Mechanical Performance and Microstructure of Bio-Based Hydrogel-Stabilized Low-Plasticity Silt

Low-plasticity silt with a loose and weakly cemented structure often exhibits low strength and poor water stability, limiting its engineering performance. In this study, a low-dosage bio-based composite hydrogel consisting of sodium alginate (SA), calcium lignosulfonate (CLS), and Ca2+ was proposed for low-plasticity silt stabilization. An EDTA–Ca/GDL-mediated internal gelation process was employed to moderate Ca2+ availability and promote gradual in situ crosslinking within the soil matrix. A three-factor, three-level orthogonal design was used to investigate the effects of SA dosage, CLS dosage, and Ca2+/SA ratio on unconfined compressive strength (UCS), shear strength, and water stability, while scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the associated microstructural and chemical changes. The best-performing combination, containing 0.6% SA, 0.8% CLS, and a Ca2+/SA ratio of 0.1, achieved a 28 d UCS of 3768.93 kPa and a cohesion of 138 kPa. Range analysis and analysis of variance showed that SA dosage was the dominant factor governing UCS and cohesion, whereas the investigated mix-proportion factors had limited effects on the internal friction angle. The optimized stabilized silt also retained its structural integrity after 24 h of static-water immersion, demonstrating substantially improved short-term resistance to water-induced disintegration. SEM analysis revealed particle coating, pore filling, and interparticle bridging, while FTIR analysis indicated changes in the local chemical environments of hydroxyl and carboxylate groups. These microstructural and spectroscopic changes, together with the macroscopic results, are consistent with enhanced interparticle bonding and modification of the pore structure, thereby contributing to improved mechanical performance and water stability in the investigated low-plasticity silt.