Influence of pozzolana on sulfate attack of cement stone affected by chloride ions

This study investigated the influence of natural pozzolana (opoka) additive on the hydration of Portland cement and the effects of pozzolana on sulfate attack of cement stone affected by chloride ions. In the samples, 25 % (by weight) of the Portland cement was replaced with pozzolana. The specimens were hardened for 28 days in water, and then one batch was soaked in a saturated NaCl solution and another in a 5 % Na2SO4 solution for 3 months at 20 A degrees C. After being kept for 3 months in a saturated NaCl solution, samples were transferred to a 5 % Na2SO4 solution and kept under these conditions for 3 months. It was estimated that under normal conditions, pozzolana additive accelerated the hydration of calcium silicates and initiated the formation of CO3 (2-)-AFm; opoka also decreased the threshold pore diameter of hardened Portland cement paste. It was found that Cl- ions penetrate to monosulfoaluminate, form Friedel's salt, and release SO4 (2-) ions, which react with unaffected monosulfoaluminate and form extra ettringite; when samples were transferred to the 5 % Na2SO4 solution, a greater quantity of new ettringite was formed. Meanwhile, pozzolana additive reduced the penetration of chloride and sulfate ions into the structure of Portland cement hydrates and inhibited sulfate attack of cement stone treated in a saturated NaCl solution.