Effect of salt solution concentration and cation types on the mechanical properties of bentonite as a barrier material

Bentonite is utilized as a barrier material in high-level nuclear waste repositories due to its superior low permeability and swelling properties. However, its engineering properties are influenced by the chemical composition of the infiltrating pore water during operation. Understanding the effect of salt solution on the mechanical properties of bentonite is crucial for evaluating the performance of buffer and backfill barriers in deep geological repositories for nuclear waste. In this study, various concentrations and types of salt solutions were used to treat Na-bentonite samples, which were then subjected to free swell test, no loading swelling ratio test, Atterberg limits test, compaction test, and analysis of the content of exchangeable cations. The results showed that the content of counterbalance cations changed significantly after the addition of salt solution, and the decrease in free swelling rate increased gradually with the increase of solution concentration. The effect of different types of salt solutions on swelling was primarily determined by the type of cation that governs charge level and hydration capacity. The inhibition of the free swelling rate was stronger for high-concentration low-valence salt solution than that for low-concentration high-valence one. Bentonites undergoing cation exchange exhibited a decreased plasticity index, a decreased maximum dry density, and an increased optimum water content. This was mainly due to the cation exchange that occurred between bentonite layers under the action of the salt solution, which affected the crystal layer structure, double electric layer structure, and intergranular stress. Finally, the van't Hoff equation was used to quantitatively characterize the differences in swelling in the test results.