This study developed a novel geopolymer (RM-SGP) using industrial solid wastes red mud and slag activated by sodium silicate, aiming to remediate composite heavy metal contaminated soil. The effects of aluminosilicate component dosage, alkali equivalent, and heavy metal concentration on the unconfined compressive strength (UCS), toxicity leaching characteristics, resistivity, pH, and electrical conductivity (EC) of RM-SGP solidified composite heavy metal contaminated soil were systematically investigated. Additionally, the chemical composition and microstructural characteristics of solidified soil were analyzed using XRD, FTIR, SEM, and NMR tests to elucidate the solidification mechanisms. The results demonstrated that RM-SGP exhibited excellent solidification efficacy for composite heavy metal contaminated soil. Optimal performance occurred at 15 % aluminosilicate component dosage and 16 % alkali equivalent, achieving UCS >350 kPa and compliant heavy metal leaching (excluding Cd in high-concentration groups). Acid/alkaline leaching tests revealed distinct metal behaviors: Cu/Cd decreased progressively, while Pb initially declined then rebounded. Microstructural analysis indicated that RM-SGP generated abundant hydration products (e.g., C-A-S-H, N-A-S-H gels), which acted as cementitious substances wrapping soil particles and filling and connecting pores, thereby increasing the soil's compactness and improving the solidification effect. Furthermore, heavy metal ions were solidified through adsorption, encapsulation, precipitation, ion exchange, and covalent bond et al., transforming their active states into less bioavailable forms, proving novel insights into the remediation of composite heavy metal contaminated soils and the resource utilization of industrial solid wastes.

The preparation of geopolymer for solidification/stabilization of heavy metal contaminated soils using industrial solid waste was a sustainable method. In this study, a binary geopolymer curing agent was synthesized from red mud and fly ash for the treatment of copper- and cadmium- contaminated soils. The changes in the properties of the cured soil were investigated by analyzing compressive strength, permeability coefficient, pH value, toxicity leaching, and the chemical forms of heavy metals. These parameters were examined under varying amounts of curing agent and curing time. The solidification mechanism of contaminated soil was revealed by microscopic experiments such as X-ray diffraction (XRD), infrared spectroscopy (FTIR), scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS). The results showed that geopolymer could significantly improved the mechanical properties and environmental safety of contaminated soil. Compressive strengths of Cu and Cd contaminated soils after 28d of curing with 30 % geopolymer were 1.27 and 1.44 MPa, the permeability coefficients were 4.2 and 3.8-6cm/s, and toxic leaching amounts of Cu2+ and Cd2+ were 4.8 and 0.21 mg/L, and pH values were 10.9 and 10.6, respectively. Geopolymer gel structures not only filled the voids between soil particles but also physically encapsulated, chemically bonded, precipitated and ion-exchanged to achieve solidification/stabilization of contaminated soils. This research provided a new technology for the management of heavy metal contaminated soil and promoted the sustainable use of industrial solid waste.