Abstract: Red mud, the largest solid waste discharged from the non-ferrous metal industry, is characterized by high alkalinity, fine particle size, complex composition, and low comprehensive utilization rate. The preparation of red mud-based backfill materials for goaf represents an efficient technological pathway for its resource utilization, low-carbon application, and green disposal. This study used solid wastes such as Bayer red mud, fly ash, and desulfurization gypsum as raw materials to prepare red mud-based composite cementitious system backfill materials. Mechanical property tests were conducted on backfill specimens with different mix ratios. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were employed to analyze the composite synergistic effects between materials and reveal the hydration products and microstructural evolution mechanisms of the backfill. The dissolution kinetics behavior of active silicon-aluminum components in fly ash are crucial factors determining the performance of red mud-based backfill materials. Through dissolution tests, using the Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) and SEM testing techniques, the dissolution kinetic behaviors of the active SiO₂ and Al₂O₃ components in fly ash were studied in an alkaline (NaOH) solution system and an alkaline-sulfate (NaOH-Na₂SO₄) composite solution system. By referencing classical dissolution kinetic reaction rate models, the kinetic processes of active silicon-aluminum component dissolution were analyzed. The results show that: with the increase of the dosage of desulfurized gypsum, the strength of the backfilling body exhibits a trend of increasing first and then decreasing. When mass ratio of red mud, fly ash and desulfurized gypsum is 4:6:0.9, the compressive strength reaches the optimum. The values of the compressive strength at 3, 7 and 28 days are 0.53, 1.29 and 1.91 MPa respectively. Under the synergistic activation of alkali and sulfate, the backfilling bodies generated main hydration products including analcime, xonotlite, ettringite, and aluminosilicate gel at 28 days of curing, filling the pore structures between materials. The dissolution kinetics behavior of reactive SiO₂ and Al₂O₃ in fly ash exhibits stage-dependent differences in the alkaline solution system, with Si dissolution predominating in the early stage and Al dissolution becoming dominant in the later stage. In the NaOH-Na₂SO₄ composite solution, the 1D-diffusion model is applicable to the dissolution of reactive SiO₂, while both the Jander model and 1D-diffusion model better describe the dissolution of reactive Al₂O₃, thereby elucidating the dissolution mechanism of silico-aluminous species in fly ash under the synergistic activation by alkali and sulfate. The dissolution mechanism of active silicon-aluminum in fly ash under the synergistic activation of alkali and sulfate was elucidated. The research results provide a preparation route for a new type of backfilling material for the green mining of bauxite under coal seams.