## 3 Results
### Mechanism of Zeolite Powder Synergized with CaO for Immobilization of Pollutants in PG  
The original PG sample exhibited a characteristic tetragonal rhombic structure (Figure 3a), which is typical of PG. This structure is formed during the production process when PG crystals stack in horizontal layers. While this arrangement imparts some structural integrity to the material, it significantly restricts the flow of interstitial liquids. This limitation hinders the efficient removal of soluble impurities, such as phosphorus and fluorine, through traditional methods such as water washing. Furthermore, PG contains a large number of free hydroxyl groups on its surface, which enhance its reactivity but also promote the agglomeration of particles, especially in the presence of high impurity concentrations. This agglomeration results in a more heterogeneous material, where particles tend to cluster together, leading to an uneven distribution of the impurities throughout the sample. Our microscopic observation of the PG structure revealed that it predominantly consisted of plate-like crystals, which form a spatial network through weak physical interactions, such as van der Waals forces, at their contact points. The weak intercrystal bonding and low compaction of PG contribute to its high porosity, which allows for fluid circulation and pollutant retention. However, this also makes PG susceptible to stress concentration when subjected to external forces, leading to material fracture and reduced mechanical strength. The low compaction and large pore size further contribute to poor water resistance and make it prone to rapid erosion and degradation, especially under wet conditions. CaO has been demonstrated to effectively react with soluble phosphorus and fluorine impurities in PG, converting them into insoluble phosphorus and fluorine precipitates. This chemical reaction occurs during the solid-phase milling process, where it interacts with the impurities, facilitating the formation of stable compounds such as calcium phosphate (Ca\({}_{3}\)(PO\({}_{4}\))\({}_{2}\)) and calcium fluoride (CaF\({}_{2}\)). However, while this chemical modification is effective in reducing the solubility of these pollutants, it does not significantly alter the overall microscopic morphology of PG (Figure 3b). After treatment, the PG particles largely retain their characteristic rhombohedral plate-like shape. Some fine particles, likely consisting of the newly formed insoluble precipitates, can be observed to be dispersed around the edges of the PG crystals but the core structure remains largely unchanged. In contrast, when PG is treated with zeolite powder, a noticeable change in the microstructure can be observed. Zeolite particles appear to surround the PG crystals, as the zeolite physically adsorbs soluble impurities, such as phosphorus and fluorine, from the surface of the PG particles (Figure 3c). When both zeolite powder and CaO are used in combination to treat PG, a more complex morphological transformation occurs (Figure 3d). The crystals of PG exhibit fine cracks, suggesting that both the physical adsorption by the zeolite and the chemical reaction with CaO contribute synergistically to the immobilization of soluble impurities. This dual approach of physical adsorption coupled with chemical modification not only reduces the mobility of pollutants but also alters the microstructure of the PG in a way that enhances its environmental stability and suitability for further applications. The treatment of PG using zeolite powder and CaO does not significantly alter the overall crystal structure of the PG, as confirmed by our XRD analysis (Figure 4). The XRD patterns of the treated PG samples closely resemble those of the untreated PG, indicating that the primary crystalline phases remain intact after treatment. This observation is consistent with our SEM imaging, which showed no drastic changes in the morphology of the PG particles upon treatment. In the XRD spectra, several characteristic peaks were identified.