## 3 Results and Discussion
### Syntheses by the Two-Step Method: Alkaline Fusion Followed by Hydrothermal Treatment  
In the alkaline melting process, the mixture of fly ash and sodium hydroxide was melted. After melting, water was added, and followed by conventional hydrothermal treatment. When performing an alkaline fusion, the quartz present in the residue is allowed to be totally or partially dissolved, depending on the temperature used [13]. The fusion with NaOH before the hydrothermal reaction was applied by [PERSON] et al. [29], where they studied different NaOH/fly ash ratios and observed that the ideal amount would be an NaOH/Ash ratio = 1.5 g/g.  
In the experiments carried out, no extra NaOH was added; maintaining the amount needed for the composition of the standard gel, the NaOH/Ash value remained at 0.9 g/g. Table 4 shows the description of the parameters used for the zeolite samples synthesized by the two-stage methodology.  
Figure 5: X-ray diffractograms of ZACV-6 samples, where q—quartz, s—sodalite, a—zeolite LTA.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{5}{c}{**Crystallinity (\%)**} \\ \cline{2-6}  & **2 h** & **4 h** & **6 h** & **24 h** & **48 h** \\ \hline ZACF-4 & 10.4 & 8.4 & 13.4 & 16.1 & 16.8 \\ ZACF-5 & 40.5 & 54.0 & 48.9 & 48.3 & 49.4 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Crystallinity of samples ZACF-4 and ZACF-5.  
Figure 6 shows the diffractogram of the residual samples melted with NaOH at different temperatures; thus, this finding verifies that at higher temperatures (500 \({}^{\circ}\)C), complete dissolution occurs in the residue, making it amorphous.  
The samples ZACF-200, ZACF-300, and ZACF-500 were all synthesized at the same conditions described in Table 3, varying only the alkaline fusion temperature. It was verified through the X-ray diffractograms (Figure 7) that the fusion alkaline followed by the hydrothermal step led to zeolitic phases. The alkaline fusion treatment provides greater availability of Si and Al, leading to more crystalline zeolitic phases. The formation of zeolite LTA was observed; at longer times and/or higher temperatures, they tend to form more stable structures in the melting stage, such as sodalite (S). The crystallization percentages (Table 5) support this, where crystallinity was observed above 70% in 2 h of synthesis for the ZACF-200 sample. The cubic morphology proves the formation of LTA zeolite (Figure 8); at longer times, the growth of sodalite crystals (spherical agglomerates) was observed.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{5}{c}{**Crystallinity (\%)**} \\ \cline{2-7}  & **2 h** & **4 h** & **6 h** & **24 h** \\ \hline ZACF-200 \({}^{\circ}\)C & 72.7 & 72.1 & 61.8 & 43.6 \\ ZACF-300 \({}^{\circ}\)C & 42.9 & 36.2 & 35.7 & 13.3 \\ ZACF-500 \({}^{\circ}\)C & 49.4 & 45.7 & 50.3 & 29.5 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Crystallinity of samples prepared by alkaline fusion followed by the hydrothermal treatment.  
Figure 6: Comparison of X-ray diffractogram of coal samples fused with NaOH at different temperatures, where q—quartz and m—mullite.  
Figure 8: SEM image analysis of ZACF-200 \({}^{\circ}\)C samples obtained after (**a**) 2 h, (**b**) 4 h, (**c**) 6 h, and (**d**) 24 h of synthesis.  
Figure 7: Diffractograms of samples prepared by alkaline fusion followed at (**a**) 200 \({}^{\circ}\)C, (**b**) 300 \({}^{\circ}\)C, and (**c**) 500 \({}^{\circ}\)C by the hydrothermal treatment, where q—quartz, s—sodalite, a—zeolite LTA.