(SiO4)4- structure change in C-S-H on dissolution processes of tricalcium silicate hydrate
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abstract
The high pH condition of aqueous solutions in the radioactive waste repository, which keeps the solubility of many radionuclides low, can be achieved by dissolution of cementitious materials. In order to clarify the mechanism to maintain this high pH for long time, we investigated the dissolution phenomena of synthesized C3S(C3S:3CaO・SiO2) hydrate. In the present research, leaching test was conducted by changing the ratio of mass of C3S hydrate to mass of leaching water (Liquid/solid ratio) from 10～2000(wt/wt). C3S hydrate were contacted with deionized water and set in the sealed bottle. After the lapse of the pre-determined time, the solid was separated from the solution. The value of pH and concentrations of Ca and Si in the aqueous phase were measured. The solid phase was characterized by using XRD, DTA/TG and 29Si-NMR. It was found that the pH and Ca concentration in the aqueous phase were converged into a constant value, which was in agreement with these reported by Greenberg et al.[1], with the leaching time. From the results of XRD and DTA/TG analysis on the solid phase and the Ca concentration in the aqueous phase, it was confirmed that Ca(OH)2 was preferentially dissolved when the liquid/solid ratio was 10 or 100(wt/wt), and C-S-H as well as Ca(OH)2 was dissolved when the liquid/solid ratio was 500(wt/wt) or more. NMR results showed that structure of silicate anion of the C-S-H became longer when the liquid/solid ratio was 500(wt/wt) or more. This indicates that leaching of C3S hydrate therefore results in polymerization of C-S-H. This research will contribute to improve the evaluating method for the long-term performance of the repository, because the change of C-S-H structure effects the dissolution and sorption of radionuclides.
