## 3 Results and Discussion
### Evaluations of Initial and Residual Strengths  
When considering the initial compressive strengths of GP specimens, majority of specimens having a lower alkaline solution to FA ratio (0.4) achieved higher initial strengths, when the Na\({}_{2}\)SiO\({}_{3}\)/NaOH ratios were kept constant. This could probably be due to the dense microstructure brought about from using a lower alkaline solution to FA ratio, which could result in fewer pores and hence higher internal strengths. In addition, when the alkaline solution to FA ratio was kept constant, the initial strength of the specimens were found to increase gradually as the Na\({}_{2}\)SiO\({}_{3}\)/NaOH ratios increased, which could be attributed to the inclusion of more sodium silicate, which in turn can enhance the geopolymerization reaction, thus resulting in higher strengths (Figure 2). After exposure to elevated temperature levels of 800 \({}^{\circ}\)C, majority of the GP specimens were found to be still intact with only mild cracking. Furthermore, no cracking was observed in GP 01-05 specimens after being exposed to both 400 \({}^{\circ}\)C and 800 \({}^{\circ}\)C and mild cracking in the latter set of samples (i.e., GP 06-10), that too only after being exposed to 800 \({}^{\circ}\)C. Owing to the high initial strengths and thermal performance of GP 05 and GP 10, they were explored for further mix designs leading to the production of RPGC specimens. When considering the initial strength developments of RPGC specimens, conditioning was found to be a highly influential factor, where specimens conditioned at 25 \({}^{\circ}\)C recorded much lower 7-day-strength readings compared to the specimens cast using the same mix design, but conditioned at 75 \({}^{\circ}\)C. The tabulated and associated graphical representation of, data are presented in Table 12 and Figure 3, respectively. Similar finding were reported previously [6,50], where it was also stated that a relatively lower percentage of calcium-silicate-hydrate (C-S-H) gels were produced, when an insufficient supply of heated water was provided, which was assumed to create a weaker microstructural matrix within RPC specimens. A previous report also showed that when RPC was water cured under room temperature, the pozzolanic activity was found to be slower, as heated water curing has the ability to rapidly increase the pozzolonic reaction of the silica \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample ID** & **Mass (kg)** & **Density (kg/m\({}^{3}\))** \\ \hline RPCG 01 & 0.2245 & 2245 \\ RPCG 02 & 0.2193 & 2193 \\ RPCG 03 & 0.2145 & 2145 \\ RPCG 04 & 0.2120 & 2120 \\ \hline \hline \end{tabular}
\end{table} 
Table 11: Density results of RPGC mixtures. Figure 2: A plot of initial and residual compressive strengths of GP specimens. amounts of hydrated compounds occurring through the formation of secondary hydrated products. This, in turn can result in an increased extent of bonding between the cement paste and the aggregate particles, thus enhancing the internal strength. In addition, the formation of ettrinigite can hinder the rate of hydration thus, reduces the rate of strength development [42]. Moreover, a continuous supply of moisture can result in a higher consumption of portlandites, which can further lead to a higher degree of hydrated products. These hydrated products could act as an inert filler material which could in turn fill the voids and gel pores within the matrix. This could ultimately a lead to denser microstructure with a better interlocking structure and fewer capillary pores- similar findings were reported previously [6; 42; 51]. Regardless of the very high initial performances, the RPC specimens exhibited completely different thermal performances. These samples displayed explosive spalling phenomenon when the furnace temperature reached approximately 360 \({}^{\circ}\)C; hence no residual strength results could be measured.