## 4 Conclusions  
This study aimed to explore the relationship between cement hydration and fluorine immobilization to better understand how fluorine content in PG affects fluorine leaching in PCPB. These findings will form a theoretical basis for the water washing pretreatment of PG. Based on various tests and analyses, the following preliminary conclusions were drawn:  
* The IFC inversely affected the strength of the CPB. The optimal strength at 28 days was achieved with a grey-to-sand ratio of 1:6.9 and an IFC of 0.009 wt% (20 mg/L), as described by the equation: \[U_{28}=-0.894+32.256C-0.01I-108.074C^{2}+4.546\times 10^{-5}I^{2}+0.007 CI,\ R^{2}=0.955\]
* Higher cement contents led to poorer leaching performance during the early stages of hydration. The minimum leaching amount at 28 days was observed at a grey-to-sand ratio of 1:6 and an IFC of 0.009 wt%, meeting the leaching criterion of less than 1 mg/L. The leaching amount is described by the formula: \[L_{28}=2.887-30.064C+0.012I+110.26C^{2}+1.298\times 10^{-5}I^{2}-0.048 CI,\ R^{2}=0.937\]
* XRD analysis showed that fluorine affects the formation of C-S-H gels and hinders the reaction of C\({}_{2}\)S and C\({}_{3}\)S. TG-DTG analyses indicated that fluorine also impacts the production of CH, which are primary factors influencing the strength of PCPB.
* SEM results confirmed the presence of hydration products and supported the strength analysis. Some granular crystals on the surface of the hydration product structure were identified as potential calcium fluoride crystals, which may be integrated within gypsum crystals or absorbed onto the hydration product structure.
* The analysis of the fluorine immobilization based on the results of the microtest and strength explains that, in the initial stage of hydration, the immobilization of fluorine is led by CaF\({}_{2}\) precipitation. As the hydration proceeds, physical encapsulation is gradually formed and ion exchange occurs in its hydration products to achieve chemical adsorption and the production of some complexes, ultimately achieving fluorine immobilization.  
Figure 13: Cement hydration immobilization mechanism.  
While replacing PG with PPG can reduce the impact of impurities, a performance gap remains between pure substances and technical grade PG. The process of PG washing has matured, and related scholars have carried out a lot of related research. Future research should focus on identifying the specific form of fluoride involved in the cement hydration process at the microscopic level, providing further guidance for PG water washing pretreatment processes.  
Conceptualization, Q.Z. and Q.C.; methodology, D.W.; validation, Y.Y., B.L., and D.W.; formal analysis, Y.F.; investigation, Y.Y.; resources, Y.Y.; data curation, Y.Y.; writing--original draft preparation, Y.Y.; writing--review and editing, Q.C. and D.W.; visualization, B.L.; supervision, Z.S.; project administration, Q.Z.; funding acquisition, Q.C. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (nos. 52274151, 552104156, 52074351, and 52004330), the Science and Technology Innovation Program of Hunan Province (no.2021 RC3125), and the 14 th Five Years Key Programs for Science and Technology Development of China (no. 2021 YFC2900400).  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.  
The authors declare no conflicts of interest.