Hydrothermal valorization of fly ash for sustainable clay soil stabilization: Linking green cementitious chemistry with micromechanical performance

Sustainable use of industrial by‐products is essential to advancing circular economy strategies and reducing reliance on conventional cementitious binders. This study investigates the hydrothermal valorization of coal fly ash activated with sodium hydroxide (NaOH) for the sustainable stabilization of clay soils. Four natural clays with liquid limits of 35.8–63.7% were treated with 15 wt% fly ash and 3 wt% NaOH and cured under conventional, thermal, and hydrothermal conditions (180 and 250°C). Engineering performance was evaluated using Atterberg limits, compaction, unconfined compressive strength (UCS), consolidation, leaching resistance, X‐ray fluorescence (XRF), thermogravimetric analysis (TGA), and micromechanical fracture parameters. Hydrothermal treatment at 250°C produced the highest UCS (1707 kPa) while reducing plasticity, compressibility, and void ratio. Microstructural analyses confirmed the formation of C–S–H and C–A–S–H gels responsible for enhanced strength and durability. The proposed waste‐to‐resource approach offers a sustainable alternative to Portland cement for environmentally responsible ground improvement and supports circular resource utilization in geotechnical engineering.