Seawater impact on permeability of compacted bentonite–sand mixtures: role of montmorillonite content and its interlayer cations

The impact of seawater (SW) on the permeability of compacted bentonite‒sand mixtures was investigated. The montmorillonite content ranged from 36% to 64%, and the ratio of the amount of extracted Na ions to the cation exchange capacity (Na/CEC) ranged from 0.01 to 0.84 for the six bentonites used in this study. The higher initial Na/CEC of the bentonite represented a large difference of ratio in the hydraulic conductivity between the SW and deionized water (DW) conditions ( k SW / k DW ). Only the bentonite with Na/CEC = 0.01 showed no difference in the hydraulic conductivity in SW and DW conditions ( k SW / k DW = 1), indicating that the crystalline swelling state was maintained constantly. Regarding the crystalline swelling as the reference state, we defined the external pore by taking the difference between the total pore and interlayer space of montmorillonite. The approximate unique relationship between k SW / k DW and the volume ratio of Ca- and Mg-montmorillonite to external pore was obtained. We found that the smaller volume of external pores, as well as the larger amount of interlayer Ca and Mg ions, played a key role in mitigating the impact of SW on the permeability of the compacted bentonite‒sand mixture.