Bio-based modification of recycled concrete aggregate for cover systems: eco-hydro-mechanical performance evaluation

Recycled concrete aggregate (RCA), derived from construction and demolition waste, represents a promising secondary resource for sustainable waste valorization; however, its limited hydraulic performance, mechanical integrity, and ecological functionality restrict its large-scale reuse in environmental applications. In this study, biochar and gellan gum were employed to enhance RCA and develop a multifunctional cover material suitable for waste management purposes. Laboratory investigations were conducted to evaluate vegetation establishment, soil-water characteristic curves (SWCCs), saturated permeability, and shear behavior of the modified RCA. Compared with unmodified RCA, the incorporation of 5% biochar increased seed germination by approximately 183% and seedling height by about 46%, whereas excessive biochar content adversely affected vegetation growth. Combined modification resulted in an overall upward shift of the SWCC, with the saturated volumetric water content and air-entry value increasing by 59% and 300%, respectively. Meanwhile, the saturated permeability coefficient was reduced by approximately 85%, and vegetation growth further enhanced this reduction, suggesting improved resistance to water infiltration. Root reinforcement further increased shear strength and produced a more ductile shear response, as indicated by increased peak displacement, reduced post-peak stress loss, and sustained residual shear resistance. Overall, RCA amended with 5% biochar and 1% gellan gum exhibited the most balanced performance among the tested formulations in terms of vegetation establishment, hydraulic regulation, and mechanical stability. These findings suggest a promising pathway for upgrading construction and demolition waste-derived RCA into value-added cover materials, although long-term and field-scale validation remains necessary before practical implementation.