Effects of alteration on shear characteristics of compacted Ca-bentonite immersed in alkaline solutions

This study evaluated the shear characteristics of compacted Ca-bentonite immersed in 0.1 mol/L of NaOH, KOH, and KOH–NaOH and 0.005 mol/L of Ca(OH)₂ at 40℃ over a maximum period of 1710 days. Triaxial compression tests were performed on the immersed specimens, and the mineral composition, mean layer charge, leachable cations, and microstructure were investigated. The dissolution of cristobalite was significant at high pH levels, whereas phillipsite was precipitated in the specimens immersed in the NaOH and NaOH–KOH solutions. The amount of leachable cations increased substantially, indicating that soluble secondary products (non-crystalline phase) were present in the specimens, as was proven by the observation of gel-like products comprised of Ca and Si on the microphotograph. An increase and decrease in the maximum deviator stress occurred as a result of the dissolution and precipitation. A structural parameter was proposed in this study by assuming the contribution of the secondary products to the cementation of the soil skeleton. This provided a series state transition of the compacted bentonite, where the maximum deviator stress increased with the cementation of the non-crystalline secondary phase. However, the progressing dissolution of the primary minerals decreased the dry density, thereby loosening the cemented structure and reducing the maximum deviator stress.