Geopolymer-Bonded Laminated Veneer Lumber as Environmentally Friendly and Formaldehyde-Free Product: Effect of Various Additives on Geopolymer Binder Features

Abstract

Environmentally friendly and formaldehyde-free laminated veneer lumber (LVL) was manufactured using geopolymer constituents as binder. The main aim of the study was to improve the bonding quality between the geopolymer binder and the wood constituents. To this end, the effect of various additives (phenol flakes, conventional silica fume, and grafted silica fume with 3-aminopropyltriethoxysilane (APTES)) in the geopolymer binder features were explored via gel time and viscosity measurements, differential scanning calorimetry (DSC), and Fourier transom infrared spectroscopy. The mechanical properties (shear, bending, and compression) of LVL panels were also determined. Results showed that adding both types of silica fume had a positive impact on the geopolymer binder features. The formation of an alkaline aluminosilicate network was proven by observing the characteristics peaks of geopolymer binder at about 683 and 970 cm−1. A peak temperature of about 98 °C was determined for the geopolymer binder curing via DSC analysis. The mechanical properties were the highest for LVL panels made of geopolymer binder with grafted silica fume. It is feasible that the APTES used as grafting agent created a better bonding mechanism with superficial wood cells. In summary, the produced LVL panel showed good properties, but it still needs to be further improved to reach the required levels for use in interior and humid application.

A Multifunctional Mineral Binder for Plywood Production: The Effect of Manufacturing Parameters on Bonding Quality

“…Previous studies also confirmed that a higher press temperature has a positive influence on the shear strength and bonding quality of the geopolymer binder [12,13]. This can be attributed to the higher dissolution of binder ingredients and the successful geopolymer network formation [24]. Notably, the shear strength of the geopolymer-bonded plywood was negatively influenced by further increasing the press temperature from 140 °C to 160 °C.…”

“…Finally, a steady increase of WA was observed after 384 h of water soaking. The increasing trend of regimes 2 and 3 are possibly due to the additional micro-cracks created in the hardened geopolymer when it was immersed in water [11,24]. As mentioned earlier, water may also cause cracking in geopolymer due to capillary forces.…”

“…where WA is the water absorption at time t, m t and m I are the weights of the samples at time t (2,24,48,96,192,386, and 768 h) and the weight of the samples prior to water soaking, respectively. Nine replications for each panel variation were tested.…”

“…With reference to Figure 3, the samples experienced an extreme change in condition after both the water soaking and boiling pre-treatments, which possibility caused more moisture to penetrate the binder line and decrease the shear strength of the plywood samples. The areas exposed to moisture influenced the binder features, deteriorated the quality of the geopolymer bonding, and consequently, reduced the quality of bonding [24]. In other words, the dimensional stability (swelling and shrinkage) of the wood caused the formation of cracks in the wood-binder interface and a decrease in the binder shear strength [11].…”

“…Although the relationship between the asymmetric stretching peak positions of various groups of T-O-Si (T = Al and Si) and the extent of the geopolymerisation process are complex, it is still informative to study the geopolymerisation mechanism [16]. The symmetrical stretching of Si-O-Si and Al-O-Si, typical for the formation of an alkaline aluminosilicate network in a metakaolin-based geopolymer, appeared at about 688 cm −1 in all binders cured with various press temperatures [18,24]. The stretching vibrations of the Si-O-R bond (R = Na, K, or Si) were observed at 990 cm −1 for the geopolymer binder cured at a 140 °C press Referring to Figure 7B, the geopolymer-bonded plywood with smooth veneers showed a MOR and MOE of about 72 MPa, and 8300 MPa, respectively.…”