## 1 Introduction  
Phosphogypsum (PG) is a by-product of the industrial production of phosphoric acid using wet acid methods [1]. Annually, nearly 300 million tons of PG are produced worldwide, with a global accumulation exceeding 7 billion tons [2, 3]. Despite a recycling rate of approximately 40%, PG is primarily utilized in construction materials and agriculture [4]. Most PG is disposed of in dry or wet stockpiles or discharged into coastal waters without any treatment [5, 6, 7]. The negative impacts of PG stockpiles can be found in groundwater, surface water, and soil and vegetation contamination [8, 9]. These hazard caused by contamination may be come from the PG leachate [10]. Research into PG recycling has intensified, particularly in using PG-based cemented paste backfill (PCPB) for mining voids. This method involves mixing PG with cementitious materials and water to form a slurry suitable for backfill, providing a large-scale disposal solution for PG [11, 12]. However, PG contains impurities, such as heavy metals (e.g., Sr, Ba, Cu, and Cd), hydrofluoric acid (HF), phosphoric acid (H3 PO4), fluoride, and radioactive elements (226 Ra, 238U) [1, 6, 9, 13, 14, 15, 16]. While ordinary Portland cement (OPC) can immobilize many hazardous substances [17, 18, 19], low OPC proportions in PCPB can increase fluoride concentrations in the leachate, sometimes exceeding safety standards [20, 21, 22]. Higher OPC contents do not resolve fluoride contamination and are economically impractical. Therefore, it is crucial to identify methods to control hazardous substances in PG, particularly fluoride, to promote sustainable remediation and address PCPB's fluorine contamination issues. Studies have shown that using different admixtures can supplement immobilization [23; 24; 25; 26; 27]. For instance, lime putty from PG can reduce setting time and improve PCPB strength [28]. [PERSON] et al. found that incorporating polyaluminum chloride (PAC) into PCPB reduces fluoride ion concentration in the leachate and enhances early hydration rates [29; 30; 31]. Several studies have validated the potential of pretreating solid waste to enhance its cementation properties [32; 33; 34]. [PERSON] et al. proposed using calcium phosphate-magnesium ammonium phosphate (CP-MAP) and solid waste micropowder to control fluoride content through the formation of magnesium fluoride (MgF\({}_{2}\)) and calcium fluoride (CaF\({}_{2}\)) [35]. Despite their effectiveness, the high cost of these materials is a concern. The pretreatment of PG, particularly through water washing, presents a more economically favorable alternative. Although, there are many other PG pretreatment methods, such as CaO neutralization modification [36], calcination [37], flotation purification [38], and ball milling [39], water washing is common and cost-effective, controlling the pH of the supernatant to meet national standards [40; 41]. However, water consumption can impact economic viability. Base washing provides an alkaline environment conducive to cement hydration and fluorine retention with calcium silicate hydrate (C-S-H) gels but can negatively affect PCPB's unconfined compressive strength (UCS) [42; 43; 44]. Balancing mechanical properties and environmental friendliness remains a challenge [45; 46]. Water washing, despite its raw material cost advantage, requires further investigation to understand its effects on PG pretreatment fully. Excessive washing can weaken PCPB and increase costs. Figure 1a shows the ideal PG water washing cycle. Fluoride in PG does not need complete elimination; reducing it to levels manageable by cement immobilization can lower production costs significantly. In addition, the water recycling process can also recycle fluoride. The establishment of standards for water washing pretreatment and the exploration of the mechanism of fluorine immobilization will become the future direction of this research field. Figure 1b illustrates the optimization of the PCPB process chain.