Study on the solidification/stabilization of cadmium-contaminated soil by red mud-assisted blast furnace slag under excitation conditions

In the context of contaminated soils and industrial solid waste utilizations, a low-carbon curing agent (RAS) was synthesized in this study using red mud (RM), blast furnace slag (GGBS), calcium carbide residue (CCR), and phosphogypsum (PG) for solidification/stabilization (S/S) of cadmium (Cd)-contaminated soils. The effects of Cd contents, RAS dosages, and curing times on the strength and leaching properties of the solidified/stabilized soil were investigated in this study. In addition, the main mechanisms governing the RAS-based S/S of the Cd-contaminated soil were revealed using an improved four-step extraction method (BCR), mercury intrusion porosimetry (MIP), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The results showed that the RAS binder significantly enhanced the compressive strength and Cd chemical stability of the Cd-contaminated soil, thereby reducing Cd2+ leaching amounts and soil porosity. The soil strength and Cd2+ leaching amounts from the solidified/stabilized soil met their corresponding standards after 14 days of curing. The main products of the solidified/stabilized soil consisted of Cd precipitates, ettringite (AFt), and calcium silicate hydrate (C–S–H). Their contents were related to the initial soil Cd content, RAS dosage, and curing time. The Cd precipitate, C–S–H, and AFt products can not only fill inter-particle voids of the soil but also exhibit physical encapsulation, surface complexation, co-precipitation, electrostatic adsorption, and ion exchange effects on Cd. The results of this study can provide a novel approach for treating and utilizing solid wastes and an environmentally friendly, economical, and efficient curing agent for the remediation of Cd-contaminated soils.