Multi-Phase Micromechanical Homogenization Approach for Predicting the Elastic Modulus of Gypsum–Fly Ash Ternary Blend Concrete

This study evaluates the efficacy of micromechanical homogenization techniques for estimating the effective elastic modulus of sustainable concrete containing fly ash and recycled gypsum. In the absence of direct experimental measurements, a multi-phase computational framework was developed to simulate the interactions among coarse aggregate, mortar, and the interfacial transition zone (ITZ). A two-phase homogenization model was first calibrated against ACI 318-19 elastic modulus predictions through a sensitivity analysis, achieving a relative error below 3% across all ternary concrete mixtures. Subsequently, a three-phase model incorporating a 30 μm ITZ was developed to provide a more realistic representation of the composite microstructure. While the inclusion of the ITZ slightly increased the prediction error for the control mix, it consistently improved agreement with ACI-based estimates for all fly ash–gypsum blends. Overall, the simulated elastic modulus values showed strong convergence with empirical predictions, demonstrating that the proposed framework consistently reproduces established empirical stiffness trends and provides a physically informed baseline for future experimental validation. These findings highlight the potential of micromechanical modeling as a practical, reliable tool for future parametric studies and experimental validation of elastic properties in sustainable concrete systems.