Modelling of the reaction kinetics and phase evolution of alkali-activated slag–fly ash blends

The reaction process of alkali-activated slag–fly ash (AASF) systems exhibits significant multi-stage characteristics and is governed by the coupled effects of the precursor composition and parameters of the activator. Existing kinetic studies have largely focused on empirical fitting of individual systems. Quantitative prediction models applicable to composite systems are needed. In this work, a coupled kinetics–thermodynamics model that enables quantitative prediction of the reaction degree and phase evolution in AASF systems was established. Based on the heat release curves of alkali-activated slag (AAS) systems and alkali-activated fly ash (AAF) systems, the relationship between kinetic model parameters and material design variables was quantified, and those for AASF blends were obtained by way of weighted superposition of the AAS and AAF models. Phase assemblage was computed by Gibbs free energy minimisation using GEM-Selektor with an extended Cemdata18 database. Based on the coupled kinetics–thermodynamics model, the predicted reaction degree of AAS at 100 days was 0.76 with 37% calcium aluminium silicate hydrate (C-A-S-H), the predicted reaction degree of AAF was 0.65 with 27% sodium aluminium silicate hydrate (N-A-S-H) and that of AASF(1:1) was 0.71 with coexisting C-A-S-H (17%) and N-A-S-H (15%), indicating a shift from calcium-dominated to sodium–calcium coexisting gel chemistry. The coupled kinetics–thermodynamics framework provides a theoretical foundation and a quantitative tool for understanding the microstructural development of alkali-activated materials.