Effect of water-to-binder ratio on the hydration kinetics of composite binder containing slag or fly ash

The calorimetric data of composite binder containing slag or fly ash at different water-to-binder ratios were measured at 298 and 333 K by an isothermal calorimeter. The influences of water-to-binder ratio on the hydration heat characteristics of composite binder were analyzed. Based on the hydration kinetics model, the controlling processes during hydration, nucleation and crystal growth (NG), interactions at phase boundaries (I) and diffusion (D), were determined, and the kinetics parameters were calculated and discussed. The results show that water-to-binder ratio slightly affects the early-stage hydration heat evolution rate and cumulative hydration heat, but a lower water-to-binder ratio results in a lower hydration heat evolution rate and cumulative hydration heat at later stage. The water-to-binder ratio has a great effect on the hydration of composite binder at elevated temperature. Decreasing water-to-binder ratio does not change the hydration mechanism of composite binder, which is NG -> I -> D at 298 K and becomes NG -> D at 333 K. The decrease in water-to-binder ratio increases the apparent rate constant of composite binder containing slag, but the inverse trend is obtained for composite binder containing fly ash during NG process at 298 K. The lower water-to-binder ratio leads to the lower apparent rate constant for composite binder containing large amount of slag or fly ash during NG process at 333 K. Decreasing water-to-binder ratio decreases the apparent rate constant during D process for all samples. The apparent activation energy of composite binder decreases with decreasing water-to-binder ratio.