Addition of zeolite to cement contributes to the alkali-aggregate reaction suppression and the densification of concrete, but there are many unclear points about the dissolution behavior of zeolite during the hardening of cement. In NaOH solution, the solubility of zeolites with various Si/Al molar ratios and pore sizes increased with increasing their Si/Al molar ratio. Meanwhile, zeolites with smaller pore sizes had higher solubility, in the mixture with Ca(OH)_2 and in the cement paste. However, solubility of a zeolite having pores that can not be passed through Ca was very small. Therefore, during the hardening of cement, it was concluded that dissolution takes place by binding OH^- to Si of zeolite, and the dissolution is facilitated by the binding of Ca to the adjacent O. When using zeolite as a concrete admixture, a suitable choice of the zeolite species should be performed by considering its solubility.
In the geological repository of radioactive waste containing transuranic nuclides (TRU waste), after a long period of time, there is a concern that bentonite making up artificial barrier is dissolved through contact with strong alkaline water of high Ca content. The first-principle quantum chemical calculation was performed using the cluster model of montmorillonite which is the main component of bentonite. Under strong alkaline condition, octahedral Al had higher solubility than octahedral Mg had. In particular, unsaturated Si-O-Al bond connecting tetrahedral and octahedral sheets at the edges of (110) and (010) planes, was suggested to easily cleavage. The dissolution of montmorillonite through the dissociation of Si-OH and Al-OH_2 groups was shown to be less likely to occur. On one hand, the adsorption of OH^- on a Si atom caused dissolution of adjacent Si and Al atoms in addition to the Si atom. At least in the early stages of dissolution, the difference in exchangeable cation species (Na^+ or CaOH^+) was suggested not to affect the dissolution behavior of montmorillonite. Thus, when evaluating the long-term stability of bentonite in the geological repository, structural factors particularly the cation composition of the octahedral sheet of montmorillonite should be taken into account.
The aim of this study is to develop a method of heat treatment for removal of Cs from the contaminated soil. This development can reduce the concentration of Cs in the soil to acceptable levels for the use as civil work materials. In a simulative experiment using clay (Bentonite) containing non-radioactive Cs for evaluating Cs removal, heightened CaO/SiO_2 ratio effectively prevent the soil from melting thereby achieving dramatically higher Cs removal rate under the conditions of high temperatures with the addition of chloride as a reaction accelerator. Based on this finding, we carried out heat treatment tests using actually contaminated soil which contained 10,000 Bq/kg of radioactive Cs and a chloride as an accelerator. Through this practice, Cs in the samples of higher CaO/SiO_2 ratio successfully volatilized showing much lower residual radioactive Cs concentration than the clearance level (100Bq/kg).