Transforming Fast-Growing Wood into High-Strength Materials via Thermo-Mechanical Densification with Hydrothermal and Alkaline Sulfite Pretreatment

Thermally compressed fast-growing wood exhibits superior mechanical properties, presenting a sustainable and cost-effective alternative to solid wood. However, to prevent structural damage and achieve higher densification during this process, effective pretreatment is essential. This study systematically evaluates the efficacy of hydrothermal and alkaline sulfite pretreatments in modifying Chinese fir (Cunninghamia lanceolata Hook.) and poplar (Populus tomentosa Carr.). The resulting compressed wood was comprehensively characterized in terms of mass loss, mechanical strength, microstructure, chemical composition, and cellulose crystallinity. Results indicate that, under the conditions tested, alkaline sulfite pretreatment was more effective than hydrothermal pretreatment in enhancing the material properties of densified wood, with peak density, compressive strength, and hardness achieved after 5 h for fir and 3 h for poplar, respectively. The results obtained under the present experimental conditions support the fact that alkaline sulfite pretreatment is an effective approach for producing densified wood with enhanced mechanical properties, suggesting its potential suitability for higher-value applications.