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

This study presents an experimental framework to isolate intrinsic biopolymer bonding from strength gains induced by densification in stabilized high-plasticity clay. Xanthan gum (XG) and κ-carrageenan (CG) were evaluated using two preparation methods: Method A, in which biopolymer replaces an equivalent soil mass to minimize densification, and Method B, in which biopolymer is added to the original soil mass. Unconfined compressive strength (UCS), secant stiffness (E50), energy absorption capacity (EAC), curing behavior, microstructure, cost-effectiveness, and binder-related carbon footprint were assessed. Results show that densification accounts for a portion of the apparent strength gains. 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 to 1255-1305 kPa, confirming the dominant role of densification. Under Method A, E50 increased with curing but decreased with dosage, while EAC rose by 269-511% for XG and 159-235% for CG. XG exhibited stable performance under natural curing, whereas CG showed moisture-sensitive degradation. SEM revealed coating, bridging, matrix formation, and dehydration-driven structural evolution. Although cement remained cost-effective and carbon-efficient, the framework supports objective evaluation and selection of sustainable stabilizers for ground improvement applications.