Ceramics-Silikáty 54, (1) 47 - 52 (2010)

Influence of natural pozzolana additive to the hydration of tricalcium aluminate and the effects of chloride ions to formed hydrates was investigated. In the samples 25 % C₃A mass weight was replaced by pozzolana and they were hardened for 28 days at normal conditions, soaked in saturated NaCl solution, and stored there for three months at 20 °C. It is estimated that at normal conditions the additive of pozzolana has no substantial effect on the course of C₃A hydration, but a part of amorphous SiO₂ penetrates into the structure of aluminium hydrates. Regular C₃A hydrates and CO₃²⁻ - AFm affected by NaCl becomes unstable and takes part in reactions producing Friedel’s salt and a complex compound - hydrocalumite - Ca₈A₁₄(OH)₂₄(CO₃)Cl₂(H₂O)_1,6·(H₂O)₈. Pozzolana additive reduces the penetration of Cl⁻ ions to the structure of C₃A hydrates and impedes the formation of aluminium chloride hydrates. In samples with pozzolana additive a part of cubic aluminium hydrates, CO₃²⁻ - AFm and C₃A hydrate substituted with Si⁴⁺ affected by chlorides remained stable.