Sorption Behavior of Cesium and Strontium During the Formation Process of Calcium Silicate Hydrate as a Secondary Mineral Under the Condition Saturated with Groundwater

Calcium silicate hydrate (C-S-H) is formed also as a secondary mineral in the vicinity of the radioactive waste disposal site by the reaction between calcium (Ca) ions leached from the cement and silicic acid dissolved from the host rock. It is well known that C-S-H significantly absorbs cationic radionuclides as reported by many previous studies. In such researches, C-S-H samples have been once dried after 7 days or more of the curing period from their synthesis. In an actual disposal environment, however, the C-S-H of secondary mineral would be formed under the condition saturated with groundwater. Additionally, C-S-H under a transient condition may also react with nuclides because C-S-H continuously forms and alters by supplying Ca ions and silicic acid. Therefore, this study examined the sorption behavior of cesium (Cs) and strontium (Sr) during the CSH formation under the condition saturated with groundwater by the sorption experiments without any drying process.<br/>The sorption experiments were conducted with the C-S-H samples adjusted Ca/Si molar ratios to 0.4, 0.8, 1.2, and 1.6. C-S-H was synthesized by mixing calcium oxide, fumed silica, and pure water. The total weight of the solid phase was set to 1.5 g, and the total volume of the liquid phase was 30 mL. The concentrations of Cs or Sr in the samples were set to 1.0 mM with stable isotopes. The sorption conditions were prepared in two types: adding Cs or Sr simultaneously with the C-S-H synthesis (hereinafter referred to as “co-precipitation condition”), and adding Cs or Sr after 30 days of C-S-H curing (hereinafter referred to as “hydration condition”). The reaction periods were selected to 1, 3, 7, and 14 days for co-precipitation conditions and 1, 3, and 7 days for hydration conditions. After the reaction period, the solid and liquid phases were separated by centrifuging and filtering with a 0.20-µm membrane filter. The concentrations of Ca, Si, and Sr were measured by ICP-AES, and that of Cs was measured by AAS. The solid phase was analyzed by Raman spectroscopy and XRD.<br/>In the analysis results of dissolved ions, the concentrations of Ca and Si were negligibly small in the comparison to the amount of Ca and Si used for the C-S-H synthesis, more specifically, the Ca/Si molar ratios of the solid phase were kept almost constant through the experimental period including just after the synthesis. This means that the soluble components quickly reach apparent solid-liquid equilibrium. On the other hand, the Raman spectra of the solid phase showed that the polymerization of the silicate chain in C-S-H progressed with time, and the lower Ca/Si molar ratio increased the degree of polymerization. In addition, the coexistence of amorphous parts and crystalline tobermorite structure was observed in the solid phase just after the C-S-H synthesis by XRD. These suggest that C-S-H continuously alters from its formation even after reaching apparent solid-liquid equilibrium. Besides, as for the sorption of Cs and Sr onto C-S-H, their sorption rates decreased with increasing the Ca/Si molar ratios because the surface charge of C-S-H was negative for the lower Ca/Si molar ratios and positive for the higher ones. Furthermore, these sorption rates were almost constant regardless of the reaction period, the curing period of C-S-H, and the conditions adding Cs or Sr to the C-S-H samples. These suggest that the sorption sites for cations such as silanol groups and interlayers of C-S-H are formed just after the C-S-H synthesis, and hardly change over 30 days or more even if C-S-H gradually alters as mentioned above. The distribution coefficients were estimated to be approximately 1 - 15 mL/g for Cs and 2 - 35 mL/g for Sr under these experimental conditions. As a result, C-S-H which continuously forms as a secondary mineral around the repository is also expected to retard the migration of Cs and Sr because of its sorption property occurring immediately after its formation.