## 3 Results and Discussion
### Physical-Mechanical Characterization
The rapid increase of temperature accelerates dehydration processes, but water release occurs at higher temperatures due to lower heat and mass transfer rates (Figure 3), giving less time for steam to escape, which in turn results in excessive vapor pressure and leads to crack propagation [15]. [PERSON] et al. [23] also reported severe cracks on Ca-rich fly ash/GGBFS mortars fired at 5 \({}^{\circ}\)C/min, suggesting some similarities in the effects of increased heating rates. However, all samples heated up to 900 \({}^{\circ}\)C maintained their structural integrity, regardless of the heating rate used. The samples heated to up 1100 \({}^{\circ}\)C with a heating rate of 10 \({}^{\circ}\)C/min present considerable molten material on their bottom surfaces (rotated in Figure 2h). Apart from the excessive vapor pressure generated, the large temperature gradients across the samples' cross-sections resulted in differential shrinkage, which increased the stresses formed and crack propagation (Figure 4), allowing for the runoff of molten material. Similar findings were reported by [PERSON] et al. [26], who observed that high heating and cooling rates form new or intensified pre-existing cracks due to the high thermal strain generated, ultimately leading to local failure. The viscosity of the melts also decreases as the temperature rises, which further facilitates their flow. Hence, the amount of molten material is expected to be significant at temperatures higher than 900 \({}^{\circ}\)C, as the increased flowability of the melts and the high number of voids and cracks in the mortar structures resulted in molten material coming out of the surfaces, as seen in Figures 2h and 4. Figure 3: Mortars’ volume contraction and mass loss as a function of temperature and heating rate. The curves are drawn as a guide to the eye. Due to deformation of the samples fired at 1100 \({}^{\circ}\)C with a heating rate of 10 \({}^{\circ}\)C/min data could not be collected. In samples heated to 1100 \({}^{\circ}\)C with a lower heating rate, no molten material was visible after thermal exposure. The more gradual heating process diminished crack formation, which allowed the molten material to be retained inside the monolith, positively affecting the mortars' structural stability and high-temperature performance. Although mortars heated up to 1100 \({}^{\circ}\)C with heating rates of 10 \({}^{\circ}\)C/min retain their cubic shape and were tested for their compressive strength, they were excluded from further physical analysis (e.g., mass loss). The physical and mineralogical modifications that occurred upon heating had clear repercussions on AAMs' residual properties. From room temperature to 600 \({}^{\circ}\)C a slight increase of compressive strength after thermal exposure was observed, as shown by Figure 5.  
The improvement of compressive strength after thermal exposure to temperatures that do not exceed 600 \({}^{\circ}\)C is not related to melting and crystallization processes (see Section 3.3), but most likely to the increment of on-going polymerization reactions. In previous work [27, 28], the authors have shown that in very similar alkali-activated systems, exothermic reactions have not completely ceased after more than 20 d of curing. Elevated temperatures stimulate these late-stage reactions and prompt the formation of more mechanically resistant binding phases. Along with the absence of significant crack formation, this beneficial effect overcomes the deleterious impact of dehydration and improves AAMs' mechanical features. The increase of compressive strength in this temperature range is often associated with densification [7, 29]. However, it can be seen in Figure 6 that the  
Figure 4: The surface of alkali activated mortar after thermal exposure to 1100 \({}^{\circ}\)C with heating rates of 10 \({}^{\circ}\)C/min. Molten material was removed from the sample surface for clarity. Figure 5: Mortars’ compressive strength as a function of temperature and heating rate. The curves are drawn as a guide to the eye.