Mechanism improvement to enhance the engineering properties of expansive soil using fly ash

Expansive soils present significant design and maintenance challenges because of their pronounced shrink-swell behaviour under changing moisture regimes. This paper investigates the mechanisms by which fly ash—an industrial by-product—improves the engineering properties of expansive soils. A comprehensive laboratory program was carried out on an expansive clay (CH) stabilised with fly ash at dosages of 0, 5, 10, 15, 20, 25 and 30% by dry weight. Tests included Atterberg limits, standard Proctor, unconfined compressive strength (UCS) at 7 and 28 days, California Bearing Ratio (CBR), and free swell index. Results, supported by microstructural interpretations, demonstrate a multimechanistic improvement in soil behaviour: particle packing and filler effects, cation exchange and flocculation reducing diffuse double-layer thickness, and long-term pozzolanic reactions creating cementitious C–S–H and C–A–H phases. Collectively, these mechanisms reduce plasticity and swelling potential while increasing strength and stiffness, with an optimal fly ash range observed around 15–20% for the synthetic dataset presented. The study also discusses the sustainability benefits of using fly ash in geotechnical practice.