Effect of seawater as mixing water on the hydration behaviour of tricalcium aluminate

This study explored the mechanism of the effect of seawater on tricalcium aluminate (C₃A) hydration. The results showed that seawater retarded C₃A hydration and reduced the reaction degree of C₃A. The co-existence or ion pairing of Ca²⁺ and SO₄²⁻ onto the surface of C₃A poisoning the reactive sites is the main reason for this retardation. Besides, the precipitation of Mg(OH)₂ on the surface of C₃A would prolong the induction period for another 30 min and then consequently decrease the dissolution rate of C₃A hydration. Trace amounts of MgSO₄⁰ and Mg²⁺ present in the alkaline solution had little retardation effect. It was also found that Cl⁻ would preferentially react with C₃A to form Friedel's salt, rather than forming hydroxy-AFm as that in the C₃A-deionized water paste. The direct formation of Friedel's salt resulted in an accumulation of Al³⁺ in solution, which would also hinder the subsequent dissolution of C₃A.