## 3 Results and Discussion
This is in line with findings from another literature precedent [61], where it was also reported that initial evaporation severely hampers the evolution in the strength  
As previously mentioned, this could also be attributed to the lower degrees of hydration of the matrices, which could lead to the breakdown of the granular structure of the matrices, thus restricting them to evolve into a more semicrystalline form. Subsequently, this effect could cause severe cracking when exposed to differential thermal gradients, especially at elevated temperature levels. Previous studies have also reported that an initial evaporation of fluids could hinder the continuous and uniform reorganization of polycondensation processes. This could in turn hinder further development in strength and structural integrity [61]. Furthermore, earlier dehydration processes can lead to less denser structures within the material, which can result in lower compressive strengths. Such effects were also reported previously [61; 70], where it was also indicated that the inhibition of the ongoing geopolymerization process could result in less denser matrices, and ones with higher porous structures. These could lead to lower compressive strengths. After exposure to 400 \({}^{\circ}\)C, unsealed Gladstone FA-based samples recorded an average compressive strength of approximately 28 MPa, while sealed samples recorded an average compressive strength of approximately 43 MPa. After the exposure to a temperature of 800 \({}^{\circ}\)C, unsealed Gladstone FA-based samples recorded an average compressive strength of approximately 18 MPa, while sealed samples recorded an average compressive strength of approximately 24 MPa. Tables 4 and 5 provide the data of the residual strength of unsealed and sealed samples, and Figures 9 and 10 show a graphical representation of these results. It was also noted that as the temperature increased to 400 \({}^{\circ}\)C, the majority of the Gladstone FA-based GP samples having a lower ratio of alkaline solution to FA (ca. 0.4), were shown to exhibit higher residual strengths. This increase could be due to less voids within the paste matrix, which would provide thermal stability, as reported previously [70; 71; 72]. Furthermore, unsealed GP 01 and 06 specimens recorded an increase in residual strength over a range of 400 \({}^{\circ}\)C to 800 \({}^{\circ}\)C, which could be attributed to the lowest Na\({}_{2}\)SiO\({}_{3}\)/NaOH ratio (ca. 0.5) of these two mixtures (GP 01 and 06). As sodium silicate has a higher thermal resistance, correspondingly at a higher alkali activator ratio, strength gains can be expected, even at higher temperature levels. It is to be noted here that the majority of the Gladstone FA specimens recorded higher initial compressive strengths compared to the Gladstone/Callide FA-based GP specimens. This could be attributed to the variances in particle sizes--Gladstone FA has a fineness of 86% passing the 45 \(\upmu\)m sieve compared to 80% of the Gladstone/Callide FA. This essentially points to the fact that the former has finer particles compared to the latter. It is also reported that [73] finer particles have a higher rate of dissolution during the geopolymerization process, thus resulting in higher compressive strengths. The difference in the composition  
Figure 10: A plot of residual strength for sealed Gladstone FA-based GP specimens. Figure 9: A plot of residual strength for unsealed Gladstone FA-based GP specimens. of the aluminosilicates has also been reported to affect the strength retention after exposure to high temperature levels [74]. A higher conversion of FA to amorphous aluminosilicate gels, and hence, a better retention of strength, has been reported at a higher Si/Al ratio. This supports the findings of the present study, where higher strengths were recorded from Gladstone FA-based GPs, which had a Si/Al ratio of 2.0 compared to 1.8 of the Gladstone/Callide FA. Table 6 and Figure 11 provide a tabulated and graphical comparison of the initial compressive strength between the two types of FA.