Abstract
In the assessment of radionuclide migration underground, the Ca/Si ratio of calcium silicate hydrate (C–S–H) gel formed as a secondary mineral is a key factor, because the sorption behavior of cationic nuclides such as cesium on the C–S–H gel depends on the Ca/Si ratio. In addition, this Ca/Si ratio of the C–S–H gel is changed by the formation of various compounds upon the reaction of Ca ions leaching from the C–S–H gel with coexisting anions. In this study, the effect of sulfate ions (up to 500 mM) on the sorption of Cs ions onto the C–S–H gel (Ca/Si ratios of 0.4, 0.8, 1.2 and 1.6) was examined and compared with that of carbonate ions. The obtained results show that the sorption ratio of Cs ions on the C–S–H gel slightly decreased with the increase in the concentration of sulfate ions under any Ca/Si ratio and that the effect of the copresence of sulfate ions on the decrease in the Ca/Si ratio of the C–S–H gel was extremely small compared with that of carbonate ions. That is, the C–S–H gel formed as a secondary mineral contributed to the retardation of the migration of Cs ions, even though the plume containing a high concentration of sulfate ions flowed around the repository.
