Influence of plasticity and porewater salinity on shrinkage and water retention characteristics of biochar-engineered clays

Clay-engineered barriers might be subjected to soil salinization issues under climate change. A recently emerged desalinization method is achieved by modifying clays using biochar. However, unsaturated soil responses of biochar-engineered clays in saline environments under drought conditions remain unknown. This study aims to investigate soil shrinkage and water retention characteristics of biochar-amended kaolin and bentonite under saline conditions. Soil shrinkage and water retention tests were conducted on clays (with and without biochar addition) with various porewater salinity (i.e., 0%–10%). Physiochemical properties (including zeta potential and porewater pH) were measured to interpret particle–fluid interactions. Shrinkage characteristics of kaolin and bentonite exhibited sensitivity and insensitivity to the porewater salinity, respectively. This phenomenon was explained by hydrogen-sodium ion exchange and deprotonation phenomenon occurring on kaolin and bentonite, respectively. Biochar significantly alleviated the salinity-induced shrinkage of clays by increasing the shrinkage limit of kaolin and bentonite by 6%–14% and 50%–107%, respectively (p < 0.05). This was attributed to the porous structure and hydrophilic functionality of biochar that immobilized sodium ions through ion exchange and protonation reactions. The air entry value of clays significantly increased with porewater salinity and biochar addition due to the reduction of void ratio and enhanced capillarity, respectively. An empirical equation was proposed to predict the shrinkage limit of clay in various saline conditions. It highlighted that the application of biochar-engineered clays could contribute to the desalination and the improvement of resistance to shrinkage damage in hydro-chemical barriers.