Ceramics-Silikáty 55, (3) 261 - 267 (2011)

This study investigated the influence of natural pozzolana additive on the hydration of C₄AF (aluminoferrite) and the effects of chloride and sulfate ions on the hydrates formed. In the samples, 25% (by weight) of the C₄AF was replaced with pozzolana. The mixture was then hardened for 28 days in water, soaked in a saturated NaCl solution for 3 months, and then soaked in a 5% Na₂SO₄ solution for 3 months at 20°C. It is estimated that under normal conditions, pozzolana additive accelerates the formation of CO₃²⁻–AFm (monocarboaluminate) and gibbsite, however, impede the formation of cubic aluminum hydrates. Also, part of the amorphous SiO₂ penetrates into the structure of hydrates of C₄AF and initiates the formation of hydrated alumino-silicate (gismondine). Monocarboaluminate affected by NaCl becomes unstable and takes part in reactions producing Ca₂Al(OH)₆Cl·2H₂O (hydrocalumite-M). After samples were transferred from a saturated NaCl solution to a 5% Na₂SO₄ solution, hydrocalumite-M was the source of aluminates for the formation of ettringite. In samples with pozzolana additive, the hydrated alumino-silicate and gibbsite compounds that were formed remained stable in an environment containing chloride and sulfate ions and retarded the corrosion reaction of C₄AF hydrates.