Stabilization of expansive soil using alkali-activated phosphogypsum and ground granulated blast furnace slag

This study evaluates the feasibility of alkali-activated phosphogypsum (PG) and ground granulated blast furnace slag (GGBFS) as a cementitious material (PG-based alkali-activated cement, PSC) to stabilize expansive soil (ES). The effects of PSC on ES properties and the underlying mechanisms were investigated through a series of tests, including Atterberg limits test, free swell ratio test, unconfined compressive strength (UCS) test, California bearing ratio (CBR) test, water stability test, leaching toxicity test, X-ray diffraction (XRD), and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). Experiments demonstrated that PSC enhances mechanical performance compared to PG alone, achieving a maximum 28-d compressive strength of 29.6 MPa. Incorporating PSC effectively reduced swelling potential and the water adsorption capacity of ES. At a 50% PSC content, the liquid limit, plasticity index, and 28-d free swell ratio of ES decreased to 32.6%, 16.78%, and 13.2%, respectively. The stabilized ES exhibited improved mechanical strength with curing age, reaching a 28-d UCS of 4.82 MPa and a CBR of 43.42%. Additionally, PSC enhanced water resistance, yielding a 28-d water stability coefficient of 0.851. Environmental assessment confirmed the effective immobilization of hazardous constituents, indicating low leaching risks. A comprehensive performance-cost evaluation identified 40% as the optimum PSC content. Microstructural analyses revealed that the stabilization mechanism involved calcium ion-induced ion exchange, which promoted flocculation and agglomeration of soil particles. Concurrently, the alkali activation of PG and GGBFS generated hydration products (e.g., calcium silicate hydrates) that filled pores and formed a cementation matrix, further reducing soil expansivity and improving mechanical integrity. These findings highlight the potential of PSC as an effective, sustainable alternative for stabilizing expansive soils.