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.

This study presents a calibration-oriented multi-phase micromechanical homogenization framework for estimating the elastic modulus of sustainable concrete containing fly ash and recycled gypsum. In the absence of direct experimental measurements, a 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 the ACI 318-19 elastic modulus relationship through a sensitivity analysis, achieving relative errors below 3% with respect to the ACI-based empirical reference. Subsequently, a three-phase model incorporating a $30\mathsf{\mu }\mathrm{m}$ ITZ was developed to explicitly account for the influence of the interfacial transition zone on the effective elastic modulus within the adopted modelling assumptions. While the inclusion of the ITZ slightly increased the prediction error for the control mix, it consistently improved agreement with the ACI-based reference for all fly ash–gypsum blends. In addition, comparison with experimentally reported elastic modulus values for similar SCM-modified concretes demonstrated that the predicted values were physically reasonable and generally consistent with published data. Overall, the proposed framework reproduced stiffness trends predicted by the ACI empirical relationship and provides a physically informed computational baseline for future experimental validation and parametric investigations of sustainable concrete systems.