## 3 Results
### Mechanism of Zeolite Powder Synergized with CaO for Immobilization of Pollutants in PG
The peaks at 11.52\({}^{\circ}\), 20.64\({}^{\circ}\), 23.43\({}^{\circ}\), 31.15\({}^{\circ}\), 47.86\({}^{\circ}\), and 56.81\({}^{\circ}\) correspond to the calcium sulfate dihydrate (CaSO4\(\cdot\)2H2O) phase, which is the main crystalline component of PG. The presence of additional peaks at 29.34\({}^{\circ}\), 31.14\({}^{\circ}\), 40.23\({}^{\circ}\), and 50.55\({}^{\circ}\) corresponds to the anhydrite phase of calcium sulfate (CaSO4\(\cdot\)0.5H2O), which is also a significant constituent of PG. Furthermore, the peaks at 26.65\({}^{\circ}\) and in the range of 32\({}^{\circ}\)\(\sim\)37\({}^{\circ}\) are attributed to the presence of silicon dioxide (SiO2) crystals, Additionally, the XRD pattern revealed some minor peaks between 60\({}^{\circ}\) and 90\({}^{\circ}\), indicating the presence of trace impurities such as calcium fluorophosphate (Ca5F(PO4)3) and sodium hexafluorosilicate (Na2 SiF6). These impurities, although present in small amounts, may contribute to the environmental concerns associated with PG. Despite the treatment with zeolite powder and CaO, these impurities remained detectable, suggesting that the treatment process primarily focuses on reducing the solubility and mobility of soluble pollutants, while the overall crystalline framework of the PG remains largely unaltered. Figure 3: Morphological changes in PG under different treatment conditions: (**a**) untreated PG; (**b**) PG treated with CaO; (**c**) PG treated with zeolite powder; (**d**) PG treated with a combination of zeolite powder and CaO. As illustrated in Figure 5, FT-IR spectroscopy was employed to analyze the functional groups and molecular interactions within the PG samples before and after treatment. In the FT-IR spectra of the untreated PG, the absorption peaks observed at 597 cm\({}^{-1}\) and 665 cm\({}^{-1}\) are attributed to the asymmetric bending vibrations of sulfate ions (SO\({}_{4}\)\({}^{2-}\)), which are a prominent feature of PG due to its calcium sulfate content. Additionally, the absorption peak at 1096 cm\({}^{-1}\) corresponds to the asymmetric stretching vibration of SO\({}_{4}\)\({}^{2-}\), further confirming the presence of calcium sulfate and its characteristic functional groups [49; 50]. The two absorption bands at 1620 and 1682 cm\({}^{-1}\) are indicative of the symmetric stretching vibrations of hydroxyl groups (OH\({}^{-}\)) in free water molecules, which are present in PG as part of its hydrated structure [51]. These peaks suggest the presence of adsorbed water in the crystalline structure of PG, which is often linked to the crystallization water in CaSO\({}_{4}\cdot\)2H\({}_{2}\)O. However, when zeolite powder and CaO were used in combination to treat PG, noticeable changes in the FT-IR spectra were observed. Specifically, the intensity of the broad absorption peak at 3392 cm\({}^{-1}\), which corresponds to the stretching vibration of hydroxyl groups in free water, was significantly weakened. This suggests that the treatment effectively reduced the amount of free water in the PG structure. The decrease in the intensity of this absorption band is attributed to the removal or immobilization of free water molecules during the physical adsorption by zeolite and the chemical reaction with CaO. The reduction in free water content highlights the successful interaction between the treatment agents and the PG, leading to a more stable material with less water retention, which could enhance the material's mechanical properties and reduce its environmental impact. Following the treatment of PG with zeolite powder and CaO, the nitrogen adsorption-desorption isotherms of all treated samples showed typical IV-type isotherms, characteristic of mesoporous materials (Figure 6a). The corresponding pore size distributions were primarily in the range of 8-12 nm (Figure 6b), indicating the presence of mesopores in the PG samples.