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 Ca3H2 [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.
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