## 1 Introduction
It is mainly applied to the fields of building materials, agriculture, and the chemical industry, such as using PG as a cement retarder; to improve acidic soil [12], with the potential to increase crop yield and safety; and to prepare wall materials and road base materials. Applied to the filling field, Na(OH)\({}_{2}\), Na\({}_{2}\)SiO\({}_{3}\), or Ca(OH)\({}_{2}\) of PG adjusts the pH value of the system and promotes the reaction between Ca\({}^{2+}\) in PG and silica-aluminum materialto form C-(A)-S-H gel, and the acidity of PG will slow down the hydration reaction of cement [13]. However, the \"disadvantages\" of PG are transformed into advantages in red mud filling, and the acidity of PG can neutralize the high alkalinity of red mud; the slow-setting property of PG can avoid premature solidification of RM slurry in the process of pumping and reduce the risk of pipeline clogging; at the same time, the coarser particles of PG are mixed with the fine particles of red mud, and the gradation is complementary so that the densification of the filling body is improved [14]. At the same time, the slurry concentration is one of the core parameters of the filling process, which directly affects the delivery efficiency of the filling body, the strength of the filling body, and cost control. Low concentration (<65%): slurry flow state close to Newtonian fluid, pipeline strength is small but the risk of solid-phase settling is high, and it is easy to lead to pipe plugging. High concentration (>75%): viscosity rises sharply, pumping pressure needs to be increased, coarse particles (such as PG) may exacerbate pipeline wear, and has a concentration of 70% when the wear rate is higher than 60%; with a concentration increase of 5%, the pumping energy consumption increases by 10-15% but can reduce the cost of dewatering and the amount of cement, and the overall benefits need to be weighed; traditional tailings filling recommended a concentration of 65-75%, except for the fine particles of red mud, etc. The concentration is recommended to be 65-75% for traditional tailing sand filling, but fine particles, such as red mud and other solid wastes, need a lower concentration to ensure mobility due to high viscosity; the concentration should be optimized in synergy with aggregate gradation and cementitious material type to improve the red mud-PG compound system due to gradation improvement. A systematic summary of the mechanisms linking slurry consistency, additives, and filling performance is given in [15], focusing on the latest research advances in red mud-based filling materials, including strategies for consistency optimization [6]. Based on this, we propose using PG to alleviate the role of RM-based cemented paste backfill due to RM's high alkalinity and high viscosity, resulting in construction difficulties and environmental risks, and PG's acidic, slow-setting, and coarse aggregate properties that can be targeted to alleviate the above problems to determine the optimal mixing concentration interval of PG and RM, balancing the environmental protection, construction, and strength requirements, so that the mechanical properties of RCPB meet the requirements of mining [16]; the high alkalinity of RM can be mitigated; the hydration process of RCPB can be investigated from a microscopic point of view, and the mechanism of F\({}^{-}\) fixation can be elucidated; the fluidity of mixed slurry can be moderately high, the coagulation time can be controlled, and the pipeline conveying strength is low; the leaching of hazardous ions (F\({}^{-}\)) can reach the national standard (<1.0 mg/L); and the optimal concentration of the PG-RM composite filling can be proposed to satisfy the practical requirements of the production. The optimal concentration ratio of the PG-RM compound filling body was proposed to meet the actual production demand.