Utilization of red mud and coal gangue for underground backfill material: Hydration and environmental characteristics

Abstract

The large-scale accumulation of industrial solid waste, including red mud and coal gangue, coupled with goafs left by underground mining activities, poses significant challenges to sustainable human development. In this study, red mud, coal gangue, and other solid wastes were used to prepare underground backfilling materials. The utilization rate of the total solid waste reached 95%, with red mud accounting for approximately 40wt% of the total. The unconfined compressive strength, setting time, and slump tests were conducted to evaluate the mechanical properties of the material. At the optimal ratio, the 7- and 28-d strengths reach 4.4 and 6.9 MPa, respectively. The initial and final setting times were 200 and 250 min, respectively, whereas the initial and 1-h slump exceed 250 and 210 mm, respectively. X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) were employed to explore the microstructure, phase composition, and chemical bonding within the material. Needle-like, clustered, and granular hydration products were observed, and the primary crystalline structures were identified as ettringite, gmelinite, C–A–S–H, and C–S–H. In addition, a thorough environmental risk assessment was conducted, complemented by detailed economic cost and carbon emission calculations. During the creation of backfill material, hazardous elements from solid waste are immobilized through adsorption, precipitation, and incorporation into the crystal lattice. The immobilization efficiencies for Ni, Al, Cr⁶⁺, and As were 97.03%, 94.32%, 86.43%, and 84.22%, respectively, at a pH of 8.49. Moreover, the use of solid waste as a raw material results in considerable cost savings and marked reduction in carbon emissions. This study innovatively promotes the green cycle of alumina production in the bauxite mining industry.

Keywords

red mud / underground backfill / solid waste / hazardous elements / Engineering / Materials Engineering / Resources Engineering and Extractive Metallurgy

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