## 3 Results and Discussion  
### Characterization of Spent Pot Lining Aluminosilicates  
The chemical composition of the SPL was determined using XRF. The results in Table 3 show that the material is almost a pure aluminosilicate, as expected from its refractory origin. The low concentration of sodium may originate from the interaction with the cryolite during operation of the pot or from the NaOH-detoxification process of the spent pot lining. Other minor elements were present as impurities in the original refractory pot lining material. The X-ray diffractogram presented in Figure 1 shows a hump between 20 and 40\({}^{\circ}\) 20. This is characteristic for an aluminosilicate amorphous phase. The quantification by Rietveld refinement, using an external standard to determine the quantity of amorphous material, is given in Table 4. The high quantity of amorphous (56 wt%) appears promising for the applications, as usually the amorphous fraction of aluminosilicate SCMs such as coal fly ash is the most reactive phase of the material [30]. The major crystalline phases are quartz, mullite and cristobalite. The Na seems to be mostly present in the crystalline phases nepheline and sodalite. Nepheline can also contain minor amounts of potassium [31]. The remaining minor elements could not be associated with a specific phase at this point. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Sample Name** & **Portland Cement (g)** & **Lime (g)** & **SPL (g)** & **Quartz (g)** & **Anhydrite (g)** & **Aluminum Powder (g)** \\ \hline Ref 5 & 30 & 15 & & 50 & 5 & 0.1 \\ \hline SPL 25-5 & 30 & 15 & 12.5 & 37.5 & 5 & 0.1 \\ \hline SPL 50-5 & 30 & 15 & 25 & 25 & 5 & 0.1 \\ \hline SPL 75-5 & 30 & 15 & 37.5 & 12.5 & 5 & 0.1 \\ \hline SPL 100-5 & 30 & 15 & 50 & 5 & 0.1 \\ \hline Ref 3 & 31 & 16 & & 50 & 3 & 0.1 \\ \hline SPL 25-3 & 31 & 16 & 12.5 & 37.5 & 3 & 0.1 \\ \hline SPL 50-3 & 31 & 16 & 25 & 25 & 3 & 0.1 \\ \hline SPL 75-3 & 31 & 16 & 37.5 & 12.5 & 3 & 0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Binder mixtures for the synthesis of autoclaved aerated concrete (AAC) blocks. The complete AAC mixture included 100 g of demi-water. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **SiO2** & **Al2O3** & **Na2O** & **K2O** & **Fe2O3** & **CaO** & **MgO** & **TiO2** \\ \hline wt\% & 68.0 & 23.8 & 2.8 & 1.8 & 2.4 & 1.4 & 1.3 & 1.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Chemical composition of the spent pot lining aluminosilicates determined using XRF. The distribution of the phases detected by XRD and additional information on the microstructure of the SPL aluminosilicates was obtained using SEM. Backscattered images are shown in Figure 2 and EDS maps are presented in Figure 3. The phases are not homogeneously distributed in the material and most particles are not single-phase particles. The latter is important as this way non-reactive phases can physically hinder the reaction of the amorphous phase. In case of the finer particles, mostly observed on the left image of Figure 2, there might be less intermixing of the crystalline phases with the amorphous phase. The EDS maps underline further the inhomogeneity of the material, especially shown by the localized nature of the Na and Fe maps. The localization of Fe suggests that a small amount of iron oxides might be present, however, this amount was too small to distinguish in the X-ray diffractogram and the presence of iron oxides can thus not be confirmed with certainty. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Phase Name** & **Phase Formula** & **Phase Content (wt\%)** \\ \hline Quartz & SiO\({}_{2}\) & 17 \\ \hline Mullite & Al\({}_{x}\)SiO\({}_{1.5\times 2}\) & 18 \\ \hline Cristobalite & SiO\({}_{2}\) & 6 \\ \hline Nepheline & Na\({}_{3}\)KAl\({}_{4}\)Si\({}_{4}\)O\({}_{16}\) & 2 \\ \hline Sodalite & Na\({}_{8}\)Al\({}_{6}\)Si\({}_{6}\)O\({}_{25}\) & 1 \\ \hline Amorphous & & 56 \\ \hline \hline \end{tabular}
\end{table}