Feasibility evaluation of mechanical and environmental properties for red mud based rapid setting filling support material

Red mud (RM), a solid waste byproduct of the alumina industry, has accumulated in significant global stockpiles. Currently, the primary application of RM involves magnetic separation to recover iron oxides, while the residual RM is predominantly landfilled. Due to its strong alkalinity, RM can serve as a substitute for strong alkalis or sodium salts in the alkali activation of concrete, thereby accelerating setting time and improving early-age strength. Building on this property, this study designs and develops an RM based rapid setting filling support material, primarily composed of RM, which features rapid setting and high early strength. This material is intended as a substitute for high-water-content materials in gob-side entry retaining for coal mining. Experimental results demonstrate that when RM content reaches 70% and cement content is 10%, the Rapid-Setting Filling Support Material (RSM) achieves satisfactory compressive strength and initial setting time, coupled with excellent economic performance. The compressive strengths at 1 day, 3 days, 7 days, and 28 days are 6.12 MPa, 8.97 MPa, 11.25 MPa, and 12.33 MPa, respectively, meeting all requirements specified in the 'Technical Requirements for Roadside Bag Filling for Gob-side Entry Retaining with High-Water Material' (GB/T 39336-2020). The RSM also exhibits good durability, with primary hydration products including C-S-H gel, ettringite, and calcium carbonate, all of which contribute to strength development. Leaching tests reveal that heavy metal ion concentrations in the RSM leachate comply with integrated emission standards for Class III groundwater and wastewater, confirming its safe application. Considering the annual increase of approximately 13,000 km in roadway length for coal mining in China, the demand for filling and support materials is substantial. Consequently, the findings of this study hold significant economic and environmental value, with the potential to promote green mining practices and the circular economy.