Decay characteristics of swelling pressure of compacted bentonite under salinity gradient cycling

During the long-term operation of a deep geological repository of high-level radioactive waste (HLW), the bentonite barrier is likely to experience the chemical cycling paths of groundwater with various chemical components. In this case, the swelling capacity of bentonite can be reduced to a certain degree. In this work, GMZ bentonite powder is densely compacted to a dry density of 1.7 g/cm³ to carry out the swelling pressure tests under the constant-volume conditions. The salinization-desalinization effect is investigated by cyclical infiltrations of NaCl solution and distilled water. Furthermore, the cation exchange effect is emphasized by sequential infiltrations of NaCl solution, distilled water and KCl solution. For both infiltration paths, the salt concentrations (0.5, 1 mol/L) are taken into account. The results show that the swelling pressure characteristics of GMZ bentonite depend on the ion species and salt concentration of infiltration solution as well as the number of chemical cycles. The salinization effect of salt solution results in the decrease in swelling pressure, while the infiltration of distilled water improves the swelling behaviors. The swelling pressure decreases with the salt concentration of infiltration solutions. The stable swelling pressure and its reduced degree gradually decrease against the number of chemical cycles. Under the infiltration of KCl solution, some of the montmorillonite is dissolved, which results in the significant decrease in swelling pressure. No swelling pressure will be measured for GMZ bentonite if the concentration of KCl solution is higher enough.