Phosphogypsum and carbide slag synergy for red mud soil stabilization: Mechanical performance, environmental impacts, and micro-scale mechanisms.

The production of phosphogypsum (PG), calcium carbide slag (CS), and red mud (RM) in global industrial development imposes serious environmental issues. Utilizing CS and PG as curing agents and incorporating RM as a soil substitute can facilitate the solid waste resource utilization. However, few studies have investigated the synergistic effects of PG and CS on the stabilization of RM and soil. To fulfill this research gap, this study explored the recycling of PG and CS for RM soil stabilization at varying RM ratio from 1:4 to 4:1. A series of tests on the stabilized RM soil composite were conducted, including the compaction test, unconfined compressive strength (UCS) test, consolidated-undrained triaxial compression test (CU), microscopic analyses (XRD and SEM), and toxicity characteristic leaching procedure (TCLP) test. The test results indicated that the maximum dry density (MDD) of the RM soil composite increased as the RM ratio increased from 1:4 to 4:1. When the PG content exceeds 8 %, the RM soil composite becomes loose, leading to a decrease in UC and ultimate deviatoricic stress. The synergistic effect of PG-CS can reduce the leaching of Cr, Cd, and Pb significantly by 54.6 %, 32.7 %, and 21.9 % respectively. When appropriate amounts of PG and CS are incorporated, considerable cementitious hydration products such as C-S-H/C-A-S-H and Aft will be generated. These hydration products can adhere to the surface of RM and soil particles or fill the gaps between particles, making the soil structure dense and stable, thereby significantly improving its mechanical performance. The optimal dosages of PG and CS were identified as 8 % and 12 %, respectively. The combined use of PG and CS is found to significantly enhance the strength of the RM-soil composite and inhibit the leaching of heavy metals, which promotes the large-scale utilization of solid waste of PG and CS.