Ceramics-Silikáty 57, (1) 45 - 52 (2013)

This study investigated the influence of natural pozzolana - opoka additive on the hydration of C₄AF rich cement and the effects of chloride ions on the hydrates formed. In the samples, 25 % (by weight) of the sintered C₄AF rich cement and OPC was replaced with pozzolana. The mixtures were hardened for 28 days in water, soaked in a saturated NaCl solution for 3 months at 20^oC. It was estimated that under normal conditions, pozzolana additive accelerates the hydration of calcium silicates and initiates the formation of CO₃^2-–AFm in the Brownmillerite rich cement. However, the hydration of Brownmillerite cement with opoka additive is still slower to compare with hydration of Portland cement. Also, opoka decreases total porosity and threshold pore diameter of Brownmillerite cement paste after two days of hydration. After 28 days of hydration threshold pore diameter became smaller even to compare with threshold pore diameter of Portland cement. Opoka additive promotes the formation of Friedel’s salt in Brownmillerite samples treated in saturated NaCl solution, because CO₃^2-–AFm affected by saturated NaCl solution become unstable and takes part in reactions producing Friedel’s salt.