Abstract
A previous study indicated that the early hydration and strength development of ordinary portland cement (OPC) delayed by the presence of high volumes of supplementary cementitious materials were compensated for by the accelerating effect of nano-CaCO3. The mechanism responsible for the accelerating effect on the early hydration and strength development was, however, not fully understood. A study aimed at understanding the accelerating mechanism of the addition of nano-CaCO3 on the hydration of tricalcium silicate (C3S) is presented in this paper. A comparison with the addition of micro-CaCO3 was made. The hydration mechanism of C3S with the addition of micro- or nano-CaCO3 was studied by conduction calorimetry, thermogravimetric analysis, and scanning electron microscopy. The conduction calorimetry results indicated that the addition of nano-CaCO3 had an accelerating effect on the hydration of C3S as well as on the hydration of OPC. Furthermore, the induction period of C3S hydration was significantly shortened by the addition of nano-CaCO3. The results of the thermogravimetric analysis indicated that the amount of nano-CaCO3 decreased as the hydration of C3S took place; the decrease was greater with the hydration of OPC. The scanning electron microscopy revealed that the accelerating mechanism in the presence of micro-CaCO3 was considerably different from that of nano-CaCO3. Calcium silicate hydrate growth was observed around the nano-CaCO3 particles. The observation suggested that the seeding effect due to the addition of nano-CaCO3 was responsible for the accelerating effect on the hydration of C3S.