AbstractA numerical framework to study the morphology change of microstructure and ionic diffusivity of leached cement pastes is proposed. According to a two-scale microstructure model, the leached cement paste in the course of portlandite dissolution and C─ S─ H decalcification is modeled. In contrast to recent leaching studies that used particle-based microstructure models to analyze micrometer-sized structures, the novel aspect of this study is the consideration of capillary pores with sizes ranging from tens of nanometers to several micrometers. The evolving microstructure was coupled with the random walk algorithm to calculate diffusion properties in capillary and gel pores of cement pastes during leaching and hydration. Results were in general accord with the reported data from leaching experiments and other simulations. The rarely discussed relationships the simulated 3D microstructure and the ionic diffusivity of leached cement pastes are investigated. In particular, the pore structure, which has different degradation characteristics in each leaching stage, affects the ionic diffusivity variously. The proposed framework has potential as a tool for multiscale simulation of leaching concrete.