Effects of seawater on properties of bentonite cut-off walls with/without cement addition

Bentonite-based cut-off walls have been widely used to protect coastal areas against seawater intrusion. However, salts contained in coastal soils affect the properties of bentonite cut-off walls, and the interactions between seawater and bentonite minerals with or without cement remain unclear. The aim of this study was therefore to evaluate the effects of seawater on the workability, strength and permeability of bentonite-based mixtures, prepared using three types of bentonite (sodium, calcium and seawater bentonite). The underlying mechanisms were further investigated through Atterberg limits, mineralogical and microstructural analyses. The results showed that cation exchange between seawater and bentonite leads to alterations in montmorillonite type and particle arrangement. This leads to a significantly reduced liquid limit and a slightly decreased plastic limit of bentonite, therefore narrowing the range of workable water content for bentonite–sand mixtures. Compared with sodium/calcium bentonite, seawater bentonite was less impacted by seawater, but showed a larger increment in workability after cement addition due to palygorskite dissolution. Under similar workability, the bentonite–sand–cement mixture with calcium bentonite yielded the highest strength and the lowest permeability. These findings illustrate correlations between bentonite mineral alterations and the properties of bentonite–sand mixtures with or without cement, and offer guidance for the construction of cut-off walls in coastal regions.