Effect of elevated Al concentration on early-age hydration of Portland cement

To understand the effect of elevated Al concentration on early hydration in Portland cement, hydration heat, the evolution of solid and aqueous composition in paste blended with reactive alumina were investigated in this study. The increase of Al concentration depletes gypsum rapidly with massive precipitation of ettringite during the pre-induction period, resulting in decreased Ca and S concentration in pore solution. The elevated Al concentration passivated the active sites on C3S surface, and the decreased Ca concentration inhibited the nucleation of C–S–H and precipitation of portlandite, hence prolonging the induction period. Meanwhile, the reaction of C3A has not been accelerated even after the complete depletion of gypsum, which is associated with the lowered undersaturation degree caused by the increased Al concentration. Additionally, the precipitation of large amounts of ettringite and foil-shaped C–S–H reduced available nucleation sites, thus hindering the reaction of both C3S and C3A during the acceleration period.