Synergistic utilization of red mud and solid wastes for aggregate-free composite stabilized materials: Process engineering design and safe disposal strategies

The global increase in industrial solid waste generation poses serious environmental and engineering challenges, particularly in the context of sustainable infrastructure development. This study developed a red mud-based cement-stabilized layer (RCSL) material by co-utilizing circulating fluidized bed fly ash and flue gas desulfurization residues, aiming to improve performance and promote large-scale waste utilization in road engineering. The effects of raw material, particularly the calcium-to-silicon (CaO/SiO₂) ratio, on the mechanical properties, durability, and microstructure of the RCSL were systematically investigated. Results show that the RCSL sample with a CaO/SiO₂ ratio of 0.53 yielded the optimal performance with a 7 days unconfined compressive strength of 9.4 MPa. The optimal sample also had a strength retention ratio of 91.26 %, and a frost resistance parameter of 94.7 %, which reflected its strong resistance to deformation, water-induced damage, and drying shrinkage respectively. Microstructural analysis of the optimal sample revealed the formation of a larger amount of ettringite and C-(A)-S-H gels beyond that of other samples, which contributed to its superior strength and heavy metal immobilization. After 28 days of curing, leaching concentrations of As, Cr, Ni, Cu, and Pb were all below the limits set by Class III groundwater and Class II seawater standards. Without using natural aggregates, the total solid waste content exceeded 95 wt%, with red mud accounting for over 50 wt%. This work demonstrates an effective strategy for high-volume industrial waste utilization and offers a scalable, eco-friendly solution for next-generation green infrastructure materials.