The swelling behavior of montmorillonite as affected by the grain size by in situ X-ray diffraction experiments

Document available in extended abstract form only. In many existing researches, the swelling behavior and impermeability of smectitic engineered barrier materials in disposal facilities of radioactive waste was investigated. In the RWMC-project, the effect of smectite content on the mechanical and hydraulic behavior of smectitic materials is investigated and modeled to introduce into THMC analysis. However, since smectite is a natural resource, physical and chemical properties are different with places of production. In order to model the swelling behavior and impermeability of smectitic materials, not only the smectite content but also layer charge and crystal size will be the primary factors. In addition, smectite types and impurity minerals contents or soluble salts affect bentonite characteristic, as well. In this research, in order to focus on the effect of grain size of smectite, the swelling behavior of smectitic materials which are the same place of production but are different grain size were investigated. The smectitic material used in this study was Kunipia-F (Kunimine Industry Co. Ltd., Japan), which is from the Tukinuno Mine, Yamagata prefecture, Japan, and is purified montmorillonite produced by hydraulic elutriation. It is considered that the montmorillonite, Kunipia-F, has large crystal size, and for example Kunipia-F is able to make a sheet when drying weak solution. The grain size was conditioned by jet mill pulverizer. The pulverizing was conducted by making each other collide with high speed. The grain size of the intact and pulverized samples was measured by using of the laser scattering particle distribution analyzer and SEM. The swelling behavior was measured by in situ X-ray diffraction using of a sample chamber which can control the temperature and humidity precisely. The result of the laser scattering analysis denoted that the pulverized sample fined down expressly. The difference of the crystal aspect ratio of the pre- and post-samples measured by SEM was ambiguous. These results show that jet mill pulverization made a stacked layer cleave rather than broke a sheet crystal. The swelling pressure measurement and X-ray diffraction measurement using of the same samples conducted in RWMC-project, was shown in Figure 2 and 3. As shown in Figure 2, the swelling behavior depended on the grain size. The swelling pressure of pulverized Kunipia-F was approximately 1.4 times larger than that of intact sample, and MX-80 was approximately 2.3 times larger. Moreover, XRD patterns of the intact and pulverized Kunipia-F and MX-80 were different, respectively. Although the swelling behavior of the intact sample showed distinct zero-, one-, and two-layer water molecular hydration states, the pulverized sample showed the continuous swelling from one- and two-layer hydration state. According to RWMC-project, pulverizing the smectitic materials increases the swelling pressure and the ratio of the low hydration states in montmorillonite in spite of the same hydrous conditions as intact specimen. This result is consistent with the result in this paper. It is thought that a small part of montmorillonite sheets is difficult to swell potentially. Since the stacked layer was cleft forcibly by the pulverization and the hydration area of montmorillonite increased, the montmorillonite hydrated uniformly. In the constant hydrous conditions, the increment of hydration area causes the montmorillonite content with low hydration state to increase, and the swelling behavior changes stepwise hydration to continuous hydration. By using of MX-80 (Wyoming bentonite), the same measurements have been carried out, and we will also report the difference between Kunipia-F and MX-80. This research is constituted from an additional experimental research and a part of 'Development of the technique for the evaluation of long-term performance of EBS, FY2011' under a grant from the Japanese Ministry of Economy, Trade and Industry (METI). (authors)