Early hydration of SCM-blended Portland cements: A pore solution and isothermal calorimetry study

In this study the hydration kinetics and the development of concentrations in the pore solution of cement pastes containing different supplementary cementitious materials (blast-furnace slag, Si-rich fly ash, limestone, quartz) at a cement replacement of 50 wt.% were investigated during the first 6 h of hydration. The results indicate that the degree of undersaturation with respect to alite is the primary factor driving the early hydration kinetics. The accelerating effect of the limestone is related to a higher undersaturation. The data reveal also that high aluminium and sulfate concentrations, which depend on the type and chemical composition of the used mineral addition, retard the reaction. High calcium concentrations had no adverse effect on hydration kinetics. The investigations underpin that the mechanisms controlling the early hydration are a complex combination of the filler effect, the pore solution chemistry as well as intrinsic reactivity of the mineral additions and their surface characteristics.