## 1 Introduction  
Red mud (RM) is a highly alkaline solid waste associated with the production of alumina by the Bayer or sintering process, and its output is 1-2 times that of alumina production [1,2]. Traditional disposal methods are limited by their strong alkalinity (pH10-13), complex composition, and high cost of neutralization, while the application in the field of building materials also faces technical bottlenecks. In recent years, the resource utilization of red mud has gradually become a research hotspot in which the mine-filling field shows significant advantages: From the geographical distribution point of view [3], alumina plants are adjacent to mines, which can realize the short-distance transportation of red mud; from the analysis of material properties, red mud is rich in silicate, aluminate, and other active ingredients, which can play the role of cementing in the filling system, partially replacing the cement to enhance the strength of the filling body [4]. At the same time, the iron and aluminum oxides contained in red mud can stimulate geopolymerization under alkaline conditions, forming a structure with self-hardening characteristics; from the engineering economic point of view, the annual average demand of hundreds of thousands of tons of materials for mine filling and the characteristics of the huge amount of red mud stockpile are highly compatible [5], providing a feasible path for large-scale consumption. The research in this direction not only reduces the cost of mine cementation and filling by 15-30%, but also opens up a new way to utilize red mud in large quantities, and the relevant results have been verified in a number of bauxite mine synergistic bases [6]. Although the application of RM in mine filling shows the potential for resource utilization, its highly alkaline characteristics still bring multiple technical challenges: On the one hand, the alkaline environment will inhibit the hydration reaction of the cementing system, leading to the lag in the early development of the strength of the filling body [7]; on the other hand, the highly alkaline slurry is prone to corrosion of the metal pipeline and equipment, and the fine-grained red mud is prone to flocculation due to its high surface charge, leading to a slurry viscosity that is too large, and pumping strength increases significantly. In order to solve the above problems, scholars proposed to introduce an acidic solid waste synergistic regulation strategy, such as phosphobgypsum (PG) [8]. As a by-product of the phosphoric acid industry, PG is not only rich in free acid and sulfate, which can effectively neutralize the alkalinity of red mud [9], but also its coarser particle characteristics (typical particle size of 0.1-0.5 mm) can form a complementary gradation with the red mud to optimize the microstructure of the filling body. Red mud with PG at a 1:1 mass ratio reduced the pH value of a mixed system from 12.3 to 8.5, and the synergistic effect of free Ca\({}^{2+}\) and SO\({}_{4}\)\({}^{2-}\) activated the dissolution of the silica-alumina phase of the red mud, which led to an increase of 40% in the generation of the hydration products of calichealumite (AFt) and C-S-H gel [10], and the 28-day compressive strength reached 9.8 MPa, which was 2.3 times higher than that of the pure red mud system [11]. However, PG also has the problem of solid waste utilization, and PG's global annual production is about 280 million tons (China accounted for 75 million tons); at the same time, fluoride-containing compounds (e.g., CaF\({}_{2}\)) [6] are easily soluble in water, and seepage into the groundwater leads to fluorine exceeding the standard, which triggers the fluorine bone disease of humans and animals and the wilting of plants; PG contains sulfate SO\({}_{4}\)\({}^{2-}\), with a pH of about 3-5, which pollutes the surrounding water bodies and soils. Therefore, the high-value utilization of PG is the key direction to solve its environmental problems.