## 3 Results and Discussion
### Thermal Stability of the Composites  
The elastic modulus of the composite and matrix when heating until 700 \({}^{\circ}\)C, monitored via impulse excitation, is presented in Figure 13. The elastic modulus of the matrix first decreased from about 5 to 2 GPa, and from about 20 to 250 \({}^{\circ}\)C. Then, an increase was observed until around 620 \({}^{\circ}\)C, where it reached a value of 9.5 GPa before starting to decrease until it reached 7 GPa at 700 \({}^{\circ}\)C. The modulus continued to decrease during the dwell time to 5 GPa and remained almost constant on cooling. The decrease in the elastic modulus of the matrix from 20 to 300 \({}^{\circ}\)C occurs in two steps and may be ascribed to microcrack formation due to residual water loss on heating. The first step, around 100 \({}^{\circ}\)C, is broad while the second step is more pronounced. The E modulus of the composite has a comparable profile of reduction until about 200 \({}^{\circ}\)C. However, for the pure matrix, the decrease is more than 50% up to 250 \({}^{\circ}\)C, while for the composite, it is only about 25%. In part, it is due to the reduced amount of matrix because of the fibers in the composite. However, there is only about 25-30 vol% of fibers; thus, this cannot explain the large difference. The effect must thus be linked to reduced cracking, because fibers stop the crack propagation and the interaction between fibers and matrix, reducing the importance of the matrix. Before 300 \({}^{\circ}\)C, the elastic moduli of the matrix and composite start to increase, but above 300 \({}^{\circ}\)C, the one for the composite decreases again. The most probable explanation for the increase is that the system starts to heal as a result of mobility increase with temperature, although a glass transition could not be observed with calorimetry in this temperature range. The modulus keeps on increasing up to about 630 \({}^{\circ}\)C after which it decreases again. A similar behavior was observed in laterite-based alkali-activated materials, where a strength increase was observed on heating between 250 and 450 \({}^{\circ}\)C [48]. One should however be aware that the strength does not necessarily follow the same trend as the modulus, because the elongation at failure can be different. The composite, however, shows a reduction in modulus in two steps above 300 \({}^{\circ}\)C. The first is probably due to the thermal incompatibility between the matrix and the fiber that led to deterioration upon heating [49]. The second one is at least in part related to the decomposition of the carbon fibers in air. The elastic modulus signal could not be recorded above 640 \({}^{\circ}\)C due to sample  
Figure 12: (**a**) photo of the composite; (**b**) microstructural cross section of laminate composite made of woven carbon fiber and matrix 80K20 MK.  
deterioration upon heating. The reduction in the mechanical properties upon heating is in line with some reported studies on alkali-activated fiber-reinforced composites [12]. However, some studies also reported an increase in strength when metakaolin geopolymer with carbon fiber reinforcement was heated at 1100-1300 \({}^{\circ}\)C, due to matrix crystallization and densification that led to an enhancement of the fiber/matrix-bonding interface [38].