Effect of synthetic water and cement solutions on the swelling pressure of compacted Gaomiaozi(GMZ) bentonite: The Beishan site case, Gansu, China

After construction of the repository, the bentonite will undergo some chemical effects resulting from the chemistry of site water and the solution produced by cement degradation in the high-level radioactive waste repository. In this study, the swelling pressure of compacted Gaomiaozi(GMZ) bentonite was investigated with three types of pore water simulating site groundwater (synthetic Beishan Site Water - BSW) and two cement solutions (Young Cement Water - YCW and Evolved Cement Water - ECW). X-Ray diffractometry (XRD) tests were also carried out to analyse the mineralogy of the samples before and after the swelling pressure tests. Results show that the swelling pressure curve is of “double-peak” pattern, which fades with the pH of solutions increase. The swelling pressure of GMZ bentonite decreases little with BSW, significantly with YCW and slightly with ECW. At a higher dry density, the swelling pressure of compacted GMZ bentonite reaches stability later and the final swelling pressure is higher. Cation exchange takes place in the bentonite and the montmorillonite of GMZ bentonite transforms to multi-cation dominant montmorillonite with the three types of pore waters. The swelling pressure reduction affected by the cement solutions is mainly due to the montmorillonite dissolution, but the secondary minerals are in amorphous phases. The swelling property of compacted GMZ bentonite can be reduced by cement solutions in the long-term operation of repository.