Development of plywood using geopolymer as binder: effect of silica fume on the plywood and binder characteristics

Geopolymers show good potential to be used as free formaldehyde-based binder to produce wood-based panels. Hence, the objective of this research was to investigate the geopolymer composition as binder on the physical and mechanical properties of multi-layered plywood. The geopolymer binder was prepared based on kaolin and metakaolin as the major aluminosilicate powder that were substituted with different contents of silica fume. The results showed that the substitution of aluminosilicate with silica fume (up to 20% based on weight) in the geopolymer mixture modified the chemistry of the geopolymerisation, and hence, improved the amorphous structures of the geopolymer binder. In summary, the substitution of aluminosilicate with silica fume (up to 20% based on weight) in the geopolymer mixture improved the geopolymer binder cohesion, reduced the binder viscosity, reduced the binder curing time, increased the binder penetration into the superficial wood cells, increased the binder shear strength, increased the bending strength of plywood, and accordingly, reduced the plywood stiffness. Notably, none of the plywood samples did delaminate even after 672 h water immersion, implying that the geopolymer binder-based products have a better stability in water compared to some organic binders (e.g., adhesives based on tannin, soya and starch), which suffer from hydrolysis after immersion in water. All in all, the geopolymer binder based on metakaolin showed promising potential to be used as formaldehyde-free binder to produce plywood.