Thaumasite formation affected by aggregate composition
in concrete in the
Miroslava Gregerová*)
Pavel Pospíšil**)
*) Masaryk University in Brno, Faculty
of Science, Institute of Geological Sciences, Kotlarska 2, 611 37 Brno, Czech
Republic, mirka@sci.muni.cz
**) Brno University of Technology,
Faculty of Civil Engineering, Dept. of Geotechnics, Veveri 95, 662 37 Brno,
Czech Republic, pospisil.p@fce.vutbr.cz
Abstract
Significance of optical study of
cementitious materials (matrix and filling material) becomes very interesting
and important during several last years. Study is focused not only on mineral
composition of matrix in concrete but especially on petrographic characteristic
of aggregates. Collection of 14 concrete samples was optically studied and
petrographically characterized. Aggregates of these samples can be
macroscopically divided in two groups, which mean that aggregates were formed
in different geological environments. Because phthanites (Paleozoic silica rock)
and porcelanites were observed in aggregates it had to be verified formation of
ASR reaction products.
Keywords
Thaumasite, concrete, degradation,
petrographic characteristic, aggregate
Introduction
Concrete degradation usually results
of parallel action of physical, chemical and biological processes, which can
closely involved with improper combination of aggregates and matrix, water and
surrounding conditions. Results of micropetrographic analyses of aggregates,
EDX analyses carried out on raster electron microscope CamScan supplemented
with chemical analyses of separated parts of highway concretes in various
stages of degradation are presented in this paper.
Petrographic characteristic
Micropetrographic aggregate analyses
of studied highway concretes proved differences in their rock composition as
shown in Fig. 1.
Basic differences were in amount of
black clay shales, granitoids and silicites in aggregate composition. It was
unambiguously proved that the lowest degraded concretes were with the highest
amount of granitoids. On the contrary the highest degraded were concretes with
prevailing portion of black clay shales and silicites in aggregate composition.
Percentage composition of aggregate in the highest degraded part of drill core
is shown in Fig. 2.
Concurrently with micropetrographic
analyses of highway concrete aggregates we focused on assessment of two
different aggregate deposits. The first of one is opened in old submarine
volcanic rocks. The second one is in granitoid rocks.
In comparison between drill cores 1,
3 and 7 are samples 1 and 7 very similar in aggregate composition. Variability
in aggregate composition for concrete production probably reflects original
rock heterogeneity in the rock massif in the quarry.
The drill core No.: 2 is in good
quality, without significant cracks, without alteration in material composition
and without formation of secondary minerals. Granitoid rocks predominate over
metabasalts, shales and cemented detrital sediments. Studied rock composition
leads to assumption about ASR, which was not anticipated in concretes in the
Aggregate alkali reaction
Presence of “active SiO2”
included in aggregate is considered as a basic condition for reactivity of
aggregate with alkali (Arya, Buenfeld, Newman [1]). They are mostly opal,
chalcedony, trydimite or crystobalite. These minerals occur usually in rocks
such as tuff, tuffite, volcanic glass, chert etc. These rock types occur in
concrete aggregate (not only of highway). Sometimes we can find porcelain
jasper (porcelanite), which is contact metamorphosed rock (burned clays and
marls) and was formed probably on the contact with basalt. It is inhomogeneous
in color, maculose with conchoidal fracture and usually contains crystobalite,
spurrite, larnite and other high-temperature minerals.
Besides the above mentioned rocks we
simultaneously verified occurrence of black clay shale or tuff and tuffite with
pyrite.
If we assume average chemical
composition of Portland clinker 21% SiO2, 5% Al2O3,
3% Fe2O3, 64% CaO, 3% MgO, 2,5% SO3 and 0,4%
of alkali oxides then will be formed approximately 0,8% CaO (free calcium
dioxide), 55,5% 3CaO.SiO2 (tricalciumsilicate), 17,8% 2CaO.SiO2
(dicalciumsilicate), 8,3% 3CaO.Al2O3 (tricalciumaluminate)
and 8,3% 4CaO.Al2O3.Fe2O3
(tetracalciumalumoferrite).
The high content of
tetracalciumalumoferrite is in all concrete samples. Separate portlandite
tables and accumulations occur in micritic matrix. Voids of the highly degraded
concrete are along edges or whole volume filled by needle-like crystals of
ettringite (Ca6Al2O6(SO4)3.32H2O
or thaumasite Ca3 H2 [CO3/SO4/ SiO4]
.13 H2O and gypsum Ca2(SO4)2. 2 H2O.
Occurrence of these minerals as verified by X-ray and EDX analyses.
The gel coatings, which were ripped
by impact of electron beams occur along the edge of some fissures – gradual
water release. As we have confirmed by element distribution maps amorphous gels
have variable composition. Gradually developed individual mineral phases formed
of these gels were verified by many images.
Conclusions
Study of concrete samples verified
following degradation factors:
Presence of
improper aggregate – silicites, black clay shales (±tuffs, ±tuffites) with
pyrite and clay minerals (Hawkins, Pinches [4]) and Ca(OH)2;
Formation of
secondary sulphates ettringite Ca4Al2[ (OH)12/
SO4]. 6 H2O, thaumasite Ca3 H2 [CO3/SO4/
SiO4] .13 H2O and gypsum Ca2(SO4)2.
2 H2O (macroscopically visible due to white margins formatted round
the black aggregate particles). Their formation closely relates to pyrite
weathering processes, which can be found especially in black clay shales.
Degradation of concrete drill cores relates also to growth pressure of
rhombohedral calcite crystals in concrete matrix (Hartshorn, Sharp, Swamy [3]).
Thaumasite
formation in experimental conditions studied (Crammond, Halliwell [2]). They
verified thaumasite formation of neutral sulphates ions added to concrete or by
sulphur acid action on concrete Oberholster, van Aardt, Brandt [7]).
It can not be
excluded also affects of surrounding environment and the bedrock. The rocks
containing sulphides and organic matters are altered by acid solutions by the
formation of more stabile mineral forms. The typical example is pyrite decaying
with the formation of limonite and sulphur acid. It can be demonstrated (Hobbs,
Taylor 2000): 2FeS2+6H2O+7O2 = 2Fe(OH)2
+ 4H2SO4. 4Fe S2 + 15O2 + 8H2O
= 2Fe2O3 + 8H2SO4 By the reaction of sulphur acid with surrounding
CSH or CH (or with gels formatted by ASR) growth pressure of neogenic minerals
breaks concrete. Limonitization of pyrite was verified in all samples.
ASR reaction
(in a limited degree – it was verified in 3 samples)
During the study of concrete samples
was found by optical microscopy that even highly degraded concrete contents
anhydrated clinker minerals. C3S and C2S are well
distinguishable in some cases. Tetracalciumalumoferrite is the most marked in
case of
It was verified that in studied
samples thaumasite often with ettringite were formed gradually of ASR gels.
Their occurrence was confirmed by microanalyses. It can be formed hypothesis
that important factor of rupture deformation of concrete matrix with formation
of fine fissures are not only ASR gels but also following reactions induced by
sulphur and hydrocarbon acids, hydroxides of Al and Ca together with ASR gels
with the formation of especially thaumasite and in case of over-abundance of Ca2+
+ Al3+ also ettringite. Gypsum needle-like crystals are formed in
case of Ca ions presence (after formation of ettringite) together with low
concentrated sulphur acid. Calcite is formed as a latest mineral in case of
presence of remains of Ca hydroxide (Photo 1-10).
It can be understood that ettringite
is usual product of hydration and occurs both in fresh and degraded concretes.
Ettringite causes concrete decaying only in case of its excessive formation,
which increases with increasing age of concrete. Ettringite can not crystallize
in free voids of concrete microstructure. Ettringite occurrence, lower than
critical can only signalize alteration of microstructure (together with
alteration of concrete properties) but need not lead to concrete decay.
Acknowledgements
The research was supported by
References
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Photo 1. Formation of radial-like
forms of portlandite of gel. SEM - |
Photo 2. Detail view – one of
formatting forms. SEM - |
|
Photo 3. Formation of first thaumasite
and ettringite crystals. SEM - |
Photo 4. Advanced state of
thaumasite formation. SEM - |
|
Photo 5. Needle-like forms of
thaumasite beside decaying gel. SEM - |
Photo 6. Ettringite, thaumasite,
gel, portlandite. SEM - |
|
Photo 7. Clumps of ettringite
needles. SEM - |
Photo 8. Columnar shape forms of
thaumasite. SEM - |
|
Photo 9. Rhombohedral calcite
crystals on thaumasite in highway concrete. SEM - |
Photo 10. Rhombohedral calcite
crystals beside thaumasite on the surface of interlocking concrete pavement.
SEM - |
