Abstract:

[Objective] This study investigates the mechanical properties and microstructural evolution of coarse-grained sulfate saline soils in the arid inland regions of Northwest China, modified with lignin fiber (LF) and polypropylene fiber (PP). It aims to clarify the effects of fiber dispersion methods, salt dissolution behavior, and fiber-salt interactions on soil strength with different sodium sulfate (Na₂SO₄) contents, thereby providing engineering strategies to mitigate soil-related hazards in construction. [Methods] Lignin fiber (hydrophilic) and polypropylene fiber (hydrophobic) were incorporated into saline soils using dry and wet mixing methods, respectively, to ensure uniform dispersion. A series of laboratory tests were performed, including fiber water absorption measurements, scanning electron microscopy (SEM) for microstructural observation, energy dispersive X-ray spectroscopy (EDS) for elemental analysis, and unconfined compressive strength (UCS) tests on soil samples with Na₂SO₄ contents ranging from 0% to 6%. The effects of salt content, fiber type (LF or PP), dosage, and their interactions on compressive strength were evaluated through multifactor analysis of variance (ANOVA) using SPSS software. [Results] (1) Fiber characteristics: The lignin fiber, hydrophilic and flat with a ribbon-like structure, had a porous, rough surface, demonstrating a water absorption ratio of 6.70 in pure water (an 8% reduction in saline solutions). Salt crystals adhered to its surface in the form of scales or layers, leaving minimal salt in soil pores. In contrast, the polypropylene fiber, hydrophobic and smooth with a cylindrical shape, exhibited a lower water absorption ratio (4.25 in pure water, 63% of that of LF), with further reduction (7%-23%) in saline conditions. Salt crystallized within the soil pores rather than on the fiber surface. (2) Salt dissolution dynamics: At salt contents ≤3%, complete salt dissolution occurred regardless of fiber type. At 6% salt content, approximately 83% dissolved, while the remaining 17% formed needle-like crystalline clusters that reinforced the soil skeleton. Lignin fiber increased the optimal water content in high-salt soils, promoting nearly complete salt dissolution. Polypropylene fiber had no significant effect on salt dissolution. (3) Strength behavior: Salt content primarily influenced UCS trends. Strength peaked at 1.5% salt (a 40.3% increase compared to untreated soil), then declined sharply, with a 37% reduction at 6% salt. Stress-strain curves shifted from strain-softening to strain-hardening behavior as salt content increased. Polypropylene fiber consistently enhanced UCS and residual strength. Optimal dosages were 0.35% for low-to-moderate salt soils (<3%, with strength increases of 150%-213%) and 0.45% for hypersaline soil (6%). Its smooth structure allowed salts to remain dissolved, while the fiber network facilitated load redistribution. Lignin fiber improved strength in low-to-moderate salt soils (12%-118% increase at 2%-4% dosage, optimal at 2%) but weakened high-salt soils (4%-17% decrease at 6%) due to salt aggregation on its rough surface. (4) Statistical significance (ANOVA): For LF-modified soils, salt content had the most significant effect on strength, followed by the interaction between salt content and fiber dosage. In PP-modified soils, salt content and fiber dosage were significant independent factors, with minimal interaction effects. [Conclusion] (1) Fiber performance hinges on microstructure and hydrophilicity. Polypropylene fiber, with its hydrophobic nature and load-distributing network, strengthens soil across all salt levels but requires higher dosages in hypersaline conditions. Lignin fiber enhances the strength of low-to-moderate salt soils by promoting salt dissolution but destabilizes high-salt soils due to surface-induced salt crystallization. (2) Salt dissolution and crystallization play a critical role in the mechanical properties. At high salt contents, undissolved crystals become part of the soil structure, while the fiber type influences both salt solubility and distribution. (3) Engineering strategies must align fiber selection with salt content. Polypropylene fiber is recommended for hypersaline soil, while lignin fiber proves more effective under moderately saline conditions. This study provides practical guidelines for managing sulfate saline soils in arid regions, emphasizing microstructure-driven design approaches.

Key words: coarse-grained sulphate saline soils, lignin fibers(LF), polypropylene fibers(PP), unconfined compressive strength, scanning electron microscopy(SEM), X-ray energy spectrometry(EDS)