Physicochemical controls on desiccation crack patterns in bentonite: A comprehensive examination of salinity effects

Climate-related extreme weather events such as drought and heat waves increase regional soil salinization and desiccation cracking that degrade the environment. Soils shrink, crack, and accumulate salts during desiccation. However, the effect of salt concentration on the characteristics of desiccation crack is not well understood. Previous experimental observations and explanations for cracking characteristics with different salt concentrations show clear discrepancies. This study presents a comprehensive examination of salinity effects on desiccation crack patterns in bentonite covering a wide range of NaCl concentrations from 0 to 2 mol/L. Crack patterns are quantified by digital image analysis. Sedimentation tests, liquid limit tests, and scanning electron microscopy are applied to elucidate clay particle interactions and microstructure effects that control the process. Results show that the degree of cracking is relatively low at low NaCl concentrations (c ≤ 0.01 mol/L) and significantly increases for 0.01 mol/L < c < 0.2 mol/L. Then the number of cracks on the surface of the sediment decreases with increasing NaCl concentrations (c > 0.2 mol/L). The change of cracking behavior with varying NaCl concentrations is related to the transition of a dispersed and homogenous microstructure of the sediment to an aggregated and heterogeneous microstructure as NaCl concentration increases. Salts can precipitate during desiccation, which lowers the evaporation rate and masks the extensive cracking developed beneath the salt cover on the sediment surface at very high NaCl concentrations. The results identify physicochemical processes that cause bentonite cracking and soil degradation as salt accumulates in soils and are relevant to the mitigation of desiccation cracking in arid and semi-arid areas.