## 2 Materials and Methods
### Materials  
#### 2.1.1 Geopolymer (GP)-Based Pastes  
The fly ash (FA) utilized in this investigation was obtained from Gladstone power station in Queensland, Australia (Class F- low calcium FA: ASTM C618 [30]. The chemical composition, obtained X-Ray Fluorescence (XRF) analysis and as reported by the manufacturer, is shown in Table 1. The various physical properties of FA, such as, particle size distribution, mean particle diameter, specific surface area, average pore diameter and pore volume are also given. The information regarding the particle size distribution and average diameter were also available from the supplier--these parameters were measured by using a CILAS Particle Size Analyzer 1190. The particle shape analyses were performed by using a Zeiss Supra 40 VP Scanning Electron Microscope (SEM); however, the actual SEM images were not made available owing to commercial sensitivity. The samples were thinly gold coated with a DYNAVAC (CS 300) deposition system prior to the SEM analysis. The specific surface area and pore properties of FA were based on the nitrogen adsorption method, by using Belsorp Max equipment (i.e., by using Brunauer-Emmett-Teller (BET) theory-BELMasterTM Version 6.3.1.0 software, which was developed by BEL Japan, Inc., Osaka, Japan) [31]. Here, the amount of adsorbed gas to a monomolecular layer on the surface corresponds to a specific surface area and pore volume value. Figure 1 shows the particle size distribution for the Gladstone based fly ash. The density of the sample was determined to be 2.29 g/cm\({}^{3}\). The largest particle diameter measured for Gladstone fly ash was 100 \(\upmu\)m, with 50% of Gladstone fly ash particle size ranging from 1 \(\upmu\)m to 100 \(\upmu\)m and 50% are under 1 \(\upmu\)m in size. The pore volume was determined to be 3.39 mm\({}^{3}\)/g and the mean pore diameter was 11.8 nm. The specific surface area of the Gladstone fly ash was 1.15 m\({}^{2}\)/g which was in agreement with previously reported results [32]. The alkaline activator that was employed constituted of Grade D sodium silicate (ratio of SiO2 to Na2O is 1:2 by weight) and 8.0 M sodium hydroxide, NaOH. The sodium hydroxide solution consisted of 26.2 wt. % of NaOH solids and 73.8 wt. % of water [33]. A total of 10 alkaline activator combinations were investigated in this study and they consisted of varying the ratio of Na2 SiO3/NaOH, from 0.5, 1.0, 1.5, 2.0 and 2.5 and at two alkaline solution to FA ratios of 0.4 and 0.57. These combinations are referred in the text as GP 01, GP 02 etc., as summarized in Table 2, which also presents the complete mix design proportions in each case. Sodium hydroxide and sodium silicate were first measured and mixed using a handheld stirrer bar until a clear, transparent solution was formed (ca. 2 min). The mixture was then kept aside to rest for 3 min before being mixed with the FA. This method was essentially adopted from a previously published work by [PERSON] and [PERSON] [34]. Subsequently, the FA was measured and mixed with the liquid solution, using a _Breville_ mixer at a working speed of 50 revolutions per minute (rpm) for 2 min and then for another 3 min at a working speed of 85 rpm, after which, 25 mm\({}^{3}\) specimens were casted. \begin{table}
\begin{tabular}{c c} \hline
**Compound** & **Oxide (wt. \%)** \\ \hline SiO2 & 51.1 \\ Al2O3 & 25.6 \\ Fe2O3 & 12.5 \\ CaO & 4.30 \\ K2O & 0.70 \\ MgO & 1.45 \\ Na2O & 0.77 \\ TiO2 & 1.32 \\ BaO & 0.09 \\ SO3 & 0.24 \\ P2O5 & 0.89 \\ MnO & 0.15 \\ Volatile fraction & 0.89 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical compositions of FA. Figure 1: The aggregate particle size distribution graph for Gladstone FA. The cubical samples were sealed immediately after casting in oven bags to minimize the effects of initial surface evaporation. The sealed specimens were subjected to oven conditioning, at 60 \({}^{\circ}\)C for 24 h in a _WEISS WVC Series Temperature and Climatic Test Chamber_. Once hardened, the cubes were removed from the molds, re-sealed and placed in the oven.