Heat Modification of Chinese Fir Wood Catalyzed by Fly Ash under Mild Temperature

Heat modification (HM) can enhance the physical and mechanical properties of wood, but conventional HM (CoHM) requires high temperatures (above 180 °C) and consumes significant energy. Herein, fly ash was impregnated into Chinese fir for catalytic degradation at mild temperatures (100, 120, and 140 °C). After combining impregnation and HM (Im-HM) at mild temperatures, fly ash was deposited as Si- and Al-related floccules on the surface of the modified wood. These floccules accelerated the degradation of hemicellulose, resulting in weight loss percentages (1.85–5.21%) similar to those achieved with CoHM treatment at 160, 180, or 200 °C. The chromaticity of the Im-HM samples approached that of the CoHM samples when treated at 200 and 220 °C. The Im-HM treatment also improved the water resistance and dimensional stability of the wood. The antiswelling efficiency of the Im-HM samples (16.50–33.80%) exceeded that of the CoHM samples modified at or below 180 °C (≤16.31%). The hydrophobicity of the wood was associated with the Im-HM temperature and duration. A greater hydrophobicity was obtained at higher temperatures or longer durations. Our findings demonstrate the feasibility of catalytic degradation through fly ash impregnation, which offers a potential method for producing sustainable wood products with enhanced properties.