## 3 Results and Discussion
### Shear Strength Analysis  
The shear strengths of LVL samples made of geopolymer binder with and without additives are shown in Figure 7. The shear strength values were significantly higher for the samples with no pretreatment compared to those that received the conditioning treatments of 24 h water soaking and 6 h water boiling that are indicators for determining their interior and humid applications, respectively. Referring to Figure 7, the samples experienced a severe change in condition after both water soaking and boiling pretreatments, which likely caused more moisture to penetrate the binder line and decrease the shear strength of the LVL samples. The average loss of shear strength after water soaking and water boiling pretreatments related to the untreated samples were about 50% and 60%, respectively. The areas exposed to moisture influenced the binder features, deteriorated the quality of geopolymer bonding, and, consequently, destroyed the bonding layers [9; 13; 28].  
Shear strength comparison of the LVL samples made by geopolymer binder with various additives showed that the addition of phenol flakes into the binder formulation significantly reduced the bonding shear strength. This can be explained through the higher viscosity of geopolymer binder, as shown in Table 4, after the addition of phenol. The higher the binder viscosity, the weaker the binder penetration, and accordingly, a weaker mechanical interlock can be feasible between the geopolymer binder and the wood constituents. On the other hand, the bonding shear strength was slightly enhanced with the addition of conventional silica fume. The higher the substitution by silica fume, the higher the bonding  
Figure 6: Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) spectra of table.  
shear strength in the geopolymer binder. The pores of the geopolymer binder can be refined by the addition of silica fume [15]. Notably, the highest bonding shear strength was obtained by using grafted silica fume with APTES. It was reported that the APTES as grafting agent can improve the interfacial adhesion between organic and inorganic substrates [32]. Hence, it can be concluded that the type of additives added into the binder formulation is an important factor influencing the geopolymerization process and, accordingly, the shear strength of the LVL samples.  
Shear strength values for most of the pre-treated samples were lower than 1 MPa, which is the upper limit value according to the European standard of EN 314-2. In EN 314-2, a correlation exists between the minimum shear strength and the percentage of wood failure. The higher the wood failure percentage, the lower is the required shear strength. A maximum of approximately 10% wood failure was observed in both pre-treated samples (cold water soaking and water boiling) produced with geopolymer binder mixed with silica fume that can be attributed to the enhanced bonding mechanism of geopolymer binder with wood. It is supposed that further addition of grafted silica fume in the binder formulation can further improve the bonding quality between wood and geopolymer binder to reach the required level of wood failure according to EN 314-2. The level of wood failure is somewhat dependent on the level and type of bonding mechanisms created between the wood cells and the binder. Wood is a porous material; hence, a mechanical interlock is the most likely bonding mechanism involved between the wood cells and the inorganic binder [9; 11; 27]. Chemical bonding formation between the wood and any mineral components, especially geopolymer binder, has not yet been proven [11]. It is feasible that the APTES used for the grafting of silica fume could have created better bonding mechanisms (e.g., a chemical linkage) with the superficial wood cells that needs to be further explored.