C-S-H phase is a carrier of a majority of properties of the hardened cement.

b-C2S (b-2CaO.SiO2) 2C2S+4H®C3S2H3+CH

The reaction is very slow.

C3A 2C3A+21H®C4AH13+C2AH8®2C3AH6+9H  hexagonal ® cubic hydrate C3A+CH+12H®C4AH13

 The reaction is fast.  At the presence of Ca(OH)2 and at a lack of gypsum, the reaction can be the reason of a „false cement-hardening“.

C3A at the presence of gypsum  C3A+3CS*H2+26H®C6AS*2H31-32 (ettringite), 1-st stage of reaction sometimes Ca6A4(SO4)3(OH)12.26H2O C6AS*2H32+2C3A + 4H®3C4AS*H12 (monosulphate), 2-nd stage sometimes proceeds [Ca2Al(OH)6].[0,5SO4.3H2O] the conversion of ettringite to monosulphate may proceed this way too: C6AS*2H32+2C4AH13® 3C4AS*H12+2CH+20H

The principal of slowing down of hardening of the Portland cement. The reaction at the beginning of a contact of the Portland cement with water: S*= SO2

Ferritic phase C2AF+2CH+11H®C4(A,F)H13 C2AF+CH+3CS*H2®C6(AF)S*3H31-32 (analogical to ettringite) C6(AF)S*3H31-32+2C4(A,F)H13®3C6(A,F)S*H12+2CH + 20H


Free CaO, MgO C+H®CH  M+H®MH

 Accompanied with an expansion, instability of the cement volume a long-lasting instability of the cement volume, slow long-lasting reactions.

A hardened cement paste (cellular concrete) has a fine-porous microstructure, which affects its properties: strength, permeability and resistance to an aggressive environment.

There is a range of methods to identify a porosity of the hardened cement paste, mortars and plasters or concrete. These are for instance:  Hg-porositometry, water vapour adsorption or optical methods (technological pores).

Chosen properties of some products of hydration of the Portland cement are listed in Table 4. As shown above, it is apparent that it is almost impossible to study the aqueous mineral phases of the Portland cement using an optical microscopy. 


Table 12 Properties of chosen aqueous products of the Portland cement.






Size of crystals



Very low


0.1.10-6 m





<0.01. 10-6 m




Lamellar, pillar-like

0.01 - 0.1 mm


cca 1,75


Long, thin acicular m




Thin hexagonal plates

1.1 - 0.1.10-6 m



15     Concrete deterioration

A survey of various forms of concrete deterioration is given in a work of  Matoušek, Drochytka (1998) aimed at an atmospheric corrosion of concrete. Therefore, characteristic features of the concrete deterioration, which we are able to identify with a polarizing or a raster microscope, are only synoptically summarized in the following chapter.


An affect of CO2 is the most surveyed one. A concentration of atmospheric CO2 is usually 0,03% vol., which corresponds to circa 60 mg of CO2 per 1m3 of the air. The corrosions that is in progress is called carbonation (a product of a decomposition of lute is calcium carbonate, i.e. carbonate).

A concrete gas and vapour permeability (ergo its permeability) depends on a number, size, shape, configuration and mutual connectivity of individual pores. A total concrete porosity consists of pores of lute and aggregates and larger cavities formed as a result of an improper processing of mixture, e.g. exsolution of aggregates and a cement mixture.

The lute contains gel pores and micro pores that if constantly filled with water (or solutions) are practically gas proof. The other pores filled with air, are permeable for gases. These mostly take part in the concrete corrosion. Gases and vapours spread by diffusion, i.e. very slowly, in the concrete stone as well as in every porous material. The diffusion speed affects a depth, which can be reached by carbon dioxide in the concrete  a certain time later. At suitable conditions, the depth of carbonation of a common more permeable concrete reaches up to 9 mm after one year and about 20 mm after 10 years.

It is possible to express chemical reactions running during the lute carbonation by a following scheme:

The carbonation of the intermediates possibly continues, e.g..:

The scheme of the process of carbonation illustrates transformations of basicl products of hydration of cement with carbon dioxide. A moisture takes an important part in the carbonation reactions. Four stages have been specified in the process of carbonation.

In the first stage, calcium hydroxide transforms to calcium carbonate in intergranular spaces (the size of the formed product corresponds to micrite) and the pores become partially filled. The concrete properties are of higher quality in this stage. In the second stage, other transformations of the products of the cement hydration proceed. CaCO3 modifications form along with an amorphous silica acid gel and stay in pseudomorphs after products of hydration of mortars and plasters, possibly formation of micrite follows again. Sparite occurs just sporadically. The concrete properties do not significantly change during the second stage of carbonation. The outstanding feature of the third stage is a graduate recrystallization of micrite. Sparite (calcite, aragonite) forms along with it. The concrete mechanical qualities worsen. The fourth stage is characteristic by a formation of sparite in such a rate that sparite replaces the lute microstructure. The process is accompanied by a loss of strenght and cohesiveness.

A moisture affectst the carbonation speed. If the concrete is dry, the carbonation process stops. The carbonation is the most dangerous, if atmospheric acid-forming gasess penetrate up to a reinforcement. Actually, a value of pH of the intergranular solution rapidly decreases due to carbonation, a reinforceing steel looses its pasivity and starts to grow significantly rusty.  This already proceeds in the second stage of carbonation. The corrosion proceeds so fast in the third carbonation stage that the reinforcement and hereby the whole construction often serves out sooner than the fourth stage of carbonation starts.

Minerals formation during carbonation (calcite, vaterite and aragonite) are characterized in the followig part of the text.

Another process leading to a desintegration of concrete is sulphatation. In comparison to carbonation, the concrete sulphatation is much more agresive precess. In a cellular concrete, air SO2 reacts with hydrated cement products, possibly with calciumhydrosilicates. At a suitable humidity the decomposition reactions run immediately. The humidity affects a speed of the reaction, a quantitative share of final products and intermediates (a concentration of SO2 and time of its attack play a role too). One may document the reaction in a following form:


The decomposition of calciumsilicates and calcite proceeds in several steps. Calcium sulphate hemihydrate is an important intermediate, which directly or secondarily becomes oxidized to gypsum during a following corrosion. A humidity presence is crutial for the process. About up to 3% vol. of humidity only sulphate hemihydrate forms and calcium sulphate hemihydrate forms from it during a following corrosion. Calcium silicate hemihydrate finally transforms to dihydrate due to another affect of the enhanced humidity. The final product of the sulphate attack on concrete is always gypsum. Gypsum forms new microstructure of crystal druses of a significant volume.

Not only environment and geological bedrock affect the process of pathological changes, but it is obvious that the deterioration proceeds in a different way in the case of a Portland clinker concrete, a wite and a clour clinker concrete, a steel concrete and an aluminous cement.  A character of the used aggregate is very important too. The last sort of deterioration is connected with it. It is represented by alkali-silicate reactios (ASR) or according to other authors alkali aggregate (aggregates - crushed gravel) reactions (AAR). The mechanism is explained according to a scheme shown in Table 13.

Three cases are usually given for a reactivity of aggregates with alkali:

- Alkali-siliceous reactions (ASR, AAR);

- Alkali-silicate-siliceous reactions (ASSR);

- Alkali-carbonate reactions (ACR).

The reaction of alkali and aggregates containing various reactive forms of silicon dioxide: opal, hornstone, flinstone and chalcedony or tridymite, cristobalite and volcanic glassess (e.g. some hornstone gravels can already cause a decrease of quality of concrete if they are present in amounts ranging 1-5%). A concrete manufactured using this aggregate is liable to a very fast disintegration due to a relative fast expansion along with a formation of alkali-silicate expansive gels.  After expiration of approx. 10 years, cracks start to exhibit in the building construction.

Reactions of alkali with tectonically deformed quartz. We sort a wide range of rocks containing tectonically deformed quartz, which is a strong reactive component of concrete as well, into this group. The tectonically deformed quartz occurs in greywackes, argillized rocks (graywackles, arcoses), in some types of silica sends, quartzites, metaquartzites, contact hornstones, gneisses, cataclased granites up to mylonites, fylites and arcoses.  The given type of reaction  is characterized by a slower expansion start. The cracks need not to be perceptible for a more than twenty-year period afrer building up the construction.

Alkali-carbonate reaction. It is a reaction of alkali that occur between dolomitic limestones with a clay addition and alkali pore solutions in concrete. The reaction is expansive and causes an intense cracking of concrete. So-called expansive dolomitic limestones are usually formed by a fine grained matrix. The matrix is formed by micrite and clay minerals. There are dolomite rhombohedrons scattered in the matrix. This reaction manifests quite soon in building constructions and the cracking starts after approx. 5 years after finishing the labour.

Alkali-silicate reactions can be caused by a composition of a geological bedrock beneath the buiding. However, a formation of gels does not terminate the process. In a presence of sulphate anions, new mineral phases of ettringite and thaumasite gradually occur in a place of gels. Their crystallization pressure leads to a total concrete disintegration. Characteristics of the minerals are given below.  


Table 13 Mechanism of the alkali expansion


Alkali distribution (comming from a cement or certain aggregate pebbles, e.g. feldspars in a concrete-water mixture)



Reactions of alkali solution (Na2O, K2O, CaO) with granulates


Soluble siliceous fragments:

Dolomitic CaMg(CO3)2 fragments:

Gel CaO-SiO2-Na2O-n H2O formation:

- decomposition by alkali:

- CaO excess

 - alkali excess

Formation of Mg(OH)2, CaCO3 and Na HCO3, K HCO3.

Non-expansive gel on surfaces of the granulates.

Gel:SiO2-Na2O-K2O expanding due to an absorption of water.



Expansion - formation of cracks- disintegration.




16 The ceramic products

16.1  Brickware and stoneware

In comparison to the other products, particulary kitchenware and building ceramics, the application of microscopic methods is rather limited in the research on brickware.

An evaluation should contain:

1          Evaluation of mineral particles:

1.1       Grainy minerals (feldspars, quartz etc.);

1.1.1    Quantitative representation, average size (particulary of large particles, if their representation is significant), shape (isometric, anisometric, spherical, rounded, angular), surface (rough, semi-rough, smooth);

1.2       Foliaceous minerals:

1.2.1    Quantitative representation of mineral species (muskovite, biotite, chlorite), average size of a cross section, shape of the particles (parallel, warped , burst);

2          Evaluation of a cementing matter:

2.1       Sintered (in CPL isotropic, but it almost always contains fine anisotropic not sharp bounded particles) - in anisotropic particles it is suitable to identify their shape: grainy, foliaceous, globuliths, trichites, microliths, skeleton-like;

2.2       Mixed (with the assumtion of the percentage of isomorphous matter and perceptible sharp bounded anisotropic particles);

2.3       Crystallic (anisotropic in crossed polarizers);

3          Microstructure - if perceptible:

3.1       Confining;

3.2       Parallel:

3.2.1    Grainy particles are parallelly  cofigurated;

3.2.2    Foliaceous particles are parallelly cofigurated;

3.2.3    All particles are parallelly cofigurated;

3.2.4    Cement is parallelly cofigurated;

3.3       Lenticular;

3.4       Reticular;

3.5       Fluidal, grains are surrounded with a fluidal cofigurated cement, which was stronger molten down;

3.6       Glomerophyric;

3.7       Ghost;

3.5       Undulated microstructure and other irregularities;

4          Fixing of particles and porosity:

4.1       Fixing of particles of grainy particles with cement;

4.2       Fixing of particles of foliaceous particles with  cement (if it is not formed by them only);

4.3       Amount of pores in 1 cm2 (these are regullar pores, which formed due to the affect of gas and vapour-releasing in matrix. The gasess and vapours were either primarily present or formed by metamorphosis and decomposition of minerals at an elevated temperature. It is necessary to evaluate pores larger than 0,2 mm);

4.4       Shape and average size of pores;

4.5       Position of pores in the microstructure and their interconnection (continuous, discontinuous porosity) and communicativeness.

So we get a complete picture of both petrografic and technological properties of the ceramic body.

It is possible to generaly describe the ceramic products according to following criterions:

Size of particles: we differ fine and coarse ceramics.

Coarse ceramics contains microscopically distinguishable particles larger than 0.1 mm. The microstructure of the fine ceramics is observable only microscopically, the size of grain reaches maximum 0.1 mm.

Porosity: at the absorption capacity over 8%, the body is porous, below 8%, it is coarse.

Colour of the body: we distinguish white-body ceramics and colour-body one.

Way of use: it characterises kitchen and building ceramics.

Presence of a specific component of non-plastics (e.g. graphite).

On the basis of the presence or the absence of some minerals in a hitorical ceramics, mica, send and lead (or graphite) ceramics are distinguished.


For the study of historical solid ceramics, using of following parameters is recommended:

Identification of a grain size of ceramics bodies distinguishing:

very coarse-grained ceramics: larger than 2.5 mm

coarse-grained: 1.5 - 2.5 mm

medium-grained: 1.5 - 0.5 mm

fine-grained: 0.1 - 0.5 mm

compact: smaller than 0.1 mm

Character of cement:

homogenous - individual particles are not distinguishable;


- submicrocrystallic (particles of cement are distinguishable only at over 50x magnification);

- microcrystallic (the particles are distinguishable at 10x magnfication).

The presence of special minerals: e.g. graphite, mica; to specify ceramics according to e.g. the character graphite components:

- with on purpose added graphite as both non-plastics and cement;

- with the substance of coal cement and pebbles of graphite material;

- with the graphite material used only as non-plastics;

- carbon of another genesis (deposition of carbon, reduction of carbnates, by the addition of grease, by burning of the other organic substances added to the ceramic body).

To specify in detail the charcter of graphite material e.g.:

- homogenous carbon substance;

- cryptocrystallic graphite (individual flakes aren not distinguishable in a common polarizition microscope, but only  in an electron microscope);

- microcrystallic (size of the individual flakes ranges approx. 0,01- 0,1 mm);

- macrocrystallic (the graphite flakes are larger than 0.1 mm).

In the case of micas, useing of their shades (dark, light), lustre - a silver one (in the case of muscovite), bronze (in biotite).

To make create cathegories according to the porosity of the body:

more than 10 % of pores -  high porous body;

8 - 10 % - porous body;

5 - 8 % - low porous body;

5 % - very low porous body.


It is possible to follow this procedure at the evaluation of the hictoric ceramics, which firing temperature, homogenity of used materials and their mixing enable to complete the pertographic characteristics.

In modern procedures, common optical microscopy is almost useless. The reasons are both the extreme finess of new phases forming from relatively inhomogenous material and not sufficient attention that has been payed to  the research of the firing processes of these products so far. The main interes has rather been concentrated at technological-economic problems. The firing temperatures are ordinarily too low to reach the state close to equilibrium. Thin sections of a common thickness do not enable more precise identification of very small authigenic forms in matrix. However, pollished sections enableing at least estimation of the microstructure and which are more useful for abnormally fine material, are hard to polish. First of all, quartz and pseudomorphs afer calcite and mica are microscopically identifiable in brickware. Fine-grained red-coloured matrix contains colour glass. The colour glass forms at 500-1000°C according to the composition of the material. At the firing over 950°C, the refraction of the glass reaches between 1.58 - 1.60. With a growing temperature, due to the affect of dissolving SiO2, its values decrease to 1.54 - 1.55 at 1150°C. The glass is inhomogenous, it contains cells, undissolved primary minerals and very fine needles and grains of devitrification products, which along with the colour of the glass determinate thea colour shade of the prodact. At the firing, yellow-brown of iron hydrates, passess into brown-red already from 250°C. Another colour change proceeds only at the crystallization of small reddish grains from glass, ordinarily over 900°C and with refraction n = 1.69. Along with the temperature increase their number increases, so they completely pigment the glass and its colour becomes more intense. On the contrary, over 1200°C, their dissolution proceeds and at 1250°C they vanish. Simultaneously, the colour of the glass passess into gray-green up to black, so the completely refired product is steel gray. At a reducing firing, crystallization of this phase does not proceed at all. Higher CaO content supports the dissolution, so the products never reachs red colour and on the contrary green isotropic crystals with n = 1.70 - 1.72 crystallize from the glass. Its consequence is the colour change of the product from pink up to green-brown.

Significantly larger amount of the glass forms in a dense stoneware body.

The phase composition of the stoneware body is in a certain interval. The glass contant is between about 30 - 40 % vol. in common products. In a fine potery, it is significantly higher (60 % vol. and more). The crystallic phase is composed first of all of mullite or a cryptocrystallic mullite phase and its contant is between 10 - 15% according to the composition ofv the initial mixture. Further, relic quartz and cristobalite are present. From crystallic phases e.g. grains of corundum, SiC, ZrO2, rutile, zircon and other minerals undissolved in the melt are identifiable. They represent relics from the initial material. In an authigenic association e.g. spinelids, cordierite etc. may form. Cryptocrystallic mullite and weak corroded quartz grains are perceptible in the glass. In a salt glaze, due to migration of dissolved Mg and Ca salts to the surface of the product, green gehlenite may appear during the drying process. In brown glazes, besides gehlenite, nepheline and egirine may appear. Only hematite has been found in red-coloured glazes.


16.2                Porcelain

In a microscope, heterogeneity of a porcelain body comes out. In fact, the body contains fragments of coarser quartz surrounded with glass matrix with mullite needles and pores. In a rough state, the grains of primary quartz are - besides a degree of homogenity of glass, particularly of pseudomorphs after feldspar - clear indicators of the finess of the matter and of the degree and the lenght of firing. In penetrating light, they are pellucid, the largest of them are cracked. In XPL, they significantly differ in light gray up to straw-yellow interferential colour from isotropic and thus dark glass matter. Very small anisotropic mulite needles ordinarily fade the glass matter. Quartz is mostly unaltered. Only exceptionally, e.g. in chalcedony material, the alteration to cristoballite was observed. However, the melting down of quartz is typical. It begins in rims and then proceeds in the whole surface, so the original angular fragment gradually gets rounded. The formation of pellucid high siliceous glass film around quartz grains is the result of the melting down of quartz. The thickness of the film reaches up to 10.10-6m and even more. At a long-time and glaze firing the complete dissolving of quartz grains appears. Islets formed by pure glass matter without mullite form instead of them. Glass margins refraction is n = 1.46 - 1.47. The stated values are somewhat higher than for apure quartz glass: n = 1.456. But, at the same time, they are lower than refractions of glass containing mullite, which are 1.486 - 1.490, due to the affect of the higher Al2O3 and Na2O + K2O contents. Except of the fragments of quartz, there are perceptible remains of feldspars in the porcelain fired at lower temperatures. In the centres of the fragments of feldspars, at a very weak firing, one can also observe birefringence. Optical properties of these feldspars correspond to sanidine, which orthoclase passess to at heating from 700°C to 1000°C. At a higher temperature formation of a melt by a reaction with a fine SiO2 from kaolinite and submicroscopic mullite proceeds. The melt fills a space of the original feldspar grains. Glass pseudomorphs after feldspar also contain mullite, but in a smaller amount and with considerably larger crystals. Because mullite can not form by the  decomposition of feldspar, it is obvious that the feldspar melt enriches not just for a siliceous but also for an aluminous component from kaolinite decomposition products. This phenomena is indicated by refraction of glass in the feldspar pseudomorphs n = 1.491 - 1.495, which is higher than the refraction which would correspond to the feldspar glass that has n = 1.485-1.488.

Matrix contains small needles of mullite and glass with n = 1.486 - 1.490. Microstructure of mullite is ordinarily completely confining, it forms a felt-like microstructure and its contants is 10 - 20 %, maximum 30 %. The size of mullite crystals is, as wellas the degree of melting down of quartz crystals a continuous dissapearing of pseudomorphs after feldspar, the orientaion scale of a firing temperature and time of durability. The lenght of mullite prisms is aprox.  3 - 30.10-6m, the largest crystals occur in feldspar pseudomorphs and around pores. Along with the increasing content and size of mullite, the homogenization of the matter glass proceeds. This manifests gradual equalizing of refraction of glass in various places of matrix. There is a direct relation among the refraction of glass, specific gravity and the Al2O3 contents in glass matter. Therefore, according to refraction or specific gravity, one is able to predict conditions of the mullite crystallization in the initial feldspar melt. Analogicly, on the basis of  Al2O3, it is possible to make predictions of viscosity and thusthe crystallization of mullite nucleuses. Glass forms about 40 - 60% vol. In matrix, there are pores, which are important for the use of porcelain. Generaly, it is possible to say that with the increasing number of pores the voltaic conductivity and strength decrease. The pores ordinarily reach 10 - 150.10-6m and they are observable both in a thin section and in a polished section. A quality electrical porcelain. should comply e.g requirements:

- fine crystallic microstructures with a mullite felt-like net;

- homogenity of matrix containing uniformly distributed mullite and glass without coarser crystals;

- contant of corroded fine quartz;

- as small volume of pores, particularly of large and communicative ones, as possible.

16.2.1 Glazes

In the case of glazes, the application of microscopic research methods allows the control of purity and the control of   size of basical components of particles, measurement of strain in the glaze, assesssment of its microstructure and identification of crystallic phases.

The microstructure of the glaze is perceptible in a fracture of the body. For the identification of defects, e.g. cells, surface defects etc., perpendicular to surface oriented sections are used.

Identification of crystallic phases is done the same way as in glass. We orientate according to shape of crystals, refraction and birefraction, extinction and lenght character. Tension and the network of cracks may be another distinguishing features. These are ordinarily not completely molten parts of the batch, foreign admixtures, passed on opacifiers and products of intentional or accidental devitrification, e.g. cristobalite from unreacted quartz. In porcelain, in the contact of the glaze and the body, more intense mullite crystallization appears, both in the electrical porcelain and in chemical porcelain. In the contact layer of low content of Al2O3 and SiO2-lead glazes, anorthite was identified.

Matte glazes most frequently contain thin lamellar anorthite, which is ordinarily twinned. In calcium-aluminium-siliseous glazes, wollastonite individual crystals up to clusters occur. Wollastonite is coarser-grained than uniformly distributed anorthite. In potassium-calcium-siliceous glaze, leucite was found too. In lead matte glazes, anorthite also prevails, besides less frequent wollastone. At high SiO2 and low Al2O3 content, tridymite forms. In the connection with increasing viscosity, higher share of Al2O3 manifests in the formation of finer crystals and thus in uniformer microstructure. Light spots in matte glazes are ordinarily caused by unreacted quartz.

In crystallic glazes, large cassiterite crystals are seldom developed (characteristics of the mineral - see the encyclopedy of minerals).

16.3  Steatite ceramics

Another group of ceramic matters is represented by so-called steatite ceramics. The basical component of the material is soapstone. The dominant component of the product is enstatite (Mg2[Si2O6]). This sort of ceramics is used in places, where a high hardness is required. Unlike in porcelain, the microstructure of the steatite ceramics is much uniformer and its mineral composition is more homogenous. In a great magnification, one finds find out that the center is ordinarily formed by crystals of isometric protoenstatite or clinoenstatite and a small amount of glass surrouns the crystals. The thermic transformation of soapstone during the firing process runs according to the following scheme:





The transformation of enstatite to protoenstatite is slow, it proceeds at aprox. 1260°C  or at temperatures somewhat lower. The formed protoenstatite may transform to clinoenstatite during cooling at 700°C. Thus, the most frequent component of the steatite ceramics is protoenstatite. A certain time later, at a normal temperature, protoenstatite may transform to clinoenstatite at a simultaneous temperature decrease. Protoenstatite represents high-temperature modification. The clinoenstatite-protoenstatite ratio is the same as the ratio of low-temperature α-cristobalite and high temperature β-cristobalite.

The optical and physical characteristics of enstatite are listed in the encyclopedy of minerals.                          

It is distinguishable from clinoenstatite by its srtaith extinction and very rare twin intergrowths on (104) and (011). It does not differ from protoenstatite in its optical properties.


Figure 12 Hardness of the steatite and the mullite-corundum ceramics according to Szymański (1997); 1- results according to Budnikov, Kantor (1961 in Budnikov 1964), 2 - Mohs hardness scale; 3 - various steatite types.


In the microscopic evaluation of the steaktite ceramics one aims at the evaluation of the modifications of enstatite, if the growth of its crystals allows it. In the microstructure of these matters, as small size of the enstatite crystals as possible, their excellent development and uniform surrounding with a pellucid glass matter without a perceptible tension are simultaneously required. Fig 8 illustratively shows the different hardness of the steatite, corundum and mullite-corundum ceramics. Large crystals display the trend towards cracking, namely under the affect of tension at rapid changes of temperature and different expansivity of the crystals and the surrounding glass matter. The glass matter shall isolate individual crystals to prevent their recrystallization and it stabilizes their undesired transformation to clinoenstatite.


16.4  Cordierite ceramics

The cordierite ceramics is very interesting. Depending on the lenght and degree of firing, cordierite forms three modifications: high-temperature (a) , low-temperature (b) and metastable (m). The a modification of cordierite is of natural genesis, synteticly it forms by the reaction of suitable material at a high temperature 1570 - 1750°C (b - modification) and m-cordierite may form in a slow-cooled glass. Syntetic cordierite called indialite crystallizes in hexagonal symmetry. Considerable rapid changes of tepmerature resistance resulting from the low coefficient of temperature expansivity is a typical property of the cordierite ceramics. Its dielektric properties are inbetween steatite and porcelain and it has a thin interval of sintering. α-cordierite is a prevalent mineral in high-quality product. Other modifications (β, μ) do not practically occur in common products. α-cordierite characteristics are listed in the encyclopedy of minerals. In the cordierite ceramics it most frequently forms xenomorphic grains, more rarely short pseudohexagonal prisms. However, refractions of the cordierite ceramics are somewhat lower than of natural rocks - usually nmedium = 1.52 - 1.53, D = 0.003 - 0.004, Chm (-). If it is elongated, Chz (-).

One can distinguish it from quartz according to D (quartz has somewhat higher birefringence and is uniaxial). In hypautomorphic up to automorphic crystals a distinguishing according to the shape of their sections is possible - squares to rectangles in longitudinal sections and hexagons in cross sections (watch quartz!) is possible.

In the cordierite ceramics, cordierite is distinctly perceptible only after firing over 1300°C, although it forms already from 1200 - 1250°C. Along with the increasing firing temperature or by its repeating, a recrystallization of crystals 1 - 2 .10-6m sized to individuals up to 20.10-6m (at 1400°C) proceeds. Over 1460°C, cordierite decomposes to mullite and a melt. Mullite forms sheaf-like aggregates, arranged into spherulithes. The aggregates have square sectionons and they are over 1 mm long and 25 - 50.10-6m wide. They are composed of needles of mullite. On the contrary, a mullite glass, devitrificated in the interval of 1460 - 1050°C, contains automorphic prismatic crystals of α-cordierite, 20 - 50.10-6m long. Some of them are twinned. At the reaction of MgO-Al2O3-SiO2 components, cordierite is not the primary compound. Spinel, MgO.Al2O3, which then reacts with SiO2 to cordierite, forms. In mixtures composed of soapstone and kaoline, possibly SiO2 and MgO, from temperatures of 1200 - 1250°C one can observe pseudomorphs after clay minerals and soapstone, possibly free quartz. The homogenity of the material plays an important role here. In lower temperatures of firing and in weakly fired matters relaively sharp separated zones occur in thin sections. They are composed not only of very fine cordierite, but also protoenstatite, mullitized clay, free quartz or  aggregates of α-corundum comming from aluminium oxide. The highest level of reacting, proceeding at the 1300 - 1400°C interval, is displayed by mixtures based on soapstone and kaoline (at lower temperatures of the stated interval) and based on kaoline, MgO and SiO2 (at higher temperatures). The ratio usually is 62 % of caoline + 38 % of soapstone, the firing temperature 1380°C. The microstrucrure is uniform, dense. Free quartz is not present (unlike in less quality matters) and cordierite 10. 10-6m sized forms distinct sheaf-like aggregates here. Individual grains of cordierite are xenomorphic. In the case of the firing of the material composed of kaoline, magnesite and ground quartz, a considerable share of 3 - 6.10-6m sized isomorphic crystals of cordierite occurs, besides small amount of free quartz. Sheaf-like aggregates cordierite do not occur in this matter, microstructure is fine confining grained.

16.5  Porcelains not containing glass - oxide ceramics

The oxide ceramics includes in fact uniphase molten products of various initial materials. The absence of glass is a condition of special physical-chemical properties. A rutile ceramics, various ferrite types based on geikielite (MgTiO3), armalcolite (Mg,Fe2+)Ti2O5 and other magnesium-titan components belong into this group of products. Mixtures of geikielite (MgTiO3) and armalcolite (Mg,Fe2+)Ti2O5 form at the melt crystallization and they may form by solid phase reactions. Geikielite itself forms as a primery compound only by the solid phase reaction. Microstructure of these matters is similar to the microstructure of the steactite ceramics. They contain no or just a minute amount of glass and prevalent crystallic phase in fine-grained crystals. The most common magnesium-titan component has the spinel microstructure, it is isotropic, n = 1.972 and grained. Geikielite is trigonal, distinct cleavable on [1011], its specific gravity reaches 4.0, hardness 6, metalloidlic lustre. It is optically uniaxial, Chm(-), ne = 1.95, nw = 2.31, D = 0.36. Armalcolite Mg0.75Fe2+0.25Ti2O5 has a metallic lustre, it is grainy, xenomorphic to hypautomorphic,  α = 2.32, γ = 2.39, D = 0.170.

Titanium dioxide TiO2 forms three modifications: rutile, brookite and anatase. From the listed minerals only one, namely rutile, is technically important.  Products of pure rutile occur very rarely. They contain admixtures of Na2O, K2O, Li2O, Cu2O, BaO, MgO, CaO, SrO, BaO, ZnO, which significantly affect their dielektric properties. On the contrary, the other trivalent metal (Fe2O3 a Al2O3) admixtures stabilize the dielectric properties of TiO2.

The most commonly used and the best surveyed ferroelectric ceramic material is barium titanate. Its microstructure corresponds to perowskite and it forms by firing of BaCO3 and TiO2. It forms several modifications: monoclinic (473°C), rhombic (from 473°C to 550°C), tetragonal (from 550°C to 370 - 673°C) and high-tremperature cubic one.

Optical constants of bariummetatitanate depend on admixtures as well. Cubic BaO.TiO2 (prepared of pure components) has n = 2.40 - 2.41. Tetragonal BaO.TiO2 (modificated Al2O3, SrO or BeO) have nε = 2.37, nω = 2.50. The cubic high-temperature modification n = 2.46. This modification forms from the tetragonal one.

16.6  The spinel ceramics

Ferrites of  spinel microstructures - spinel ceramics. A complete character of microstructure of these prodacts is to some extent similar to the steatite ceramics. They are distinct at the  prevalence of crystallic phase. The crystallic phase is often surrounded with minute, often microscopically unidentifiable, amount of glass. An microscopical assessment should be aimed at the arrangement, shape, size and granularity of a prevalent mineral, at the amount and distribution of glass or accessory minerals and porosity. Optical parameters of the majority of the minerals occuring in the oxide ceramics are discussed above. 

The term comes from the natural mineral - spinel MgAl2O4, where Al3+ substitues Fe3+ at the formation of ferrite spinelide. Using the formula of a common spinelide MFe2O4 one is able to replace M with bivalent cations as Co2+, Fe2+, Ni2+, Zn2+, Mg2+ and Mn2+. There is the practical application particularly for ferrites with the spinel type microstructure as: FeFe2O4, CuFe2O4, MgFe2O4, Li0,5Fe2,5O4, ZnFe2O4, CoFe2O4, NiFe2O4.

Ferrites with a magnetoplumbite (hexagonal) microstructure. Natural magnetoplumbite Pb1.1Fe3+7.7Mn3+2.6Mn2+0.6Ti0.6Al0.4Ca0.1 O19 is represented by only one corresponding form, namely  hexaferrite BaFe12O19 in the group of ferrires. The other identified ferrites considerably differ from the natural mineral.

In the M-position, there may be any bivalevt cation represented: Co2+, Fe2+, Ni2+, Zn2+ and Mn2+. Fe3+ may be replaced with trivalent cations (Cr3+, Al3+, Sc3+, In3+, Ga3+, Mn3+ etc.). Analogicly, Ba2+ ions Ca2+, Sr2+ or Pb2+ ions may be represened there. Similar ferrites with garnet-like microstructure may exist.

Further, Al2O3, MgO, BeO, CaO, ZrO2,ThO2, UO2, CeO2 and a series of other oxides  first of all from the lanthanum and actinium-group belong  among the oxides of heatproof materials.

High temperature resistant heatproof materials can be prepared of materials corresponding to following minerals: corundum (a-Al2O3), baddeleyite (ZrO2), bromelite (BeO) and other components, without using high-grade manufacturing technologies, as well as of specially prepared components.

Bromelite BeO crystallizes in hexagonal symmetry, if authomorphic, it is ordinarily prismatic. However in the oxide ceramics, it is most frequently xenomorphic. It is pellucid, perceptibly cleavable on {1011}, nε = 1.719, nω  = 1.733, D = 0.014, Chm (+).

Thorianite ThO2 is cubic, in a thin section, it is red-brown, gray or pellucid, n = 2.20.

One is able to study the oxide ceramics using thin sections, polished thin sections or pollished sections. At using the thin sections, it is necessary to make the sections as thin as possible, aprox. 10 - 20.10-6m thick. In a series of them, their high hardness is a serious barrier of grinding. Therefore using of fine boron carbide instead of siliciumcarbide is convenient for finishing the grinding. For making crystal rims more conspicious, etching the thin section with HF vapours or acide alkaline sulphates and fluorides or NaOH for several minutes at relative high temperatures (300 - 350°C) is recommended. On the contrary, pores come out after filling with a foreign component with a different refraction ant thus by making the  differences of the refractions more conspicious. 

Products of these type are used in a rocket industry and a nuclear power-engeneering, where the products must have both heatproof properties and ability to slow down e.g. neutrons. 

A typical representative of the oxide ceramics is also sintered corundum. In its crystals, pores are almost always perceptible in a polarization microscope. The pores are rounded, often mutually connected with thin canals.  Their size usualy reaches between 10.10-6m and 5.10-6m. Unlike in pores of melted corundum, the canals are neither filled with glass nor crystals were found in them. In another case - in BeO - the identic orientation of individual pores, which were bounded with basal, prismatic and pyramidal surfaces, was identified. In sintered corundum, the pores most frequently occur in relatively large crystals in distinctly recrystallized matters. No difference between recrystallized and pure Al2O3 fired at high temperatures and admixtures, which distinctly positive affect the sintering, but at the same time support the recystallization, was found. Admixtures of MnO, TiO2 and CuO are applicated this way. Extra ordinary large pores occur in corundum crystals in matters with the most efficient admixture - a mixture of MnO and TiO2. The distribution of the crystal pores is not uniform, particularly in matters with sporadic prsmatic autigenic crystals. Pores are concentrated inside the grains and their number decreases towards margins.

The absence of pores close the crystal borders is explained in the connection with the diffusion of vacant crystal lattice positions. The vacant positions occur close the pores. The concentration gradient is the condition of their diffusion to crystal inerfaces, where they disappear. In oter words, from the crystal interface they difuse to neighbouring pores, fill them and these gradually get smaller up to disappearing. It leads to the densifying of individual grains and consequently to shinkage, which manifests even after reaching the zero absorbability. The proceeding of the shinkage, which is manifestation of the decreasing of discontinuous porosity and it corresponds to the increasing of bulk density. Along with increasing firing temperature, pores inside the crystals get larger. This sort of pores may work the same as the crystal inetrfaces. Here, both vacant positions extinct and pores grow larger. The interpretation of the genesis of the original Ryschkewitsch pores as intercrystal relics was in fact exeperimentally verified.  At the sintering, the orignal intercrystal pores gradually decrease, as long as the recrystallization proceeds. Both pores itself and present foreign phases, if they work as a growth barrier, prevent the recrystallization. As soon as the total volume of pores decreases below a certain level and the sufficient amount of a foreign phase is not present, the movement of the crystal inerfaces towards the centres of their curvature appears and the recrystallization of corundum begins. Although pores in a close distance to the moving interface disappear, closing of the pores into the crystal centres, particularly at the rapid, non-uniform growth of sporadic crystals due to the affect of the mineralizers, may proceed. In the uniformly coarsened pure Al2O3, the amount of pores closed in the crystals is in fact lower than in the intensively recrystallized matters with  the MnO and TiO2 admixtures. In fine-grained matters, the crystal cavities do not occur at all or are less frequent and reach smaller extents. This is the an evidence of the fact that pores in these materials disappeared before the recrystallizetion began or they disappeared afterwards, sitll during the movement of the crystal interfaces. Bulk density, which is generally higher than in matters with large crystals, supports it too. 

A sintered corundum is microstucturally devideable into several categories. One usually considers a shape, crystal distribution and the level of  uniform mass-ditribution in a larger space. According to the crystal shape one distinguishes following types of microstructures: paving, microstructure formed by rounded grains, confining grained one, formed by long columnar crystals without a preferential orientation one. The isometric microstructure may occur in both molten corundum materials and oxide materials. The confining microstructure occurs only in the oxide matters. According to crystal size we further devide them into fine-grained (below 10.10-6m) ones and coarse-grained ones (over 10.10-6m). However, this way of division may be applicated only in monodispersed matters with crystals of relatively uniform size. Polydispersed matters contain crystals many times larger than the average grain-size of matrix.  Microstructure of monodispersed matters may be both isometric and confining. There may be both molten corundum and oxide matters represented among them. By deviding the microstructures into four groups (paving, confining, monodispersed and polydispersed one), it is possible to illustrate the basical differences between molten corundum and oxide matters. Microstructure of the molten corundum matters is first of all given by granularity of the initial material, which stays preserved in coarse grains of fired and even refired matter. The original granularity of the material only changes in the smallest particles disappearing at the exclusion of relatively little increase of medium and large particles. In the unsintered matter, there are smaller grains of corundum still perceptible and the average size of the crystal stays approx. the same as in the initial material. In the sintered matter, the crystal size increases to aprox 10.10-6m and the fine grains practically disappeare.

The oxide matters may, unlike the molten corundum matters, display much more significant microstructural differences. The cause of distinct differences is both the character of the oxide matter and the sensitivity of the oxide matters to the affect of the sintering admixtures. We have to consider the sintering admixtures affect specificly. The affect of some admixtures, e.g. MnO, CuO and TiO2, leads to the formation of distinct polydispersed microstructure, even in materials, which, in a pure state, form monodispersed microstructure even at a complete sintering. Contrawise, the other sintering admixtures slow the growth, namely both in matter naturaly prone to polydispersed microstructure and matters with artificial made one. The artifical microstructure may be caused by e.g. MnO. The affect of MnO is then canceled. The uniformity of the molten corundum refired matters increases and they become significantly monodispersive, while the uniformity of the oxide matters dissapears and they pass into polydispersed ones.

According to the distribution of crystals in the whole volume of the matter, one distinguishes uniformly grained and ununiformly grained matters. The ununiformly grained microstructure forms either at a certain way of formation of the matter or at faults of preparing of the matter. It ordinarily manifests at casting and drawing of matters containing anisomertic Al2O3 particles, further at the mixing of the matter, at the admixtures coarse-granularity and possibly due to the formation of oriented aggregates of Al2O3 at the matter drainage. In matters containing anisometric, e.g. lamellar, material particles, the most distinct crystal orientation appears at the casting. The direct (parallel) arrangement very well comes through in CPL in the combination with the l - plate. In this case, prismatic crystals in a suitable combination with an optical compensor come through in a single shade, e.g. in the blue one and perpendicular to it in the yellow one. It is possible to assume that the formation long prismatic corundum crystals is caused by intergrow in oriented formations of the individual particles. At the drawing of the plastic body the streaming of the particles and mobility is considerably more difficult than in a casting mould, so the orientation of the Al2O3 plates according to basal faces does not manifest so distinctly. The plates tend to take up a position of as little resistance as possble, considering the extrusion aperture and in a surface layer they orientate parallel to the axis of the driven product. The corundum crystal growth proceeds particularly on the c-axis-direction. The growth in this direction is facilitated by sliding on basal faces.  This corresponds to the tangential arrangement of prismatic crystals in the driven pipe of the oxide matter. Considering the fact that these are light walled products, the orientation is traceable in the entire length of the body. At the semidry pressing, the conditions for the orientation of the material plates are not so suitable as at the casting and drawing. Therefore the pressed matters ordinarily do not display orientation in the entire section of the body. The shape orientation is traceable particulary in the growth of crystals. In the cast matter, where the orientation of original Al2O3 plates is the most distinct, the largest prismatic crystals gradually form. On the contrary, in a not oriented cast matter, the cystals are significantly smaller of sizes reaching approx. 1/10-1/20 of the large crystals of the cast matter. Only less ferquently, in the pressed matters, smaller zones composed of directly oriented crystals, which form so called mosaic-like microstructure may appear. These nonuniformities are the best identifiable using the gypsum plate in CPL as well. An explanation of the orientation in the pressed matters is not completely distinct. Most likely, the orientation of particles formed in prepared matter in the wet environment plays a role here (this sort of orientation is known e.g. from the kaoline drainage). A sliding on anisometric particles, possibly certain „streaming of matter“ at pressing may also be considered. However, the ununiform, particulary directly oriented microstructure always worsens the required properties.


17 Summary

Technolithology is inter-disciplinary branch standing between material engineering and geological disciplines. Geological disciplines (mineralogy, petrology, and geochemistry) stand ahead in technolithe research. It has become clear that with technolithes, a new research scope for technical disciplines (particularly for material engineering section) has emerged. Recently, basic research perspectives have been determined for geological science disciplines with some minor successful issues (Rovnaníková  et al  1999, Gregerová 2000, 2000b, 2000a, 2000c, Gregerová  et al 2000, Gregerová, Pospíšil 2000). The issues include research of rule-based relationships between ettringite- thaumasite formation and alkali silica gels in concrete construction exposed to specific environmental conditions. Using microscopy methods, RTG, DTA, electron microscopy, and microanalysis, certain rule-based relationships among stable, metastable, and secondary minerals in building construction cements have been revealed (Gregerová 2000a, 2000c, Gregerová, Pospíšil 2000a, Gregerová, Rovnaníková 2002 Witzany et al. 2002).

Technolithe minerals require specific approach of mineralogists. The crystals form solid solutions of several isomorphic miscible components and feature multiple modifications with unknown relationships.

Special alloys, superconductors, and monocrystalline layers represent an interesting group of technolithes.  Microscopic research may include for example inclusions in alloys, monocrystals, glass, and ceramic products. Mineral fibre study information (for example, whiskers, endless fibres) may be significant too.

Technolithology methods can be also used to inspect historic materials in petroarchaeological study (Gregerová, Kristová 1995, Gregerová Hložek 2000, 2002). Prospective research types include study of inorganic phosphate fertilizers.

It is obvious that the technolithology may affect many disciplines in several ways (see Figure 10).

Figure 10 Position of technolithology and relationships with industry branches.


It is worth stating that technolithology is not the only perspective option for mineralogists, petrologists, and geochemists. The other trends include biomaterial/biomineralization study.

The results have been obtained on a long-term basis in GACR projects No. 103/00/0607 and 103/02/0990.


18 Bibliography

1.      Bartuška, M. (1964)“ Mikroskopie minerálů.- Skripta ČVUT. Praha.

2.      Bartuška M. (1987): Technická mineralogie.- Skripta VŠCHT. Praha

3.      Berry, L. G., Mason, B. (1959): Mineralogy, concepts, descriptions, determinations.- W.H. Freeman and Co, San Francisco.

4.      Bielankin, D.S., Ivanov, B.V., Lapin, V. V. (1957): Petrografia kamieni sztucznych.-Wyd. Geol., Warszawa.

5.      Cohen, M. (ed.) (1980): Materials Science and Dist Engineering – Its Evaluation, Practice and Prospects.- Elsevier, Amsterdam.

6.      Deer, W. A., Howie, R. A. and Zussman, J. (1992): Rock-forming Minerals - An introduction to the rock-forming minerals 2nd. – ed. Longman, 687 str. Essex.

7.      Deer, W. A, Howie, R. A. and Zussman, J. (1997- 2001): Rock-forming Minerals Series:

8.      Volume 1A: Orthosilicates (second edition)

9.      Volume 1B: Disilicates & ring silicates (second edition)

10.  Volume 2A: Single-chain silicates (second edition)

11.  Volume 2B: Double-chain silicates (second edition)

12.  Volume 4A: Framework silicates: Feldspars (second edition).

13.  The Geological Society. London.

14.  Ginsberg A.S. (1951): Eksperymentalnaja petrografija.- Nauka. Leningrad.

15.  Gregerová, M. (1996): Petrografie technických hmot.- Učební texty, PřF MU v Brně.

16.  Gregerová, M. (2000): Petrografie technických hmot.- Habilitační práce, nepublikováno, VUT Brno.

17.  Gregerová, M. (2000a): Petrografické rozbory malt, jejich relativní datování podle stupně degradace pojiva.- Geologické výzkumy na Moravě a ve Slezsku v roce 1999. ISSN 1212-6209, 2000, vol. VII, no. 1, s. 149-151.

18.  Gregerová, M.(2000b): Uplatnění digitální fotografie v aplikované mineralogii a petrografii.- In Mikroskopie 2000. Vyd. 1. Praha : ČVUT Praha, 2000. Mikroskopie 2000, ISBN 80-01-02244-7, s. 13-15.

19.  Gregerová, M. (2000c): Vzdušné malty historických staveb, jejich identifikace, příčiny degradace a návrh sanace.- Geologické výzkumy na Moravě a ve Slezsku v roce 1999. ISSN 1212-6209, 2000, vol. VII, no. 1, s. 152-154.

20.  Gregerová,  M., Fojt, B., Vávra V. (2002): Mikroskopie horninotvorných a technických minerálů.- Moravské zemské muzeum – Přírodovědecká fakulta MU v Brně. 325 str. ISBN 80-7028-195-2.

21.  Gregerová, M., Hložek, M. (2000): Technologické posouzení keramické produkce na sídlišti s vypíchanou keramikou v Pavlově.- In Sborník Miroslavu Buchvaldkovi. Most: Ústav archeologické památkové péče severozápadních Čech v Mostě, 2000. ISBN 80-901828-6-8, s. 81-86.

22.  Gregerová, M., Hložek, M. (2002): Stanovení společných technologických znaků dvou částí antropomorfní nádoby lengyelské kultury z Těšetic-Kyjovic. - Nitra : Archeologický ústav SAV, 2002. 430 s. Archeaologica Slovaca Monographiae, Tomus.IV. ISBN 80-88709-57-1.

23.  Gregerová, M., Kristová, L. (1995): Probleme der Interpretation der differenz-thermischen Analysen der Graphittonkeramik.-  II Internationale tagungen  in Mikulčice: Slawische Keramik in Mitteleuropa von 8. bis zum 11. Jahrhundert - Terminologie und Beschreibung, Spisy Archeologického ústavu AV ČR Brno, 4, 213-223.  Brno.

24.  Gregerová, M., Pospíšil, P. (2000): Microscopic methods applied to study of historical mortars and plasters.- In Proceedings of the 8th Euroseminar on Microscopy Applied to Buildings Materials. Vyd. 1. Atheny : Atheny, 2001. s. 353-358.

25.  Gregerová, M., Pospíšil, P. (2000a): Možnosti digitální rastrové technologie pro studium degradačních procesů. In Mikroskopie 2000. Vyd. 1. Praha : ČVUT Praha, 2000. Mikroskopie, ISBN 80-01-02244-7, s. 25-30.

26.  Gregerová, M., Pospíšil, P., Rovnaníková, P. (2000): Identification of the Roman cements in Bohemian and Moravian Historical Buildings. In 5th International congress on restoration of architectural heritage Firenze 2000. Vyd. 1. Florencie, Itálie : University of Florence, 2000. s. 1527-1536.

27.  Gregerová, M., Rovnaníková, P. (2002): Tvorba výkvětů na povrchu betonu. In VII. Vedecká konferencia s medzinárodnou účasťou. Zborník prednášok 10. sekcia: Stavebné materiály. Košice : Technická univerzita v Košiciach. Stavebná fakulta, 2002. Zborník príspevkou zo VII. Vedeckej konferencie, ISBN 80-70-99-816-4, s. 185-189.

28.  Gregerová, M., Sulovský, P. (1994): Možnosti využití elektronové mikroskopie a mikroanalýzy.- Geol. výzk. Mor. Slez. v r. 1993, 103-104. Brno.

29.  Hejtman, B.(1956):Všeobecná petrografie vyvřelých hornin. 376 s. Nakladatelství CSAV, Praha. 

30.  Hornbogen, E. (1983): Metall, 37(5), 472 – 475.

31.  Huang, W.T. (1962): Petrology.-  McGrow-Hill Book. New York.

32.  Kohl, W., H. (1977): Mat. Electron., 3 (19). 45-46.

33.  Konta J. (1982): Keramické a sklářské suroviny.- UK  Praha.

34.  Matoušek, M., Drochytka, R. (1998): Atmosférická koroze betonů.- IKAS. 161 str. Praha.

35.  Rovnaníková, P., Gregerová, M., Krmíčková, N. (1999): Složení a náhrada omítek historických staveb.- In XI. Mezinárodní końference, 18.-20. října 1999 - Vysoké učení technické, Fakulta stavební, Sborník příspěvků. Vyd. 1. Brno : VUT Brno, 1999. s. 157-160-4.

36.  Ryshkevitch,  E. (1960): Oxide Ceramics.- Akad. Press. New York.

37.  Vražičová, J.,(2002): Reakční mechanismus tvorby pojiva na bázi fosforečnanu hořečnato-amonného, Sborník příspěvků ze IV. semináře Pokroky v anorganické chemii, MU v Brně, 2002, s. 68-69, ISBN 80-210-2951-X.

38.  Szymański, A.(1997): Mineralogia techniczna.- Wydawnictvo Naukowe PWN. 459 str. Warszawa. ISBN 83-01-12208-0.

39.  Witzany, J., Gregerová, M., Pospíšil, P., Wasserbauer, R., Hruška, A., Cikrle, P. (2002):
Projekt Karlova mostu. I. etapa - diskuze.- Projekt měsíčník pro stavebnictví a interiéry. ISSN 1211-9490, 2002, vol. VI, s. 27-35.