Enhancing mechanical performance of red soils via lime–phosphogypsum stabilization: A Box–Behnken design approach

This study evaluates the feasibility of storing phosphogypsum (PG) on lime-stabilized red soils (RS) and quantifies the synergistic stabilization capacity of PG-hydraulic lime (L) blends. Mortar specimens with variable RS/L/PG ratios underwent comprehensive physicochemical (pH, electrical conductivity [EC], X-ray fluorescence [XRF]), geotechnical (Atterberg limits), mineralogical (X-ray diffraction [XRD], Fourier transform infrared [FTIR]), microstructural (scanning electron microscopy [SEM]/energy dispersive spectroscopy [EDS]), thermogravimetric (differential thermal analysis coupled with thermogravimetric analysis [DTA-TG]), and mechanical (unconfined compressive strength [UCS]) characterization. Box–Behnken design (BBD) was applied to delineate the influence of varying proportions of RS, L, and PG on the mechanical performance of stabilized soil composites. The results establish that 10 wt% L with ≤32 wt% PG significantly enhances soil performance. The UCS increased from 1.67 MPa (RS + 2%L) to 4.48 MPa (RS + 10%L + 32%PG), and the plasticity index decreased from 17.47% (untreated RS) to 12.64% (RS + 10%L + 10%PG). Critically, PG addition did not induce ettringite formation despite available sulfate ions (SO₄²⁻), aluminol/silicate groups, Ca²⁺, and OH⁻ ions, eliminating the risks of sulfate-induced expansion. Scanning electron microscopy (SEM) revealed rod-shaped gypsum microcrystals (CaSO₄·2H₂O) on particle surfaces, accelerating hydration kinetics and strengthening mechanical performance through microstructural densification. This study establishes PG as a sustainable co-additive that concurrently mitigates industrial waste liabilities and enhances geotechnical performance in marginal red soils. Component synergies rigorously quantified via BBD provide a mechanistic blueprint for eco-engineered infrastructure and circular waste management strategies.