## 3 Results and Discussion
### Crystallinity  
The mineralogical composition and the high-temperature mineralogical transitions are of utmost importance to fully describe the thermal behavior of AAMs and to safely classify them in terms of high-temperature resistance. At room temperature, AAMs are predominantly amorphous to semi-crystalline materials, and the amount and type of crystalline phases present are mainly dependent on the parent precursors. The powdered slag (PS) and silica fume (SF) used in this work as starting materials were almost completely XRD amorphous materials. Only one crystalline peak of small intensity was detected in the XRD patterns for SF, identified as moissanite (SiC) which was an impurity (SF and PS XRD patterns available in [27]). Not surprisingly, the AAMs made thereof exhibited an entirely amorphous structure at room temperature, with a pronounced hump between 20-40\({}^{\circ}\) 20 in their XRD patterns. Heating the samples to 105 \({}^{\circ}\)C did not lead to any crystallization regardless of the heating rate used (not shown here for the sake of brevity). The first signs of the formation of crystalline phases were visible in the samples heated at 600 \({}^{\circ}\)C. Operating at the heating rate of 1 \({}^{\circ}\)C/min, no crystalline peak could be detected, as shown by Figure 7a; however, increasing the heating rate to 10 \({}^{\circ}\)C/min likely promoted the formation of some crystalline phases, visible from the peaks at 20-26-28\({}^{\circ}\) (Figure 7b). Given the weakness of the signals, a clear identification is impossible, but the positions are consistent with the main diffraction lines of leucite (KAlSi\({}_{2}\)O\({}_{6}\)) polymorphs (Leucite 1, PDF#76-2298, and Leucite 2, PDF#85-1626). Leucite is a well-known product of the thermal transformation of K-based AAMs [9]. The formation of crystalline phases was more pronounced as the temperature increased, stimulated by the separation of iron oxide phases. As with many waste-derived iron-rich glass-ceramic systems [34,35], a temperature increase to 750 \({}^{\circ}\)C involved the precipitation of magnetite (Fe\({}_{3}\)O\({}_{4}\), PDF#89-0951), whereas a pyroxene phase (esseneite, PDF#78-1546), i.e., an Al-doped calcium-iron silicate, crystallized at 900 \({}^{\circ}\)C. Magnetite, embedding iron in both Fe\({}^{2+}\) and Fe\({}^{3+}\) forms (Fe\({}_{3}\)O\({}_{4}\)=FeO\({}_{. }\)Fe\({}_{2}\)O\({}_{3}\)), is a typical nucleating agent, separating at 650-800 \({}^{\circ}\)C from 'basaltic' glasses (glasses of the SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\)-Fe\({}_{2}\)OCaO system, including also alkali oxides), later transformed into pyroxene glass-ceramics. Magnetite may persist or be reabsorbed in the solid solution [34]. Heating to 900 \({}^{\circ}\)C and above promoted some oxidation, shown by the fact that both additional phases detected, i.e., CaAlFeSiO\({}_{6}\) (PDF#78-1546) and hematite (Fe\({}_{2}\)O\({}_{3}\), PDF#89-0599) host only Fe\({}^{3+}\) ions [34]. Finally, it can be noted that the different heating rates determined a different evolution at the maximum temperature (1100 \({}^{\circ}\)C), with the appearance of calcium alumino silicate phases: while the sample heated at 1 \({}^{\circ}\)C/min exhibited a limited precipitation of anorthite (CaAl\({}_{2}\)Si\({}_{2}\)O\({}_{8}\), PDF#73-0264), the sample heated at 10 \({}^{\circ}\)C/min featured both anorthite and gehlenite (Ca\({}_{2}\)Al\({}_{2}\)SiO\({}_{7}\), PDF#72-2128). Differences in the phase assemblage, with consequences for the thermo-elastic stresses developed upon cooling, could be a further explanation for the lower strength of samples heated at 10 \({}^{\circ}\)C/min: in particular, gehlenite is recognized for its high coefficient of thermal expansion [36]. The formation of esseneite and magnetite as the predominant phases in all samples can be seen as a natural consequence of the abundant availability of Si, Ca, and Fe species provided by the mix design.