Seawater effects on properties of bentonite cut-off walls with/without cement addition

Abstract

Bentonite-based cut-off walls have been widely used to protect coastal areas against seawater intrusion. However, salts contained in coastal soils affect properties of bentonite cut-off walls, and interactions between seawater and bentonite minerals with or without cement remain unclear. Therefore, this study aims to evaluate seawater effects on the workability, strength, and permeability of bentonite-based mixtures, prepared using three types of bentonite, namely sodium, calcium, and seawater bentonite. The underlying mechanisms were further investigated through Atterberg limits, mineralogical, and microstructural analyses. Results show that the 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, the seawater bentonite is less impacted by seawater, but shows a larger increment in workability after cement addition due to palygorskite dissolution. Under similar workability, the bentonite-sand-cement mixture with calcium bentonite yields the highest strength and the lowest permeability. These findings illustrate correlations between bentonite mineral alternations and properties of bentonite-sand mixtures with or without cement, and offer guidance for cut-off walls construction in coastal regions.