Mechanistic insights into mineral-specific interactions in biopolymer-stabilized soils

Biopolymer stabilization potentially offers a low-carbon alternative to traditional binders, yet mineral-specific soil–biopolymer interaction mechanisms remain unclear. This study examines four mineralogically distinct soils montmorillonite (M), kaolinite (K), their composite (KM), and silty sand (SM) treated with xanthan gum (XG), polyhydroxyalkanoate (PHA), and their binary blend (XG_PHA). To isolate mineralogical effects, biopolymer dosage, compaction energy, and curing regime were kept constant. Mechanical, thermal, spectroscopic, and microstructural analyses were conducted to resolve mineral-specific interaction pathways. Strength responses followed intrinsic mineral reactivity (M > KM > K), whereas SM exhibited the largest relative gain because of its initially loose fabric. Ductility decreased in mineral-reactive soils but increased in SM owing to fabric modification. XG_PHA consistently provided the greatest strength, stiffness, and energy absorption. Thermal analysis showed a distinct PHA degradation peak (≈250–300 °C) in M and KM, but a weak or absent peak in K and SM, indicating mineral-dependent polymer retention. BET, PSD, and FTIR confirmed mineral-specific interactions, with reactive soils showing pronounced surface area reduction, mesopore constriction, and stronger adsorption-related spectral features, whereas low-reactivity soils showed moderate surface modification and weaker chemical signatures. XRD indicated non-intercalative, surface-controlled interactions, while SEM revealed densified polymer-associated fabrics, consistent with adsorption-derived gels in reactive soils and surface film encapsulation in low-reactivity soils. Three stabilization pathways emerged: adsorption-derived gel network bridging and pore densification (M, KM), adsorption-limited surface gel formation and film encapsulation (K, SM), and mineralogy-dependent synergistic dual-polymer stabilization in XG_PHA. These findings identify XG_PHA as a mineral-responsive stabilizer for sustainable ground improvement.