## 3 Results and Discussion
### Binder Characterisation  
Viscosity and gel time of the binders with various types and contents of additives are presented in Table 4. The viscosity and curing time of the geopolymer binder with no additives were 1040 cP and 83.7 s, respectively. The viscosity was noticeably increased with the addition of phenol flakes into the  
Figure 4: Fractured surface of the geopolymer binder mixed with various additives using the ABES instrument. Figure 3: Bonding shear strength of geopolymer binder mixed with various additives using an Automated Bonding Evaluation System (ABES) instrument. binder recipe, reaching to about 6280 and 10,740 cP after adding 5 wt % and 10 wt % phenol based on the metakaolin powder, respectively. Phenol donates a proton to the hydroxyl ions in the alkaline solution, with the phenol becoming the phenolate anion (phenoxide anion). In other words, the phenol will absorb some water in the binder and accordingly increase the binder viscosity [39]. Consequently, the diffusion and mobility of the geopolymer network are reduced. Therefore, the curing reactions of geopolymer may be influenced by diffusion because the mobility of the molecules decreases when the molecular weight increases. The addition of both conventional and functionalized silica fume also increased the geopolymer binder viscosity, but their increasing impact was not as significant as those for the phenol flakes. The final solid content of the geopolymer binders were slightly increased after the admixture of additives into the binder formulation. The higher the binder solid content, the higher the viscosity that was achieved. Furthermore, the differences between the viscosities of the binder with similar content of the additives could be due to their various effects on the geopolymerization process. The lowest viscosity of the binder after addition of the additives belonged to the functionalized silica fume. Viscosity is an essential factor influencing the binder penetration into the porous structure of the wood [9; 11]. The effects of additives on the curing time of geopolymer binders are presented in Table 4. The addition of phenol delayed the gel time of the geopolymer binder to about 90 s. The pH level of the geopolymer binder strongly influenced the geopolymerization process, as well as the final binder characteristics. A high alkalinity is needed for the appropriate dissolution of the aluminosilicate powder. Phenol is a weak acid that negatively influences the binder curing. Addition of both conventional and functionalized silica fume accelerated the solidification of the geopolymer binder. Silica fume is favored in geopolymer formulation because of its acceptable early-stage strength development [15]. Notably, the gel time in geopolymer binder based on functionalized silica fume was shorter in comparison to those of conventional silica fume. This can be attributed to the alkaline nature of the APTES, leading to faster geopolymerization. The DSC data are also presented in Table 4. The onset and peak temperatures of the reference geopolymer binder (no additives) were recorded at 79 \({}^{\circ}\)C and 98.8 \({}^{\circ}\)C, respectively, that is, somewhat like that of urea formaldehyde adhesive [5; 40]. The onset temperature was not significantly changed with the use of various additives in the geopolymer formulation. The peak temperature of the geopolymer binder was not affected either after the addition of conventional and functionalized silica fume, whereas the addition of phenol flakes slightly reduced the peak temperature to about 95 \({}^{\circ}\)C. The curing reactions of the geopolymer binder was generally exothermic and became the main contribution to heat evolution during DSC measurements (Figure 5). The DSC results showed that the heat of reaction obviously decreased with the addition of phenol into the binder formulation. Conversely, the heat of reaction was slightly increased with the addition of both conventional and grafted silica fumes into the geopolymer binder formulation.