Development of Red Mud-Based Binder and Experimental Study on Solidifying Lead-Contaminated Soil

In environmental geotechnical engineering, the use of red-mud-based binders for the remediation of heavy-metal-contaminated soils can yield significant economic and environmental benefits. Three types of red mud-based binders were synthesized with 30% red mud and 40% slag/fly ash/steel slag, by adjusting the ratios of carbide slag and phosphogypsum. Their mechanical properties and micro-mechanisms were investigated. On this basis, the developed materials were further applied to stabilize Pb-contaminated soils, and the mechanical performance and leaching toxicity of the stabilized soils were evaluated. The results show that: the mortar prepared with 30% red mud, 40% slag, 20% carbide slag, and 10% phosphogypsum exhibited the best strength performance, achieving an unconfined compressive strength of 23.8 MPa after 28 days of curing; excessive phosphogypsum induces excessive formation of ettringite, which triggers pronounced expansive cracking and consequently leads to a reduction in mechanical strength; compared with untreated contaminated soil, the stabilized soil exhibited a significant improvement in unconfined compressive strength from 0.28 MPa to 5.22 MPa, while the Pb²⁺ leaching concentration decreased from 3.83 mg/L to 0.34 mg/L.