Chemical effect of nano-alumina on early-age hydration of Portland cement

For a full understanding of effects of nano-alumina on early-age hydration, hydration of a Portland cement blended with 5% of nano-alumina (gamma-phase) was studied. Except for the well-known filler effect, a chemical effect of nano-alumina was identified in the study, which is related to its dissolving behavior in pore solution. The dissolution of nano-alumina leads to a high initial ettringite content even in the first 10 min, thus a higher initial heat release. The continuous dissolution of nano-alumina contributes to formation of additional ettringite, which consumes aqueous S species, promotes the depletion of gypsum, thus accelerating reaction of aluminate (C(3)A). The dissolution of nano-alumina may also be influenced by S species in pore solution during the induction period. Afterwards, the slow dissolution of nano-alumina only results in a slightly increased aqueous Al concentration for the further increase would be limited by Al-bearing hydrates such as ettringitc and AFm phases.