## 3 Results and Discussion
### Morphological Analysis  
Figure 8 shows representative backscattered electron imaging micrographs of the reference mortar before being exposed to thermal treatment. A binder phase was formed (Figure 8a), but the existence of embedded unreacted particles confirms that complete dissolution was not achieved. The high solid-to-liquid ratio used (2.85) limited the precursors' dissolution, but the samples show a homogeneous matrix in which undissolved particles acted as small-sized aggregates, (Figure 8b). Extensive microcracks have been observed with undissolved particles acting as physical barriers that limited crack development and propagation. Similar microcracks have been reported in the literature, and their origin associated with drying shrinkage processes [28]. EDS analyses revealed a binder phase predominantly composed of Si, Al, Fe, Ca, and K, while the spectrum of undissolved particles was in line with the chemical composition of the slag used as the main precursor and as aggregates (Table 3). Some spherical pores were also observed that can be attributed to entrapped air during mixing. Figure 8: Backscattered electron imaging micrographs of alkali activated mortar before being exposed to elevated temperatures showing: (**a**) binder phase; (**b**) microcracks. The effects of high-temperature exposure and heating rate on mortar morphology are illustrated in Figure 9. Independently of the heating rate used, a more compact microstructure with few large cracks was formed relative to the parent mortar. Partial softened aggregates have been reported to contribute to form homogenous mortars [23] and morphological analysis indicates that undissolved particles previously visible in green samples were almost completely molten (Figure 9a,b). Their previous homogeneous structure was transformed into a texture of finely resolved phases. However, the contours of undissolved particles remain identifiable as white lines. A clearer picture of interfacial transition zones between molten aggregates and previous existing binders is better seen at higher magnification (Figure 9c,d). EDS analyses revealed the predominance of iron elements in \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & **SiO2** & **CaO** & **FeOx** & **Al2O3** & **MgO** & **TiO2** & **K2O** & **Na2O** & **Others** \\ \hline
**B1** & 44.6 & 13.8 & 18.8 & 14.4 & 1.0 & 0.7 & 6.2 & 0.5 & 0.0 \\
**B2** & 44.9 & 13.7 & 18.4 & 12.7 & 1.0 & 0.5 & 8.8 & - & 0.0 \\
**B3** & 46.3 & 10.5 & 20.2 & 13.0 & 1.0 & - & 8.4 & 0.6 & 0.0 \\
**A1** & 34.7 & 22.9 & 23.0 & 16.8 & 1.4 & 0.5 & 0.6 & - & 0.1 \\
**A2** & 34.2 & 23.6 & 23.1 & 16.4 & 1.3 & 0.6 & 0.6 & - & 0.2 \\
**A3** & 34.2 & 23.5 & 23.2 & 16.4 & 1.4 & 0.7 & 0.6 & - & 0.0 \\
**F1** & 11.5 & 7.8 & 73.2 & 5.8 & Nd & 0.7 & - & - & 1.0 \\
**F2** & 9.2 & 6.2 & 77.6 & 4.8 & 0.7 & 1.4 & - & - & 0.1 \\
**F3** & 11.8 & 9.6 & 72.4 & 4.5 & 0.7 & 0.7 & - & - & 0.3 \\ \hline \end{tabular}
\end{table} 
Table 3: Representative SEM-EDS results describing the chemical composition of binder and aggregates at room temperature (B1–B3, and A1–A3, respectively) and white areas outlining the aggregates’ contours after thermal exposure to 1100 \({}^{\circ}\)C (F1–F3). Data collection location as indicated in Figures 8a and 9b. Figure 9: Backscattered electron imaging micrographs of mortars exposed to 1100 \({}^{\circ}\)C: (**a,c**) with a heating rate of 1 \({}^{\circ}\)C/min; (**b,d**) with a heating rate of 10 \({}^{\circ}\)C/min. the white areas outlining the aggregates' contours, which suggests the concentration of oxidized iron phases at these locations (Table 3 and Figure 9d). At higher magnifications, the morphological differences between samples heated with distinct heating rates become more evident. Mortars heated at 1 \({}^{\circ}\)C/min exhibit a continuous interfacial transition zone with no visible cracks or microscopic pores.