Phosphogypsum coupled enzyme-induced calcium carbonate precipitation for soil stabilization: Mechanical performance...

Enzyme-induced carbonate precipitation (EICP) has emerged as a promising bioremediation technique for improving soil quality. However, its widespread application is constrained by reliance on expensive calcium sources (e.g., CaCl₂), which limits economic feasibility. To address this limitation, phosphogypsum (PG), a calcium sulfate-rich industrial byproduct from phosphate manufacturing, was introduced as an alternative calcium source to develop a cost-effective EICP system while achieving waste valorization. The synergistic mechanisms governing EICP‒PG interactions were investigated through a comprehensive experimental program focused on examining the effects on the mechanical and ecological performance of soil. Mechanical, microstructural, and biological characterization was conducted after 7 days of curing through unconfined compressive strength (UCS), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and vegetation growth was also assessed to evaluate the influences of PG and EICP treatment on germination and early-stage vegetation development. PG-only treated specimens exhibited UCS values of 48.8-86.58 kPa. Conversely, the synergistic EICP-PG treatment using a mechanical premixing method yielded enhanced mechanical performance across all tested urea concentrations (0.5 to 2.0 mol/L). The UCS of the treated soil increased significantly, achieving an overall range of 77.94 to 134.34 kPa (representing improvements of 21.83% to 110.00%). Polynomial regression models were developed to quantify the relationships among urea concentration, PG content, and soil strength, demonstrating high predictive accuracy. Microstructural analysis revealed that CaCO₃ crystals generated by formed interconnected networks with PG particles, resulting in a dual-cementation mechanism that substantially increased soil matrix integrity. Ecological assessments indicated that ryegrass growth was optimized under moderate treatment conditions (1.0 mol/L urea +32% PG), highlighting a favorable balance between mechanical reinforcement and biological compatibility. Overall, this study demonstrates the feasibility of utilizing industrial waste streams in bio-cementation applications and provides a sustainable strategy, thereby advancing resource-efficient and environmentally responsible geotechnical engineering practices.