Hybrid Sticky Rice-Xanthan Gum Biopolymer Stabilization of Clayey Sand: Complementary Hydrophobic and Gel-Bridging Mechanisms Under Wet-Dry and Freeze-Thaw Cycling

The development of sustainable ground improvement techniques for transportation infrastructure has motivated increasing interest in biopolymer-based soil stabilization. However, hydrophilic biopolymers show limited durability under cyclic wet-dry and freeze-thaw exposure, a key constraint for cold-region subgrade applications. This study investigates the performance of a clayey sand stabilized with hybrid combinations of xanthan gum (XG) and sticky rice (SR). SR is an amylopectin-rich natural starch with hydrophobic surface characteristics. Performance was quantified using unconfined compressive strength (UCS), ultrasonic pulse velocity (UPV), water absorption, wet-dry and freeze-thaw resistance, and SEM-EDS microstructural analysis. Surface wettability decreased systematically as SR content increased, with contact angles rising from 66° (XG100) to 155° (SR100), indicating reduced susceptibility to moisture ingress. Under wet-dry conditions, XG100 failed during the initial wetting phase, whereas hybrid mixtures exhibited improved durability, with SR60-XG40 showing the highest resistance with a cumulative mass loss of only 2.25%. Under freeze-thaw exposure, a non-monotonic performance trend was observed, with SR60-XG40 retaining 94.5% of its initial UCS after 12 cycles, showing the highest UCS retention among hybrid ratios. SEM-EDS analysis revealed that SR60-XG40 developed the most continuous composite binder matrix, evidenced by oxygen enrichment and silicon signal. UPV measurements indicated progressive stiffness degradation under both exposures, broadly consistent with UCS-based trends. Overall, SR60-XG40 emerges as a favorable dual-mechanism formulation for cyclic durability in SC soil at 1% dosage.