Ferrochrome slag: A critical review of its properties, environmental issues and sustainable utilization

Ferrochrome slag (FCS) is a by-product of ferrochrome industries and is produced during the extraction of ferrochrome from chromite ore. The chemical composition of FCS comprises of 27-33% SiO₂, 15-25% Al₂O₃, 20-35% MgO, and 10-15% iron-chromium compounds. The high chromium content of FCS and the possibility of its leaching into the environment categorize FCS as hazardous waste material. For each ton of ferrochrome production, nearly 1.2-1.5 tons of FCS is generated, which becomes a significant challenge for the ferrochrome producers while managing this hazardous waste. Therefore, several research attempts have been made to observe the leaching characteristics of chromium (VI) in FCS, its stabilization, and subsequent potential utilization. The high mechanical properties of FCS have led many researchers worldwide to utilize it as a construction material. This review work has undertaken FCS's physical, chemical, and microstructural characteristics and its following utilization as a fine and coarse aggregate in producing green and sustainable concrete. Different methods of stabilizing chromium (VI), including the physical, chemical, and biological methods, are extensively discussed in this review. This article also accommodated FCS as a precursor material in geopolymer and alkali-activated binders. However, the compressive strength achieved with FCS as a binder in geopolymer is very low, and thus more studies are needed to establish the possibility of strength enhancement. The leaching aspects of geopolymers with FCS also need to be studied extensively for their successive application. Lastly, the conclusions and discussion of this study have keenly addressed the significant challenges to the safe utilization of FCS in construction applications. Also, it deliberates on how the emerging research on FCS, such as refractory, composites, and coating material, can be new avenues for its utilization without any potential threat to the environment.