Micromechanical study of alkali-activated GGBS/FA binders: Insights from microscale testing and homogenization analysis

Alkali-activated slag/fly ash (GGBS/FA) binders exhibit complex heterogeneous microstructures, making it challenging to quantitatively relate phase evolution to mechanical performance across multiple scales. This study develops an experimentally validated multiscale micromechanical framework to investigate the elastic and strength evolution of alkali-activated-GGBS/FA pastes with different slag contents. Mechanical properties, phase assemblage and pore structure were systematically characterized from the nano- to micro-scale and incorporated into a two-step homogenization framework for elastic prediction and an elasto-brittle upscaling model for strength prediction. Increasing GGBS content led to pore refinement and significant improvements in mechanical performance, which can be attributed to the transformation from porous N-(C)-A-S-H gel to denser C-(N)-A-S-H gel as inferred from the evolution of phase fractions and pore structures. The homogenization model accurately predicted the elastic modulus with errors below 10%, while the strength model successfully captured the experimentally observed strength evolution. The results demonstrate that hydrate phase assemblage and porosity primarily govern the mechanical behavior, whereas unreacted particles enhance stiffness but also induce local stress concentration that limits strength development. The proposed framework provides a physically based approach for predicting the mechanical properties of alkali-activated materials directly from intrinsic phase characteristics and offers guidance for the microstructure-informed design of sustainable cementitious materials.