Durability and environmental performance of Bayer red mud-coal gangue-based road base material

Red mud is the solid waste residue of bauxite ore digestion with caustic soda for alumina (Al₂O₃) production. China is currently the biggest producer of alumina worldwide. However, the comprehensive utilization rate of red mud is very low. Moreover, coal gangue is also a complex industrial solid waste, which is one of the largest in China. In this study, red mud, coal gangue, fly ash, and other industrial solid wastes were used to prepare a road base material in a laboratory to solve the environmental issues caused by these solid wastes. The mechanical and environmental properties and durability of Bayer red mud-coal gangue-based road base materials were investigated. Accordingly, 20 drying and wetting cycles and five freezing and thawing cycles were used to study the durability of the road base material. An inductive coupled plasma emission spectrometer was adopted to analyze the leaching properties of the road base material, while scanning electron microscopy-energy-dispersive X-ray spectroscopy techniques were used to investigate its element distribution. The results show that the unconfined compressive strength of the road base material, which cures for 7 days, is over 6 MPa and meets the strength requirement of the Chinese national standard when 75% red mud-coal gangue and 97% total solid wastes are added to the road base material. The strength of the road base material reaches 5.98 MPa after 20 drying and wetting cycles, thereby meeting the Chinese national standard. The strength still reaches 6.89 MPa after five freezing and thawing cycles. The leaching results indicate that the Na⁺ content in leachate is much lower than that required by the Chinese national standard for drinking water quality. Meanwhile, the radioactivity results of the pure red mud used herein meet the Chinese national standard for outdoor building materials. The energy spectrum and the mechanism of the alkali fixation results show that the silicon aluminum system could be more effective in consolidating Na⁺. The silicon and aluminum contents in the red mud-coal gangue-based road base material are relatively high, explaining the better Na⁺ consolidation. The results show good environmental and social benefits that can provide more options for road base materials.