Investigation on Macroscopic and Microscopic Properties and Application of Straw Fiber-Reinforced Red Mud Unfired Bricks

To address the environmental hazards caused by massive bauxite red mud stockpiles, phase-change unburned bricks have been developed. The slump flow and initial setting time of fresh mortar were tested, while the flexural strength, compressive strength, splitting tensile strength and rebound hardness of hardened mortar specimens were measured. The synergistic influence of stearic acid on mechanical strengths, rebound hardness and thermal conductivity was revealed, and the corresponding indoor simulation tests were performed. X-ray diffraction (XRD), scanning electron microscopy (SEM) and Ultra-depth-of-field microscope cross-section scanning were adopted to interpret the intrinsic microstructure and inner mechanism. Results indicate that slump flow, initial setting time and all mechanical indices follow cubic functional relationships with red mud mass ratio. Specimens incorporating 20 wt.% red mud achieve the optimal mechanical strength, rebound hardness and thermal conductivity, with the maximum growth rates of up to 32.7%, 18.7% and 27.0%, respectively. Appropriately, straw fibers improve the mechanical properties and rebound hardness yet reduce thermal conductivity. In simulated thermal cabin tests, wall temperature continuously rises under heating and declines after heat termination; red mud and straw fibers jointly slow the heating-up rate and post-heating cooling rate. Samples with 5 wt.% red mud possess the densest hydration matrix. Red mud promotes the generation of ettringite (AFt), calcium carbonate and dolomite crystals, and elevates the content of dicalcium silicate (C2S) within the binder system. This study provides a reference for fabricating functional wall materials using industrial solid waste (red mud) and agricultural solid waste (straw fibers).