Microstructural Characterization of Expansive Soil Stabilized with Coconut Husk Ash: A Multi-Technique Investigation into Mineralogy, Pore Architecture, and Surface Interactions

Black cotton soil (BCS) is unsuitable for construction due to its high plasticity, low shear strength, and significant volume changes upon wetting and drying. The present study investigates the effectiveness of an alkali-activated coconut husk ash (CHA) binder in improving the geotechnical properties of BCS. CHA is derived from coconut husk and serves as a sustainable binder. Microstructural characterization of untreated and CHA-treated BCS was carried out using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Fourier transform infrared spectroscopy (FTIR). The specific surface area (SSA) and porosity were evaluated using nitrogen gas adsorption methods based on the Brunauer–Emmett–Teller (BET) and Langmuir techniques. The Barrett–Joyner–Halenda (BJH) method demonstrated a decrease in mean pore diameter from 6.7 nm to 6.2 nm following CHA treatment. The SSA diminished from 40.94 m2/g to 25.59 m2/g, signifying the development of calcium silicate hydrate (C-S-H) gels that occupied the pore spaces. The formation of pozzolanic reaction products enhanced the microstructural integrity of the treated soil. Unconfined compressive strength (UCS) test results at 24 h and 28 days of curing for CHA-treated soil have been incorporated to analyze the optimum binder content. The UCS values enhanced significantly from 182 kPa to 305 kPa and 1030 kPa, respectively, at 9% binder content after 24 h and 28 days of curing. The microstructural and mechanical strength test analysis results indicated that CHA is a feasible and environmentally sustainable substitute for BCS stabilization. CHA-based AAB will be an eco-friendly alternative to cement and lime, reducing CO2 emissions and construction costs.