Exploring the effect mechanism of Bayer red mud on cement hydration and mechanical properties

Bayer red mud (BRM) was used as the supplementary cementitious materials (SCMs) of Portland cement in this study. The compressive strength, heat release, reaction products, microstructure, and gel phase assemblage of the blended cement were investigated. The results show that the presence of high concentration of CO32-/HCO3- (carbonate ions) and Al(OH)(4)(-) in BRM significantly influences the cement hydration and mechanical properties. The carbonate ions in BRM can accelerate the hydration of calcium silicate, and promote the formation of hemicarbonate together with Al(OH)(4)(-). Carbonate ions also lead to decalcification of calcium silicate hydrate (C-S-H) into a compact alkali silica gel (N(C)-(A)-S-H). BRM can enhance the early-age compressive strength of cement through its effect on hydration, with the strength activity index of mortar incorporating 50% BRM (P50B) reaching 84.1% at 1 day. However, this enhancement may also hinder the growth of C-S-H and the continuous hydration of clinker, with a mere increase of only 1.18% in the bound water content of P50B from 1 day to 7 days. This study contributes to a better understanding of the synergistic use of BRM and cement.