## 3 Results and Discussion
### Physical-Mechanical Characterization
apparent density slightly decreases at 600 \({}^{\circ}\)C. Although more resistant and probably denser polymeric chains have been formed, this decrease is the combined effect of water loss and shrinkage. As mass loss (\(\approx\)8.5 wt.%) was significantly higher than the volumetric contraction up to 600 \({}^{\circ}\)C (<2 vol%), apparent densities decrease slightly. The increment of on-going reactions is further supported by comparing the mechanical strength of samples thermally treated up to 600 \({}^{\circ}\)C with different heating rates. As can be seen, lower heating rates--meaning more prolonged residence time at elevated temperatures--favor the formation of more resistant AAMs without significantly modifying the samples' mass and volume (Figure 3) or promoting the significant formation of new crystalline phases (Figure 7). At 750 \({}^{\circ}\)C, considerable deterioration of AAMs' mechanical properties occurred with both heating rates. This process may be related with modifications on the iron- and aluminosilicate networks occurring even before sintering [30]. The shrinkage of the aluminosilicate network induces internal cracks and increases the mean pore size, which leads to poorer mechanical performances [30]. Nonetheless, totally dehydrated charge balance cations (Na\({}^{+}\) or K\({}^{+}\)) act as flux at high temperatures and reduce the temperature at which melting and crystallization start. This can imply the formation of partially sintered phases at around 750 \({}^{\circ}\)C [26]; however, these are not fully able to compensate for the decline in compressive strength caused by dehydration-related shrinkage. An additional factor that has been reported to contribute to the loss of strength at temperatures around 800 \({}^{\circ}\)C is the expansion of coarse aggregates [31]. Due to the type and granulometric distribution of the amorphous aggregates used in this work, however, a similar phenomenon seems unlikely to be significant. The effects of the network shrinkage could be seen at the macroscopic level by the steepest increase in volumetric contraction observed around 750 \({}^{\circ}\)C (Figure 3). At this temperature, increasing the heating rate was found to be detrimental to AAMs' residual strength. Along with higher crack formation due to the higher vapor pressure generated, the considerable difference in mechanical performance between mortars heated at different rates may indicate more deteriorated network structures. At 750 \({}^{\circ}\)C, AAMs start to lose their predominantly amorphous character, leading to the appearance of some broadened peaks of magnetite (Fe\({}_{3}\)O\({}_{4}\)) and esseneite (CaAlFeSiO\({}_{6}\)) (Figure 7). These crystals could not be clearly identified in morphological analysis and does not seem able to compensate for the decrease of AAMs' mechanical strength; however, they may have contributed to some degree to the densification of the samples. As mass loss was mainly associated with dehydration processes occurring mostly  
Figure 6: Mortars’ apparent density 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 collection was not possible. before 300 \({}^{\circ}\)C, the mass remained practically unchanged at temperatures higher than 600 \({}^{\circ}\)C. Therefore, the apparent density followed a similar trend to the volumetric contraction. A minor increase of mass was visible from 750 \({}^{\circ}\)C and above (Figure 3) that can be attributed to further oxidation and crystallization of iron species into magnetite and afterwards to hematite. The reference mortar presented a density of 2.46 \(\pm\) 0.02 kg/m\({}^{3}\) and a compressive strength of 85 \(\pm\) 3 MPa after 28 d of curing.