Bio-mediated ternary stabilization of an expansive clay matrix using waste eggshell powder, xanthan gum, and cotton fibers: Mechanical behavior, swelling control, and microstructural evolution

Expansive clay soils utilized in compressed earthen construction often exhibit low strength, severe volumetric swelling, and high moisture sensitivity, limiting their structural utility in sustainable building practices. Although conventional stabilizers such as cement and lime improve engineering properties, their extensive application increases carbon emissions and environmental impacts. This study investigates the stepwise bio-mediated stabilization of an expansive clay matrix using waste eggshell powder (ESP), xanthan gum (XG) biopolymer, and recycled cotton fibers (CF) as a foundational material-scale evaluation for sustainable earth building applications. Initially, standardized cylindrical specimens containing 1.5%, 3%, and 4.5% ESP were prepared and cured for 1, 7, 14, 28, and 56 days to identify the optimum mineral baseline. Using the selected 3% ESP content, XG was incorporated at 1%, 1.5%, and 2%, followed by CF additions of 0.2%, 0.4%, and 0.6% to systematically address matrix brittleness. Performance was assessed through standard Proctor compaction, unconfined compressive strength (UCS), indirect tensile strength (ITS), ultrasonic pulse velocity (UPV), and free swell tests. Microstructural evolution was characterized via SEM, EDS, and XRD. Results indicated that 3% ESP provided optimum calcium-mediated particle flocculation, while 1.5% XG produced substantial cohesive gain via hydrogel pore-bridging. Incorporating 0.4% CF maximized tensile capacity and post-peak ductility, whereas higher fiber contents reduced efficiency due to fiber clustering. The optimized ESP–XG–CF system enhanced short-term compressive and tensile strength, elastic stiffness, and UPV while reducing free volumetric swell from 18.4% to below 2.5%. Microstructural observations confirmed a multi-scale stabilization mechanism governed by calcium-induced aggregation, biopolymer matrix webbing, and tensile fiber bridging, establishing the bio-mediated ternary composite as an effective, zero-cement matrix for compressed earth applications.