Influence of carbonated additives on Portland cement hydration in chloride environment

This study investigated the influence of natural pozzolana (opoka) and limestone additives on the hydration of Portland cement and the effects of chloride ions on the formed hydrates. In the samples, 25% (by weight) of the Portland cement was replaced with additive. The specimens were then hardened for 28 days in water and then soaked in saturated sodium chloride (NaCl) solution for 3 months at 20°C. It is estimated that, under normal conditions, pozzolana and limestone additives accelerate the hydration of calcium silicates and initiate the formation of monocarboaluminate (CO₃^2––AFm). Monocarboaluminate affected by NaCl becomes unstable and takes part in reactions producing Friedel's salt. More Cl⁻-containing compounds were formed in the samples with limestone than in the Portland cement samples after treatment in saturated NaCl solution. Meanwhile, pozzolana additive reduced the penetration of Cl^– ions into the structure of Portland cement hydrates and impeded the formation of aluminium chloride hydrates.