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6.4. Wood–Cement Composites Wood–cement composites are a panel for structural and building-related applications, which use Portland cement as the binder and the wood particles of various dimensions and shapes [87–89]. The panels have been used for thermal and acoustic insulation and for structural applications due to their fire resistance or termite resistance [88–92]. In an early study on possible re-use options for CCA-treated wood in Florida, cement- bonded particleboards and wood–cement composites were identified as having potential for several reasons. The wood may reduce the density of the pure cement, giving better insu- lating properties, while the cement may contribute to the stabilisation of the metals within the wood, minimising potential for leaching [93]. Other studies have assessed the proper- ties of cement-bonded wood composites manufactured with demolition wood [90,94,95]. The leaching of copper and arsenic was greatly reduced for Portland cement composites, in- cluding CCA-treated wood particles; however, chromium remained leachable [96]. Schmidt et al. [97] indicated that CCA-treated wood had greater compatibility with the cement than untreated wood, with greater resistance to fibre pull-out. A more recent study considered the hygrothermal properties of a wood fibre–cement system as permanent formwork for a structural system [98]. However, the wood used was shavings, i.e., post-industrial material, rather than post-consumer recycled wood. Wang et al. [99] considered wood that had been used as formwork in construction as a feedstock for cement-bonded particleboard materials and demonstrated that lightweight panels could be achieved by the suitable choice of admixtures that prompted crystal formation within the wood cells. It is most likely that particleboard is the most suitable format to accommodate recycled wood, as this reduction technology is better developed. However, Qi et al. [95] showed that MDF can be recycled into wood–cement composites with good results. Wood wool has also been formed from waste wood to demonstrate the potential in wood wool–cement boards (WWCBs) [100]. WWCBs are widely accepted due to the lower density and higher sound and thermal insulation achieved through use of wood wool. The heat flow was identical for the recycled and the raw wood wool in WWCBs, although the mechanical properties of the strands were different. This led to a slight decrease in WWCB panel strength as the proportion of recycled wool increased and fresh strands decreased. Some recent studies on wood cement composites have highlighted the beneficial effect of wood as a low carbon material to reduce the emissions or increase the carbon storage of the cement or concrete [101,102]. This may provide renewed impetus to developments within this area. 6.5. Extraction of Nanocrystalline Cellulose from Wood Waste Cellulose nanocrystals (CNCs) have diverse applications in fields such as bioplastics, composites, and biomedical materials [103]. Waste MDF has been identified as a potential source for extracting CNCs through sequential chemical degradation processes [104,105]. This method involves fractionating the MDF and isolating the CNCs. Couret et al. [105] used MDF waste fibres and other fibres from processes that replicate recycled wood fibre feedstocks to evaluate the effect of adhesives present on the nanocrystal generation process. The cellulose recovery yield was good for all four fibre types; however, in the MDF fibre types, a small percentage of the adhesives or non-wood starting material were still present in the nanocrystalline cellulose sample at the end of the process. TEM revealed good quality nanocrystals from all feedstocks.
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