Bio-based polyelectrolyte complexes for customized soil stabilization: Linking polymer adsorption to soil mechanical performance

Various chemical processes are used for soil stabilization, which aims to improve soil performance and durability. However, the variability in soil mineralogy, morphology, and composition introduces significant challenges for soil stabilization. This study evaluates two bio-based polyelectrolytes (PEs), cationic chitosan (CHI) and anionic sodium lignosulfonate (LS), and their complexes (PECs), as potential customizable, sustainable stabilizers for two soils – silty sand (SM) and lean clay (CL) containing smectites (e.g. , montmorillonite (MMT)). To determine the engineering properties of PE- and PEC-stabilized soils of different types, the unconfined compressive strength (UCS) and direct shear strength under dry and wet conditions were evaluated. These tests provide complementary performance metrics, which are directly applicable to subgrade design and offer insights into soil behavior. Individually tested LS, CHI, and CHI/LS PECs at molar ratios of 0.3 and 3.5 (in moles of polymer units), abbreviated as PEC-0.3 and PEC-3.5, showed different improvement levels in dry UCS, which were tailorable to specific soil types by a selection of specific PEs and PEC compositions. When tested in wet conditions, CHI- and PEC-stabilized soils retained their structural integrity and demonstrated superior moisture resistance (up to 68 % higher UCS retention) compared to the untreated samples. In direct shear tests, PECs outperformed individual PEs in shear strength improvement. The improvement in shear strength of the silty sand was due to an increase in cohesion, while it was due to increasing both cohesion and friction angle for the clay-rich soil. In addition, to establish correlations between PE-induced soil stabilization and binding of PEs with mineral particles, this work quantitatively assessed the adsorption of CHI, LS, and their PECs onto surfaces of the soil components (MMT and quartz, QZ) and established the molecular basis for the high effectiveness of CHI in soil stabilization. This work highlights the promise of using bio-based polyelectrolytes for soil stabilization as an eco-friendly alternative to traditional chemical stabilizers.