Insight into volume change behavior of GMZ01 bentonite over cyclic salinization–desalinization processes

In the long-term operation of a deep geological repository for disposal of high-level radioactive wastes, bentonite material will inevitably undergo cyclic salinization–desalinization processes, which exert significant effects on performance of the engineered barrier. In this study, cyclic salinization–desalinization tests were performed on compacted bentonite specimens at a dry density of 1.7 Mg/m3 under vertical stresses of 0.15, 0.20, 0.20–0.26, 0.34 and 0.34–0.47 MPa. Meanwhile, MIP tests were conducted on an as-compacted specimen and the specimens experienced hydrations with de-ionized water (DW) and 2.0 M NaCl solution, as well as cyclic salinization–desalinization (2.0 M NaCl–DW) processes, respectively. It was found that the specimen volume change is governed by both of the salinization–desalinization processes and the vertical stress. The accumulative strain measured over cyclic salinization–desalinization processes for specimens tested under vertical stresses trends to be zero, suggesting that there exists an equilibrium state at which the shrinkage strain induced by the salinization is equal to the swelling strain generated by the subsequent desalinization. Based on these, a swelling equilibrium line and a shrinkage equilibrium line were proposed. Meanwhile, a swelling zone and a shrinkage zone were divided with a critical separating vertical stress of 0.47 MPa. In the swelling zone (< 0.47 MPa), the specimen exhibited prevailing swelling behavior, while in the shrinkage zone (0.47–12.82 MPa), the specimen was prevailed by shrinkage behavior. Finally, these phenomena were interpreted from the microstructural point of view according to the MIP test results.