Pretreated unbleached cellulose screen reject for cement-bonded fiberboards

Unbleached screen reject (USR) is a byproduct widely available from the kraft pulping that is rich in cellulose and requires a sustainable destination. Therefore, the purpose of this work was to determine the best condition to apply USR as reinforcement of cement-bonded fiber boards (CBFBs); using raw or pretreated USRs through water or sodium hydroxide immersion followed or not by refining. The size distribution, fiber morphology and chemical composition of the USR were investigated. The CBFBs were produced using the following parameters: wood:cement ratio of 1:2.75; water:cement ratio of 1:2.5; hydration rate of 25%; 4 wt% of the CaCl₂ additive (based on cement mass); target density of 1.4 g/cm³; and thickness of 15 mm. Physical and mechanical properties of the USR-containing CBFBs were evaluated. Obtained results indicated that USR consists of preserved fiber bundles weakly attached to each other. Their main proportion (> 95 wt%) is composed of particles greater than 20 mesh. The refining process slightly affects morphology and size distribution whereas immersion treatments partially removed lignin precipitates. The alkaline pretreatment of the waste followed by refining proved to be the best condition to improve the composites’ physical and mechanical properties. Overall, the mechanical and physical strength of the boards was higher or close to that of commercial products, except for MOR and PC. Nonetheless, such challenges could be overcome by improvements in the composite’s composition and the USR pretreatments. Thus, USR waste from cellulose industry showed great potential to be applied as reinforcement of CBFBs.