The two suppression mechanisms for Cs leaching, which contain fly ash (FA), are refinement effect on capillary pores and adsorption effect that are dependent on the mix proportion of FA. If the leaching suppression by the adsorption effect is expected, FA should be mixed with the cement matrix at a replacement ratio of approximately 70 mass% and the Ca/Si ratio in the C-S-H phase should be adjusted to approximately ≤1.0 (0.6–1.0). Meanwhile, if the leaching suppression by the refinement effect on a capillary pore is expected, FA should be mixed at a substitution ratio of approximately 30 mass%.
