Optimization of particle size distribution of sulfur-free lignin in enhancing disintegration resistance of saline soil

Sulfur-free lignin (SFL), a byproduct of the corn ethanol industry, effectively improves soil strength; however, its inherent solubility limits saline soil treatment in dry-wet environments. To solve this problem, this paper proposes the selection of a granular SFL (GSFL) with a relatively large particle size, but with its chemical properties retained, to replace the conventional powdered SFL (PSFL). Among the processes of water-induced erosion failure of earthen structures, disintegration is commonly deemed the primary cause. Therefore, the disintegration characteristics of both PSFL- and GSFL-modified soils with 0%, 5%, 7%, 10%, and 12% dosages were tested under dry-wet conditions. Moreover, several physicochemical properties, including the particle size distribution, bound water content, pH value, cation exchange capacity, functional groups, and microstructure, were explored. The results revealed that the water stability of the GSFL-modified soils was greater than that of the PSFL-modified soils. All of the PSFL-modified soils and natural soil completely disintegrated at the first immersion stage, whereas the GSFL-modified soils resisted 7 dry-wet cycles, notably at the 10% dosage. Microscopic analysis revealed that the SFL particle size affected mainly the cementation strength and microstructural homogeneity. Both the GSFL and PSFL particles strengthened the intergranular cementation through ion exchange, electrostatic interactions, and reinforcement effects. However, the PSFL preferentially filled the intragranular pores, causing a nonuniform microstructural arrangement and ineffectively enhancing the disintegration resistance of the saline soil. This preliminary study suggests that the particle size of a soil modifier may affect certain properties of the modified soil, necessitating future focused research.