Preparation of adhesive-free composite boards from agricultural and forestry solid waste: pretreatment-promoted enhancement of wet hot-pressing compatibility

Fabricating binder-free composite boards upcycles agro-forestry residues. To enhance the low strength and poor weatherability of wheat straw, four hard shells were screened as biomineralizers. A differential pretreatment was developed: alkali treatment for wheat straw to facilitate cross-linking, and acid-ZnCl₂ treatment for shells to preserve lignin rigidity and expose hydroxyl groups. Particle size critically affected mechanical properties; fine particles compromised physical interlocking. Oil-tea oleifera shells exhibited superior performance due to their porous structure, which adsorbed ZnCl₂ and formed dense regions under pressure. Optimal composite boards (wheat straw and 2 mm oil-tea oleifera shells) achieved tensile and bending strengths of 28.62 MPa and 35.58 MPa, increasing by 83.93% and 46.42% over pure straw boards, respectively. Water absorption decreased to 32.30%, reductions of 34.10% and 74.01% compared to pure shell and straw boards, respectively. This differential pretreatment method provides new insights for multi-source biomass board fabrication.