Strength improvement in biopolymer-treated clay: decoupling the role of densification and bonding

This study presents a controlled experimental framework to isolate intrinsic biopolymer bonding from strength gains induced by densification (void ratio reduction) in stabilized high-plasticity clay. Xanthan gum (XG) and κ-carrageenan (CG) were evaluated using two preparation methods: Method A, in which the biopolymer replaces an equivalent soil mass to minimize densification, and Method B, in which the biopolymer is added to the original soil mass. Unconfined compressive strength (UCS), secant stiffness ( E 50 ), energy absorption capacity (EAC), curing behavior, microstructure, cost-effectiveness, and binder-related carbon footprint were systematically assessed. Results show that densification accounts for a substantial portion of the strength gains often attributed to biopolymer stabilization. The maximum strength improvement ratio increased from 41.3% (8% CG, Method A) to 163.6% (8% XG, Method B). At 28 days and 8% dosage, UCS increased from 639–647 kPa (Method A) to 1255–1305 kPa (Method B), confirming the dominant role of densification. Under Method A, E 50 generally increased with curing but decreased with dosage, while EAC rose by 269%–511% (XG) and 159%–235% (CG) relative to untreated soil. XG exhibited stable mechanical performance under natural curing, whereas CG showed moisture-sensitive degradation. Scanning electron microscopy analysis revealed coating, bridging, matrix formation, and dehydration-driven structural evolution. Although cement remains more cost-effective and has a lower binder-related carbon footprint within the adopted boundaries, the proposed framework enables a more objective evaluation of biopolymer stabilization and supports the informed selection of sustainable ground improvement strategies.