## 3 Results
### Fluoride Leaching  
Figure 6 illustrates the fluoride leaching behavior of samples with varying cement-to-sand ratios. For samples with a cement-to-sand ratio of 1:6, fluoride leaching decreased significantly with increased curing time. The most notable reduction occurred within the first 7 days, followed by a slower decline between 7 and 28 days. This pattern highlights the substantial role of early-stage hydration in reducing fluoride leaching. Samples with cement-to-sand ratios of 1:8 and 1:10 demonstrated similar trends, with steady decreases in fluoride leaching as curing time increased. However, samples with lower IFC showed a more pronounced reduction in leaching during the initial seven-day curing period. Conversely, samples with higher IFC exhibited relatively stable leaching rates over time. Overall, there is an inverse relationship between cement content and fluoride leaching: higher cement ratios lead to lower fluoride ion concentrations in the leachate. After 28 days of curing with deionized water, the fluoride concentration in the samples ranged from 1 to 2 mg/L, exceeding the Chinese Class III groundwater quality standard of 1 mg/L (GB/T14848-2017 [54]). This standard was only met when the IFC was 20 mg/L or lower. Figure 5: Response surface curve and contour graph of the UCS curing at (**a**) 14 days and (**b**) 28 days. Figure 7a,b provides a detailed comparison of leaching results for samples with IFCs of 280 mg/L and 20 mg/L across different cement-to-sand ratios. At 0 days of curing, the samples with a cement-to-sand ratio of 1:6 exhibited the highest fluoride leaching. However, after 28 days of curing, these samples leached less fluoride than those with other ratios. For the IFC of 280 mg/L, the difference in leaching among the samples diminished significantly after 14 days of curing. In contrast, for the IFC of 20 mg/L, the leaching levels of the 1:6 ratio samples approached those of other samples as early as 7 days into the curing period. Some studies suggest that Ca\({}^{2+}\) from aggregate dissolution during early hydration is the primary source of bound fluoride. The solubility difference between calcium hydroxide and calcium fluoride may explain this [55]. Samples with higher cement concentrations showed significantly more fluoride leaching initially. However, after 14-28 days of curing, the difference in fluoride leaching between samples with IFC of 280 mg/L and 20 mg/L diminished (Figure 7b). Initially, the difference in fluoride leaching rates was less than the actual difference in IFC. It is hypothesized that gypsum content primarily determines fluoride immobilization during early hydration, with further immobilization occurring as structural changes in the hydration products develop. While PCPBs with higher cement contents showed lower fluoride leaching after 28 days, those with lower cement contents had lower fluoride leaching at early hydration stages. This might be due to higher Ca\({}^{2+}\) concentrations controlling fluoride leaching early on. As hydration progresses, fluoride immobilization increases, resulting in lower leaching rates in PCPBs with higher cement content. However, the formation of insoluble calcium fluoride during hydration can impede curing due to its adsorption on the cement particle surfaces, replacing hydroxyl groups in the C-S-H gel structure [13]. Studies have shown that fluoride ions are mainly present as CaF\({}_{2}\) crystals in hydration products [56]. In summary, the fluoride immobilization in the initial stage of hydration (0 day) comes from CaF\({}_{2}\) precipitation, which is caused by the solubility difference between CaSO\({}_{4}\)-H\({}_{2}\)O (S = 1.29 \(\times\) 10\({}^{-2}\) mol/L) and CaF\({}_{2}\) (S = 2.05 \(\times\) 10\({}^{-4}\) mol/L) [5, 57]. The fluoride immobilization effect of PCPB with a higher cement content in the initial stage of hydration in the leaching results better illustrates this situation.