## 3 Results and Discussion
### Physical-Mechanical Characterization
Using a heating rate of 1 \({}^{\circ}\)C/min the maximum apparent density was observed at 900 \({}^{\circ}\)C (2.78 \(\pm\) 0.03 g/cm\({}^{3}\)); however, the decrease of apparent density by 1100 \({}^{\circ}\)C was not detrimental to mechanical performance as the samples resulting from a heating rate of 1 \({}^{\circ}\)C/min presented the maximum compressive strength recorded, 184 \(\pm\) 13 MPa (Figure 5). In those samples, when the temperature exceeded 750 \({}^{\circ}\)C and the crystallization of new phases became more intense, the compressive strength decrease was fully reversed. The compressive strength of sintered samples was progressively increased as the temperature rose, with an increase relative to the parent mortar of 85% and 115% at 900 and 1100 \({}^{\circ}\)C, respectively. Such strength gains contrast with the losses reported by several authors in low-iron systems, with proclivity towards lower mechanical performances as  
Figure 7: XRD patterns of alkali activated mortars after heating to different temperatures: (**a**) with a heating rate of 1 \({}^{\circ}\)C/min; (**b**) with a heating rate of 10 \({}^{\circ}\)C/min. the temperature rises [23,32]. [PERSON] et al. [32] investigated the influence of the Si/Al ratio in the thermal performance of fly ash geopolymer mortars and reported loses in compressive strength ranging from 54-77% when firing at 1000 \({}^{\circ}\)C. Conversely, [PERSON] and [PERSON] [33] reported significant increases in the mechanical strength of metakaolin geopolymers, but the maximum strength reported was approximately 160 MPa and required firing at 1200 \({}^{\circ}\)C for 2 h.  
Increasing the heating rate to 10 \({}^{\circ}\)C/min not only provoked a higher decrease of compressive strength at 750 \({}^{\circ}\)C, but also significantly impaired the performance recovery at higher temperatures. On those samples, the maximum compressive strength was halved (97 \(\pm\) 8 MPa) and attained at 900 \({}^{\circ}\)C. Despite the recovery from the decay observed at 750 \({}^{\circ}\)C and the 13% increase relative to the parent mortar, much higher compressive strengths were observed in samples heated up to 900 \({}^{\circ}\)C with the lower heating rate (158 \(\pm\) 13 MPa). At 1100 \({}^{\circ}\)C, the mechanical strength of samples heated with different heating rates became even more divergent. Unlike samples heated at 1 \({}^{\circ}\)C/min that yielded the maximum strength at this temperature, when high heating rates were imposed, a considerable degradation of the AAMs' performance occurred. As shown in Figure 3, these samples were not dimensionally stable, with some molten material coming out of the samples' structure. This explains the much lower compressive strength. The reason for material run-off in some rapid heated samples might be that crystallization is not fast enough, while the viscosity decreases due to the increasing temperature. At the same time, capillary forces generated at the solid surface may further favor some material outflow from the molten core. At 1 \({}^{\circ}\)C/min, there seems to be a better match between the heating and crystallization rate, as the significant residence time and formation of more liquid phases does not promote such phenomena. With the exception of samples exposed to 1100 \({}^{\circ}\)C with a heating rate of 10 \({}^{\circ}\)C/min (17 \(\pm\) 9 MPa), all sintered samples presented compressive strength values higher than 40 MPa.