Effects of cation species (Na(I), K(I), Ca(II), Mg(II)) and concentrations on swelling strain of bentonite-sand mixtures

Self-sealing is a design requirement for buffer materials used in the geological disposal of high-level radioactive waste. In coastal areas of Japan, groundwater with diverse cation compositions and a wide range of salt concentrations is anticipated. In this study, swelling-deformation tests were conducted on a bentonite-sand mixture (70% bentonite, 30% silica sand), which is one of the candidate buffer materials, using salt solutions containing the major cations found in groundwater (Na+, K+, Ca2+, and Mg2+). The effects of cation type and salt concentration on swelling strain were systematically compared. In addition, leached cation analysis was performed on the specimens after permeation to assess changes in the exchangeable cation composition. The maximum swelling strain followed the order Na+ > K+ > Ca2+ ≈ Mg2+. Calculations using a theoretical equation for swelling characteristics that accounts for the exchangeable cation composition suggested that these differences in swelling behavior among cation types were attributable to changes in the exchangeable cation composition caused by cation exchange reactions. Moreover, it was shown that, under high salt concentration conditions, the difference in swelling strain among cation species became less pronounced as salt concentration increased, even though changes in exchangeable cation composition were observed.