Multicompound Model for the Hydration of Portland Cement–Fly Ash Binders

In this study, traditional reaction equations (based on the theory of Powers) are applied to the hydration reactions of portland cement and the pozzolanic reaction of fly ash separately. Moreover, portland clinker is considered as a mixture of four minerals, each with its own sensitivity to the presence of fly ash. The kinetics of the reactions of each clinker mineral have been analyzed by fitting generally known models such as the Avrami and Jander equations to isothermal heat measurements on pastes of cement, fly ash, and water. The proposed model therefore consists of different stages, in which nucleation, phase-boundary, and diffusion reactions become rate controlling. The kinetics of the pozzolanic reactions have been described with similar equations, implementing parameter values based on measured selective dissolution data. Fly ash may accelerate the reaction of a clinker mineral, while at the same time it can decelerate another mineral. Depending on the relative proportions of the clinker minerals, the method can explain the apparent contradiction found in literature related to the acceleration or deceleration effect of fly ash on the cement hydration.