A hybrid kinetic model for the heat release of Portland cement: fly ash binders

Based on the independent hydration concept, the reaction rates of the four important cement clinker minerals and fly ash are modelled. The kinetics of the reactions of each clinker mineral have been analysed by fitting Avrami and Jander equations to isothermal heat measurements on fly ash - cement pastes. Control processes describing nucleation, phase-boundary and diffusion reactions, subsequently become rate-controlling. The kinetics of the pozzolanic reactions have been described with similar equations, implementing parameter values based on measured selective dissolution data. This method allows fly ash to accelerate the reaction of a clinker mineral, while decelerating another. The influence of water availability and temperature are explicitly taken into account. For some fly ash - cement combinations, depending on the amount of sulphate ions, a third hydration peak appears, relating to the conversion of an AFt phase into an AFm phase. For Portland cement with a low amount of calcium aluminate, this peak cannot be distinguished since no significant conversion from AFt to AFm takes place.