Isothermal calorimetric study on the heat evolution and the apparent activation energy of alkali-activated slag/fly ash pastes

Alkali-activated slag/fly ash (AASF) as an environmental-friendly binder system for construction materials has recently attracted great attention from both academic and industrial communities. Towards its wider engineering application, it is crucial to have a better understanding of the temperature induced effects by different curing regimes and the temperature sensitivity on the thermal properties of this system, for instance the apparent activation energy (Ea). However, the available information on Ea of AASF system is still quite limited.
The present study is aimed at investigating the role of alkaline activator chemistry on the reaction kinetics of AASF at early age. The binder is made of 50 wt.% blast furnace slag and 50 wt.% fly ash. Four alkaline activator silicate moduli (SiO2/Na2O ratio = 0.8, 1.0, 1.2 and 1.5) were used for the mixture preparation. The effect of activator modulus on the heat evolution was studied by performing isothermal calorimetry test up to 160 h at both 20°C and 40 °C. The cumulative heat release and ultimate total heat were studied through curve fitting using exponential model. Furthermore, the Ea of AASF pastes was determined using incremental methods and its variation over wide range of early age reaction was studied. It was found that the activator modulus evidently influences the heat evolution of AASF. The cumulative heat release reached the maximum value at activator modulus of 1.0, followed by at 0.8, 1.2 and 1.5. This trend is inversely related to the changes of Ea of AASF mixtures. In addition, it was confirmed that the Ea of AASF was not only related to the chemistry of reactants but also reaction-stage dependent. Particularly it varied significantly at the very early age of reaction.