Influence of metakaolin and acetic acid on sodium silicate-based inorganic bonded wood composites for additive manufacturing

Aluminosilicates such as geopolymers show promise as formaldehyde-free binders for inorganic-bonded wood composites. Understanding the curing behavior, mechanical properties, and wet performance of these composites is necessary to advance inorganic-bonded wood composites for construction additive manufacturing (AM) applications. This study examined the effects of metakaolin content, curing temperature, and acetic acid treatment on the dry and wet properties of wood-sodium silicate composites for AM. Wood-sodium silicate-metakaolin composite (WSSMC) formulations with 50 wt% wood fiber and 50 wt% binder we prepared with varying sodium silicate to metakaolin content (0, 5, 10 wt%). Dynamic rheology demonstrated suitable shear-thinning behavior and extrudability for formulations containing up to 10 wt% metakaolin. These composites displayed glass transition temperatures ranging between 181 and 201 °C (peak of Eʹʹ), and true densities increased with increasing curing temperature. Cold-pressed and extruded composites were subjected to acetic acid treatment and subsequent water soaking. Composites after acid treatment displayed changes to aluminosilicate bonds identified using Fourier transform infrared spectroscopy. Acid treated composites exhibited higher flexural (3–10 MPa) and compressive strengths (9–23 MPa) and lower weight gain and thickness swelling after water soaking compared to untreated composites. Flexural and compressive strengths were highest for dry, untreated composites (8–13 MPa and 12–34 MPa, respectively), especially when cured at lower temperatures (60 °C and 105 °C). This work provides insight into the use of acetic acid to improve the wet performance of inorganic bonded wood composites and demonstrates the potential for using metakaolin in inorganic bonded wood composites for novel wood composite additive manufacturing for construction.