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Phase assemblage and strength are essential engineering properties of hybrid blends of cement and siliceous fly ash (FA). This study presents a kinetic and thermodynamic model for evaluating reaction level, phase assemblage of reaction products and strength progress of hybrid blends of cement-fly ash with various water/binder ratios and FA/binder ratios. First, the reaction level of cement and glass phase of FA are determined using a kinetic model that considers the cement hydration, the glass-phase reaction of FA and the dilution effect of fly ash. The crystalline phase of FA is assumed to be inert. Moreover, the reaction level of binders and concrete mixtures are used as input data for a thermodynamic model. Second, the phase assemblages of hydrating binary blends are determined using GEM-Selektor (Gibbs energy minimization). The mass conservation of the reactants and products of hybrid blends with fly ash are considered in thermodynamic model. Finally, by using calculation results of calcium silicate hydrate (CSH) content from GEMS, the strength progress of composite concrete is determined. The correlation coefficient between prediction and experiments for the CSH-based model is 0.98. The analysis results clarified that the use of fly ash in concrete with a low water/binder ratio is a suitable option.