## 3 Results and Discussion
### Synthesis of Zeolites  
Figure 4 shows the XRD images of the solid substances which were crystallized from the alkali washing solution of the activated CFA sample naturally settled at an ambient temperature of (0 h), and then for 1, 3, and 5 h. By comparing the XRD images of zeolites at different crystallization times in the parallel experiment, although the peak intensities of the two groups of experiments were slightly different, the coexistence of NaA and sodalite zeolites as well as the mechanism from NaA zeolite to sodalite zeolite during the phase-transformation process can be clearly manifested. The XRD spectra for 0 h clearly indicated a small characteristic peak of NaA zeolite in addition to a large bulge corresponding to the most-amorphous solid phase. [PERSON] et al. (2018) investigated the early reverse crystal growth of zeolite A and its phase transition to sodalite, and found that in the early phase transition stage, crystallization will occur on the surface of amorphous spherical particles formed by precursors and polymers, forming a typical cubic morphology related to zeolite A. It has a very thin crystalline cubic shell and amorphous core. When the solution was crystallized at 100 \({}^{\circ}\)C, the amorphous material started to transit toward the structure of NaA zeolite. This was clearly shown in its XRD spectrum of the sharp and narrow characteristic peak of NaA zeolite after 1 h of crystallization. With the extension of crystallization from surface to core, sodalite zeolite nanoplates were crystallized within the amorphous nucleation of these NaA zeolite cubes. As validated in Figure 4, the crystallization time was extended to 3 h and reversely weakened the intensity of the NaA characteristic peak; however, it strengthened a new characteristic peak corresponding to the sodalite zeolite. This was likely due to the fact that OH\({}^{+}\) was produced during the formation of NaA crystal form, resulting in an increase in alkalinity of the solution system. The environment with higher alkalinity was more conducive to the nucleation of sodalite zeolite. In addition, the skeleton energy of NaA structure was higher than sodalite, leading to a trend of spontaneous transformation toward sodalite ([PERSON] et al., 2018). As expected, the extension of the crystallization time to 5 h completely transformed the NaA zeolite to be the sodalite zeolite, as evidenced by the only narrow and sharp sodalite characteristic peaks that remained in spectrum of the obtained zeolite product. Combined with the research of [PERSON] et al. (2018), the phase transformation of NaA zeolite to sodalite zeolite mainly includes the following steps: Dissolution of NaA zeolite particles, supersaturation of liquid phase and aluminosilicate ions, and nucleation of hydroxy sodalite from the solution and crystal growth of sodalite. Meanwhile, the D-spacing in Table 5 showed an increasing trend with the crystallization time, indicating that the atomic arrangement of the synthesized zeolite was more compact. Figure 5 shows the SEM images of the zeolite crystallized from the alkali washing solution of the activated CFA sample for 1, 3, and 5 h, and then naturally settled at an ambient temperature. It indicated the variation of morphologies of zeolite by controlling the crystallization reaction time. It shows spherical aggregates of amorphous silicate aluminate particles in small particle sizes with only a very small amount in the NaA zeolite structure. After crystallization for 1 h at the elevated temperature, the spherical particles in the solid phase disappeared and cubic grains of complete NaA zeolite clearly appeared ([PERSON] et al., 2018), showing the \"stacking\" growth phenomenon. After crystallization for 3 h, only a small part of regular crystal shape NaA grains was found in the solid phase, in contrast to the majority in sodalite structure with an irregular shape, chimeric with each other, and in flower cluster. The SEM image further revealed that sodalite grains considered the surface of NaA crystal