Synergistic Stabilization of Aeolian Sand Using Phosphogypsum-Based Materials: Mechanical Performance and Microstructural Evolution

Large-scale desert ecological restoration is constrained by three coupled problems: aeolian sand aggregates poorly and cannot sustain vegetation; stockpiled phosphogypsum imposes a persistent environmental burden; and importing and ameliorating exogenous topsoil is costly. This study evaluated wet-ground modified phosphogypsum (PG-A), as-received phosphogypsum used as aggregate (PG-B), aeolian sand (AS), and phosphogypsum-based supersulfated cement (PPSC). The liquid–plastic limit, compaction, direct-shear, unconfined compressive strength (UCS), and leaching tests were performed. The plasticity index of the PG-B-AS mixtures ranged from 6.62 to 12.31, and the maximum dry density increased with the AS fraction and with the PPSC content under the tested conditions. For PG-B:AS mass ratios from 1:3 to 3:1, the leachate pH ranged from 6.79 to 7.79. When AS constituted at least 25% of the aggregate, soluble phosphorus met the Class V limit of GB 3838-2002, fluoride met the Class I limit, and the phosphorus stabilization efficiency exceeded 93.5%, reaching 96.9% for GL-16 (PG-B:AS = 25:75, 15% PPSC). Room-temperature 14- and 28-day UCS values ranged from 0.602 to 1.950 MPa and from 0.853 to 2.250 MPa, respectively. The mechanical performance improved with a higher PG-B fraction, whereas the environmental fixation improved in the opposite direction. The XRD and SEM revealed quartz, gypsum, ettringite, and C-S-H development during curing. Within the tested range, PG-B:AS = 25:75 with 12–15% PPSC provided the clearest balance between leachate compliance and retained mechanical improvement.