Hydration kinetics of composite binder containing fly ash at different temperatures

Fly ash has been widely used as supplementary cementitious material in concrete industry. Hydration mechanism of composite binder containing fly ash is much more complicated due to the mutual effect of the hydration of cement and the pozzolanic reaction of fly ash. This paper involves the hydration kinetics of composite binder containing up to 65 % of fly ash and comparison of the results with data on composite binder containing slag that are previously published. The hydration heat evolution rate and cumulative hydration heat of composite binder containing fly ash were measured at 298, 318 and 333 K with an isothermal calorimeter. Based on the hydration kinetics model, three hydration processes, namely nucleation and crystal growth (NG), interactions at phase boundaries (I) and diffusion (D) were characterized, the relationship between the hydration rate and hydration degree was discussed at different stages, and kinetics parameters, n, K and E-a, were calculated and analyzed. Results show that the hydration heat evolution rate and cumulative hydration heat of composite binder obviously decrease with increasing the replacement ratio of fly ash. Elevated temperatures promote the hydration process, especially for composite binder containing high amount of fly ash. The kinetics model could simulate the hydration process of composite binder containing no more than 65 % of fly ash, whose hydration process sequence is NG -> I -> D at 298 and 318 K, but it becomes NG -> D at 333 K. Fly ash has relatively smaller effect on the overall reaction of composite binder than slag. The reaction rates of composite binder containing fly ash at different stages are higher than those of composite binder containing slag at the same replacement ratio. The value of E-a for the overall reaction of composite binder decreases first and then increases with increasing the content of fly ash, and it is lower than that of composite binder containing slag at the same replacement ratio.