Ceramics-Silikáty 58, (2) 138 - 144 (2014)

This study investigated the influence of natural pozzolana (opoka) additive on the hydration of Portland cement and the effects of chloride and sulfate ions on the formed hydrates under 4^oC temperature. In the samples 5, 15 and 25 % (by weight) of the Portland cement was replaced with pozzolana additive - opoka. The specimens were cured in water at 20^oC for 28 days. Then all samples were soaked in saturated NaCl solution at 4^oC for 3 months. Later they were immersed into 5 % Na₂SO₄ solution and held there for 3 month at 4^oC. All specimens were examined by thermal, X-ray diffraction and FT-IR analysis after every immersion in different solutions. It was estimated that under normal conditions opoka promotes the formation of calcium silicate hydrates and initiate the formation of monocarboaluminate. Monocarboaluminate affected by a saturated NaCl solution, becomes unstable and takes part in reactions producing Friedel salt at 4^oC temperature. It was obtained, that opoka takes place in the pozzolanic reaction even at law temperature in a saturated NaCl solution. However, ettringite remained stable under the examined conditions and Thaumasite is mostly depends on decomposition of ettringite in 5 % Na₂SO₄ at 4^oC temperature.