Cement-based materials used at radioactive waste disposal sites are required to possess long-term stability. However, when these materials come in contact with groundwater, calcium leaching from the solid occurs, and the material becomes porous. The use of mineral admixtures is recommended to minimize porosity. However, few studies have focused on the diffusion performance of cement-based materials blended with mineral admixtures after leaching. Therefore, in this study, the diffusion performance of such materials using blended cement after leaching was evaluated. It was found that the diffusion coefficient of the blended cement increased with leaching, and when leaching progressed considerably, the diffusion coefficient of the blended cement was close to that of ordinary Portland cement. Furthermore, the diffusion coefficient after leaching demonstrated good correlation with the pore volume when the pore diameter was 50 nm or larger.