In order to improve the adverse engineering properties of saline soil and to comply with the concept of sustainable development, a novel green and pollution-free soil solidification method consisting of a composite of sulfur-free lignin (SFL), basalt fiber (BF), and hydrophobic polymer (HP) was designed for the solidification of saline soil. A quantitative model of solidified material and strength was developed by the response surface method using a central composite rotatable design (CCRD) for the unconfined compressive strength (UCS) test. A series of physicochemical and microstructural analysis tests were conducted to elaborate the hydrophobic mechanism of the composite-solidified soil and to investigate the pore evolution and microstructural characteristics of the composite-solidified saline soil. The results showed that the optimum ratio of composite-solidified saline soil is 9.87%, 0.24%, and 6.29%, and the strength increase rate is 98.87%. The hydrophilic groups (-OH) on the surface of soil particles and lignin are replaced with hydrophobic groups (-CH3) in the composite-solidified soil, increasing the erosion resistance and durability of the soil. At the microscopic level, BF and soil particles provide the spatial conditions and binding forces for SFL and HP, and the gel groups formed by HP attach to the SFL-induced soil particles to form cemented agglomerates and particle clusters. Overall, the composite material together improves the internal structure of the soil.
