## 3 Results
### Mineralogical and Microstructural Characterization  
Figure 8 shows the XRD diffractograms of selected pastes and anhydride FA and MT. Based on the size of the humps in the FA and MT diffractograms, associated with their amorphous content, it appears that the MT has a higher crystalline content than the FA. The pastes showed peaks associated with the crystalline phases of the FA (quartz and mullite) and MT (arsenopyrite, chamosite, muscovite, quartz, and pyrite), only with a lower intensity due to a dilution effect. This indicates that such phases are mostly inert. Some of the peaks associated with the chamosite phase (Fe\({}^{2+}\); Mg; Fe\({}^{3+}\))\({}_{5}\)Al(Si\({}_{3}\)Al)O\({}_{10}\)(OH;O)\({}_{8}\) on the MT showed lower intensities after activation (paste P0), suggesting that these phases partially participated in the reactions. This intensity reduction appears to have also occurred on the remaining four pastes analysed; however, in this case, the original MT peaks are diluted with the FA peaks. The only new phase that was found to develop in the activated MT (paste P0) was _burkeite_ (Na\({}_{6}\)(CO\({}_{3}\))(SO\({}_{4}\))\({}_{2}\)), which is basically a sodium sulphate carbonate. Although the _burkeite_ peaks were less intense when FA was added to the precursor combination, this mineralogical phase was still visible, suggesting that the released S ions combined with the sodium, forming sodium sulphate. The FTIR spectra of the same pastes are presented in Figure 9. The FA presents two main bands, at 1010 cm\({}^{-1}\) (Si-O-T stretching vibrations, with T = Si or Al) and 410 cm\({}^{-1}\) (O-Si-O). The MT shows a classic and very intense Si-O-T band at around 980 cm\({}^{-1}\), corresponding to asymmetric stretching modes of the SiO\({}_{4}\) and AlO\({}_{4}\) tetrahedra; this band was maintained in the pastes. The bands at 872 cm\({}^{-1}\) and 773 cm\({}^{-1}\) (also on the FA), typical of Si-O-T (T = Si or Al) stretching vibrations, and the bandat 690 cm\({}^{-1}\) (also in the FA), typical of Al-O, appear on the original MT and in the all the resulting pastes, which reflects the partial crystalline nature of the MT. The FA band at around 1010 cm\({}^{-1}\) was partially dissolved in the reactions, indicating the presence of soluble material, while the peak at 410 cm\({}^{-1}\) was maintained or partially dissolved (it is difficult to interpret since it is overlapping with an intense peak from the MT). Regarding the MT, the original peak at 1060 cm\({}^{-1}\), associated with Si-O or Al-O, was still present in paste PRef, but not in the remaining three pastes, suggesting that parts of the silicon and aluminium species dissolved from the MT were able to form new compounds. The intensity of the 872 cm\({}^{-1}\) peak (MT) was maintained in PRef, but showed a slight decrease in P1A, P2B, and P3B, suggesting that some Si-O-T links were broken. A similar situation was found at 1102 cm\({}^{-1}\) (typical of S-O asymmetric stretching vibrations in SO\({}_{4}\) tetrahedra), i.e., it was still present in the PRef paste but was eliminated in the remaining pastes, suggesting the presence of soluble sulphur species. The intensity of the peak at 610 cm\({}^{-1}\), representing O-S-O deformation vibrations in SO\({}_{4}\), increased for pastes PRef and P2B, suggesting the formation of new molecules involving dissolved S, and slightly decreased for pastes P1A and P3B relative to the original MT. Bands at 529, 460, and 410 cm\({}^{-1}\) (MT), indicating O-Si-O vibrations, were also found in the pastes, with similar intensities. A brief summary of the bands involving sulphur is presented in Table 4.  
SEM and EDX of P0, PRef, P1A, P2B, and P3B are presented in Figures 10 and 11, respectively. The presence of Fe in the P0 is, as expected, very intense. The gel phase, characterised in Figure 10, essentially constituted Na, Al, Si, and S. Based on these semi-quantitative values, it is possible to