Experimental and theoretical study of the effect of layer charge on shrinkage behavior of bentonite

The principal clay mineral in bentonite is montmorillonite. Its layers carry fixed negative charges arising from isomorphic substitution. The resulting layer charge is the key factor controlling bentonite’s shrinkage behavior. This study combines experimental and theoretical methods to investigate the mechanism by which layer charge affects bentonite shrinkage. Sodium bentonite was used as the starting material. A series of charge-reduced samples with progressively lower layer charges were prepared using the lithium fixation method. Shrinkage characteristic curves were obtained by digital image analysis. Soil–water characteristic curves and water distribution were measured using a dewpoint potentiometer and nuclear magnetic resonance (NMR). Results indicate that decreasing layer charge transforms bentonite from highly to weakly shrinkable and leads to the emergence of a structural shrinkage stage in the shrinkage curve. Based on the inflection point, the shrinkage curve can be divided into a capillary stage and an adsorption stage. This division agrees with the observed water distribution patterns. Reducing layer charge also decreases water retention capacity. However, after normalizing by cation exchange capacity (CEC), the soil–water characteristic curves converge at high suction, indicating that the adsorption stage is primarily governed by hydration of interlayer exchangeable cations. Drying-induced compression curves, expressed as intergranular stress, show that most shrinkage in the capillary stage is elastoplastic, whereas deformation in the adsorption stage is predominantly elastic. Overall, the boundary between the capillary and adsorption stages identified using shrinkage curves, water distribution, soil–water characteristic curves, and desiccation compression curves is consistent across these methods.