Effect of Phosphoric Acid on the Properties of Sodium Bentonite and Its Mechanism

Expansive soils, widely distributed in nature, often pose challenges to construction stability due to their low unconfined compressive strength (UCS), poor shear strength, and high expansibility. This study investigates the application of phosphoric acid (H₃PO₄) in modifying sodium bentonite, focusing on its effects on the mechanical properties and swelling behavior of bentonite, as well as the underlying mechanisms. H₃PO₄ was added to bentonite at mass ratios of 1% to 8%. Compared to unmodified bentonite, the plastic index of the modified bentonite decreased by 39.9%, and the UCS value increased by 92.24% when the H₃PO₄ dosage was 2%. Notably, at an H₃PO₄ dosage of 8%, the free swelling rate of the modified bentonite decreased by 38.1% relative to the control sample, and the cohesion increased by 165.35%, indicating significant improvements in both the expansibility and bearing capacity of modified bentonite. The results on the physical and chemical properties of modified bentonite revealed an ion exchange involving hydrogen ions from H₃PO₄ and metal cations in sodium bentonite. The zeta potential of bentonite decreased with H₃PO₄ addition, reflecting a reduction in the double electric layer thickness due to hydrogen ion exchange with metal cations. This enhanced the gravitational attraction between soil particles, leading to their closer proximity and a significant increase in the UCS value of the modified soil. Additionally, the XRD results confirmed that the addition of H₃PO₄ facilitated the formation of a new mineral, aluminum phosphate, which is hard and insoluble, filling soil pores, contributing to its densification. This study demonstrates that H₃PO₄ can effectively enhance the swelling resistance and strength of sodium bentonite, offering a promising method to improve its application performance.