## 3 Results and Discussion  
### Effect of Processing Parameters on Binder Performance  
The data from experiments no. 1 to 9 (Table 2) were used to develop a regression model (Table 3) for predicting the shear strength of the geopolymer binder. Referring to Table 3, the model had a coefficient of variation below 10%, a coefficient of determination (\(R^{2}\)) and adjusted \(R^{2}\) over 96%, and an adequate precision over 4. Hence, the model is acceptable according to available reports on the RSM study [32]. The optimum levels of pressing temperature and time for evaluating the effect of additives content on geopolymer performance, as well as the LVL production, were determined and validated, which were 140 \({}^{\circ}\)C and 112.5 s, respectively. Figure 1: Structure of the produced laminated veneer lumber (LVL) panels and testing specimens. The effect of hot-pressing temperature and time on the bonding shear strength of the geopolymer binder was simulated by using the regression model, as shown in Figure 2. As shown in Figure 2, the hot-pressing temperature and time were significantly influenced by the bonding shear strength of the geopolymer binder. At a lower press time than 112.5 s, the bonding shear strength of the geopolymer binder was increased by elevating the press temperature to 160 \({}^{\circ}\)C. At a lower press temperature than 140 \({}^{\circ}\)C, the bonding shear strength of the geopolymer binder was also increased by extending the press time to 360 s. According to [PERSON] et al. [14], elevating curing temperature will accelerate the dissolution, polymerization, and re-precipitation processes of the geopolymerization reaction. Hence, it can be noted that a high press temperature and time, in these ranges, intensifies the geopolymerization process through a higher dissolution of binder ingredients, and accordingly, the appropriate completion of geopolymer network formation [26; 38]. Conversely, the bonding shear strength tended to stay constant when both the press temperature and time are increased beyond 140 \({}^{\circ}\)C and 112.5 s, respectively. The reason might be that the dissolution and polycondensation processes may not have developed completely whereas some free water in the binder slurry evaporated due to the fast heat development [14]. The optimum levels of pressing temperature (140 \({}^{\circ}\)C) and time (112.5 s) obtained by RSM modelling was used to evaluate the effect of additives content on the bonding shear strength of geopolymer binder using the ABES instrument (Figure 3). As shown in Figure 3, the additives content had obvious effects on the bonding shear strength. The shear strength of geopolymer binder with no additives was about 3.5 MPa. Adding various amounts of phenol flakes into the geopolymer binder formulation showed no significant influence on the bonding shear strength. The bonding shear strength of the reference binder was increased about 45% after the addition of 10% conventional silica fume, reaching \begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Model 1**} & \
\begin{tabular}{c} **Coefficients of** \\ **Variation (\%)** \\ \end{tabular} &
\begin{tabular}{c} **Coefficient of** \\ **Determination (R2)** \\ \end{tabular} & **Adjusted R2** & **Adequate Precison** \\ \hline \(\mathrm{Y}=-11.49+0.16\mathrm{X_{1}}+\) & & & \\ \(0.027\mathrm{X_{2}}-0.00015\mathrm{X_{1}}\mathrm{X_{2}}-\) & 6.15 & 0.970 & 0.967 & 35.52 \\ \(0.00040\mathrm{X_{1}}^{2}-0.000012\mathrm{X_{2}}^{2}\) & & & \\ \hline \hline \end{tabular}  
* Y = value of bonding shear strength (MPa); \(\mathrm{X_{1}}=\) press temperature (\({}^{\circ}\)C); \(\mathrm{X_{2}}=\) press time (s).  
\end{table} 
Table 3: A regression model developed on the basis of the experimental data of bonding shear strength of the geopolymer binder. Figure 2: The interactive effect of hot-pressing temperature and time on the bonding shear strength of geopolymer binder determined by RSM simulation using the regression model, (**A**) three-dimensional picture and (**B**) two-dimensional picture.