Geotechnical and Microstructural Mechanisms Associated with Valorization of Waste Phosphogypsum in Lime-Treated Expansive Soil Stabilization

In the present study, phosphogypsum (PG), a by-product from the fertilizer industry, has been utilized to stabilize expansive soil. In order to enhance the pozzolanic activity of PG stabilized soil, 3% and 6% of hydrated lime have been used as a secondary additive. The lime-treated specimens were subjected to a mellowing period of 7 days. Comprehensive laboratory tests were conducted to investigate the behavior of clay treated with different proportions of PG and lime. The optimum admixture content was determined based on unconfined compressive strength results obtained at curing periods of 0, 7, 14, 28, and 56 days. The results indicate that PG-stabilized soil exhibits a progressive increase in strength up to 14 days of curing, followed by a marginal decrease in strength at 28 and 56 days. In contrast, lime-treated PG-stabilized soil demonstrates sustained improvement in strength with curing. The optimized PG and lime content were experimentally determined to be 25% and 6%, respectively, resulting in enhanced strength, reduced compressibility, and increased bearing ratio. The compression index of optimum stabilized soil decreased by approximately 10 times, whereas the California bearing ratio values increased by about 9.4 times relative to the unstabilized soil. The microlevel investigation reveals that the progressive development of ettringite minerals with time in PG-stabilized soil corroborates the adverse effects of utilizing higher doses of PG alone as a soil stabilizer. However, the lime-treated sample with a mellowing period shows well-compacted, agglomerated soil particles, and the presence of cementitious compounds affirms a stable stabilizing mechanism.