Mechanical and microstructural performance of residual sandy soil treated with different polymers

Sustainable management of environmentally vulnerable, moisture-sensitive soils is a critical challenge because they are susceptible to moisture-induced degradation. Biopolymer (BP) stabilisation offers an environmentally sustainable approach to transforming residual sandy soils into green geomaterials with reduced environmental impact. Consequently, engineering properties of these residual sandy soils vary spatially, and moisture conditions strongly influence their shear strength. This study investigates the efficacy of four categories of soil additives, i.e., (i) biodegradable polyester-based BPs (BPBs), (ii) natural polysaccharide BPs, (iii) synthetic polymers, and (iv) amino-polysaccharide BPs to enhance the mechanical performance of residual sandy soils. Specifically, the BPBs, namely polybutylene succinate (PBS) and polyhydroxyalkanoates (PHA) are evaluated and compared with natural polysaccharides; xanthan gum (XG), guar gum (GG), gellan gum (GLG) and sodium alginate (SA); a synthetic polymer, i.e., polyacrylamide (PAM); and cationic amino-polysaccharide biopolymer, i.e., chitosan (CS). To this end, varying proportions of BPBs were incorporated into the residual sandy soil to optimize stabilization performance and evaluate its effectiveness against conventional BP-based stabilizers at 28-days under isothermal curing at 16 °C using unconfined compressive strength test and advanced material characterization techniques i.e., Scanning Electron Microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric Analysis (TGA). It was observed that the highest single improvement in UCS 28−d was obtained with a natural polysaccharide (2.0% GG, ≈ 8.8× the untreated strength), although performance within the polysaccharide group varied by an order of magnitude (334–1936 kPa) and overlapped with the amine-based and synthetic polymers. Despite BPBs showing comparatively lower UCS 28−d yet offering dosage efficiency; TGA confirmed that 0.4% PBS specimens demonstrated higher thermal stability among all treated samples while consolidated undrained triaxial test confirmed that PBS proves to be very effective in enhancing shear strength of residual sandy soil thus making it suitable for performance-specific applications.