## 3 Results and Discussion  
### Extraction of Silicon and Aluminum in CFA  
After the sample was calculated with Na\({}_{2}\)CO\({}_{3}\), the original silicate aluminate and glass phase structure in CFA could be destroyed, and new silicate or silicate aluminate structures with high activity were likely generated. This activated CFA sample mainly contains a large amount of insoluble aluminosilicate, some water-soluble Na\({}_{2}\)SiO\({}_{3}\), and a small amount of NaAlO\({}_{2}\). The specific experimental reactions were likely as follows:  
\[\mathrm{Al}_{6}[\mathrm{SiO}_{4}]_{2}\mathrm{O}_{5}+4\mathrm{SiO}_{2}+3 \mathrm{Na}_{2}\mathrm{CO}_{3}\to 6\mathrm{Na}\mathrm{Al}\mathrm{SiO}_{4}+3 \mathrm{CO}_{2}\mathrm{\uparrow} \tag{1}\]  
\[\mathrm{Al}_{2}\mathrm{O}_{3}+\mathrm{Na}_{2}\mathrm{CO}_{3}\to 2\mathrm{Na} \mathrm{AlO}_{2}+\mathrm{CO}_{2}\mathrm{\uparrow} \tag{2}\]  
\[\mathrm{SiO}_{2}+\mathrm{Na}_{2}\mathrm{CO}_{3}\to\mathrm{Na}_{2}\mathrm{SiO}_{3 }+2\mathrm{CO}_{2}\mathrm{\uparrow} \tag{3}\]  
Among them, the majority of Na\({}_{2}\)SiO\({}_{3}\) and a small amount of NaAlO\({}_{2}\) can be easily removed with simple water washing. The remaining solution is mostly insoluble silicate aluminate, which was likely continuously washed-out using an alkaline solution. The alkaline solution can easily decompose the silicate aluminate into soluble aluminate and silicate, in order to achieve the extraction of silicate aluminate, following a reversible reaction of the alkali dissolution process as follows:  
\[\mathrm{Aluminosilicate\ (solid\ phase)}\rightleftharpoons\mathrm{Aluminate\ ion\ (liquid\ phase)}+\mathrm{Silicate\ ion\ (liquid\ phase)} \tag{4}\]  
Experimental results showed that the water-washed CFA achieved the Si extraction efficiency of 51.46% and Al of only 2.07%. The alkali washing treatment was carried out after the orthogonal experiment as shown in Table 3. It was found that the extraction efficiency of Si and Al elements changed significantly under different levels of factors. The range R helped in judging the influence degree of each factor in the experiment, showing that the significance of factors followed the sequence of the solid-liquid ratio (C), the NaOH concentration (A), the holding time (D), and the heating temperature (B). The k-value was calculated according to the extraction efficiency of the Al element in each group. The effect curves of each factor were drawn leading to the optimal combination of conditions, in order to achieve the maximum extraction of Al. When the NaOH concentration is 20% (A3), the heating temperature is 60 \({}^{\circ}\)C (B1), the solid-liquid ratio is 1:20 (C3), and the holding time is 1 h (D2), then the extraction efficiency of Al and Si can be achieved by 62.51% and 36.82%, respectively.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Group**} & \multicolumn{4}{c}{**Factor**} & \multirow{2}{*}{**Al Dissolution Rate**} & \multirow{2}{*}{**Si Dissolution Rate**} \\ \cline{2-2} \cline{4-7}  & **A** & & & & & \\ \hline
1 & 0.1 & 60 & 1:10 & 0.5 & 14.91\% & 7.26\% \\
2 & 0.1 & 75 & 1:10 & 1 & 24.24\% & 12.65\% \\
3 & 0.1 & 90 & 1:10 & 1.5 & 29.59\% & 14.23\% \\
4 & 0.15 & 60 & 1:15 & 1.5 & 33.02\% & 17.72\% \\
5 & 0.15 & 75 & 1:15 & 0.5 & 39.73\% & 27.10\% \\
6 & 0.15 & 90 & 1:15 & 1 & 27.68\% & 11.24\% \\
7 & 0.2 & 60 & 1:20 & 1 & 62.51\% & 36.82\% \\
8 & 0.2 & 75 & 1:20 & 1.5 & 14.87\% & 7.81\% \\
9 & 0.2 & 90 & 1:20 & 0.5 & 42.42\% & 25.05\% \\ K1 & 22.91\% & 36.81\% & 19.15\% & 32.35\% & & \\ K2 & 33.48\% & 26.28\% & 33.23\% & 38.14\% & & \\ K3 & 39.93\% & 33.23\% & 43.94\% & 25.83\% & & \\ R & 17.02\% & 3.59\% & 24.79\% & 6.52\% & & \\ Best combination & A3 & B1 & C3 & D2 & 62.51\% & 36.82\% \\ \hline \end{tabular}
\end{table}
Table 3: Alkali leaching orthogonal experiment.