A Study of the influence mechanism of plasticity index on the fluidity of soil-based CLSM and mix ratio optimization

This study systematically investigates the effects of soil plasticity index (PI), manufactured sand coarse aggregate content, and cement dosage on the fluidity of soil-based controlled low-strength material (CLSM). Natural silt was mixed with 0%, 6%, 12%, 20%, and 22% bentonite to prepare fine-grained soils (serving as soil-based fine aggregates) with plasticity indices of 9.7%, 11.7%, 13.2%, 15.4%, and 17.7%, respectively. The manufactured sand content was varied at 0%, 20%, 40%, and 60% (by weight), and the cement content at 0%, 4%, 6%, and 8%. The experimental results indicate that an increase in plasticity index significantly reduces fluidity and increases water demand. When the PI rises from 9.7% to 17.7%, the fluidity under the same water-to-solid ratio decreases by 40%–50%. The manufactured sand content regulates fluidity by altering the aggregate packing structure: At low sand content (0%–20%), aggregates are discretely distributed, requiring higher water demand. At moderate content (38%–52%), a “filling-lubrication” synergistic effect forms, reducing flow resistance. At high content (>52%), dense aggregate packing further decreases water demand. A fluidity mix ratio equation was derived through regression analysis based on the aggregate packing structure.