## 3 Results
### Mineralogical and Microstructural Characterization
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Band** & **Bond** & **Type** & **Comment** \\ \hline
1102 (cm\({}^{-1}\)) & S-O & \(\upsilon_{\text{as}}\) & Showed some traces of the original MT peak in the PRef paste, and disappeared in the remaining pastes \\
610 (cm\({}^{-1}\)) & O-S-O & \(\delta\) & Increased (decreased) intensity in pastes PRef, P2B (P1A, P3B) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of the sulphur-related peaks presented in Figure 9.  
Figure 9: IR spectra of the original materials and most effective pastes. conclude that a N-A-S-H-type gel was developed, responsible for the strength gain. However, at this stage, the possible inclusion of sulphur ions in the gel structure was also considered, even if the exact role of such ions was not yet established. In terms of morphology, the micrographs indicate fairly compacted material due to the formation of gel with a SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) molar ratio of approximately 2 (Table 5). Nevertheless, several intact pyrite and ash particles are still visible. The P2B paste presented an apparent lower amount of unreacted FA particles (Figure 11c) and a higher volume of gel compared with the remaining three pastes. This higher reaction degree, responsible for the highest UCS reached by this paste (Figure 5), is a consequence of the higher concentration of the activating solution (12.5 M, Table 2). \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Paste**} & \multirow{2}{*}{**Point**} & \multicolumn{4}{c}{**Element Content (wt.\%)**} & \multicolumn{4}{c}{**Molar Ratios**} \\ \cline{3-10}  & & **Si** & **Al** & **Na** & **S** & **SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\)** & **Na\({}_{2}\)/O\({}_{3}\)** & **Na\({}_{2}\)/O\({}_{3}\)** \\ \hline PRef & A1 & 17 & 16 & 8 & 6 & 2.161 & 0.560 & 0.957 \\  & A2 & 21 & 21 & 4 & 2 & 2.093 & 0.248 & 1.405 \\  & A3 & 21 & 20 & 4 & 3 & 2.155 & 0.262 & 1.155 \\  & A4 & 22 & 21 & 2 & 1 & 2.133 & 0.105 & 2.031 \\ \hline P1A & B1 & 20 & 19 & 3 & 3 & 2.213 & 0.206 & 0.711 \\  & B2 & 18 & 18 & 5 & 5 & 2.067 & 0.339 & 0.706 \\  & B3 & 16 & 17 & 4 & 6 & 1.950 & 0.269 & 0.449 \\  & B4 & 11 & 12 & 4 & 7 & 1.936 & 0.423 & 0.440 \\ \hline P2B & C1 & 3 & 3 & 35 & 31 & 1.837 & 13.565 & 0.803 \\  & C2 & 7 & 6 & 30(7) & 18(7) & 2.483 & 5.790 & 1.204 \\  & C3 & 3 & 2 & 31(7) & 37(7) & 3.576 & 17.874 & 0.572 \\  & C4 & 18 & 25 & 5 & 3 & 1.497 & 0.258 & 1.161 \\  & C5 & 1 & 1 & 41(7) & 40(7) & 1.931 & 73.893 & 0.708 \\  & C6 & 38 & 2 & 5 & 3 & 38.087 & 2.650 & 1.155 \\ \hline P3B & D1 & 15 & 13 & 7 & 3 & 2.319 & 0.585 & 1.642 \\  & D2 & 23 & 17 & 5 & 2 & 2.832 & 0.348 & 1.900 \\  & D3 & 15 & 16 & 4 & 3 & 1.937 & 0.331 & 1.009 \\  & D4 & 19 & 14 & 4 & 9 & 2.691 & 0.314 & 0.293 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Characterisation of the points presented in Figure 11.  
Figure 10: SEM micrographs and EDX analysis of paste P0. Based on the gel composition, assessed from a total of 120 EDX points from which a representative sample was selected and presented in Figure 11 (with corresponding data summarised in Table 5), a typical N-A-S-H gel was formed, although with significant percentages of sulphur included. The presence of sulphur in the gel indicates that this element had some influence on the composition of the gel structure, probably encapsulated in the aluminosilicate matrix [50]. The high sulphur percentages found in paste P2B were always associated with high sodium contents, indicating the possible development of sodium sulphate, which was found in the mineral form of _burkeite_ (Figure 8). The majority of the gel points from the remaining three pastes presented a significantly higher content of Si+Al than Na+S, meaning that the formation of sodium sulphate was less intense in these pastes.