INFLUENCE OF POZZOLANA ON C4AF HYDRATION AND THE EFFECTS OF CHLORIDE AND SULPHATE IONS ON THE HYDRATES FORMED

The influence of natural pozzolana additive on the hydration of C4AF (aluminoferrite) and the effects of chloride and sulphate ions on the hydrates formed were studied. In the samples, 25 wt% 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 CO3(2-); AFm (monocarboaluminate) and gibbsite, however, impede the formation of cubic aluminium hydrates. Also, part of the amorphous SiO2 penetrates into the structure of the hydrates of C4AF, initiating the formation of hydrated alumino-silicate (gismondine). Monocarboaluminate affected by NaCl becomes unstable and participates 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 sulphate ions, and retarded the corrosion reaction of C4AF hydrates.