Value-added recycling of construction waste wood into eco-friendly cement-bonded particleboards

Abstract

Landfill disposal of waste wood formwork from construction sites presents significant environmental burden and economic wastage in Hong Kong. Upcycling the construction wood waste into value-added cement-bonded particleboards is a promising option, which capitalize the merits of wood and cement as individual components and address their respective shortcomings. However, several technical difficulties limits the commercial application, such as low wood-cement compatibility, degradation of wood particles, and low production efficiency. This study proposed new approaches to effectively produce well-compatible and high-performance particleboards by improving cement hydration chemistry and microstructure characteristics. The results showed that at suitable mixture design (wood-to-cement mass ratio of 3:7), cement hydrates in the porous structure ensured acceptable dimensional stability (< 2% swelling) and flexural strength (9 MPa). The chloride incorporation accelerated precipitation of oxychloride while sulphate addition produced calcium sulphoaluminate for promoting early strength development. The use of 2% CaCl₂ proved to be sufficient for improving the wood-cement compatibility. Besides, eco-friendly CO₂ curing could accelerate cement reaction and enhance physical properties. The 24-h CO₂ curing significantly facilitated cement hydration and accelerated Ca(OH)₂ transformation into CaCO₃, which contributed to strength development and carbon sequestration (as high as 9.2 wt%) in the particleboards. The results also illustrated the vital role of moisture content of particleboards in cement hydration and accelerated carbonation, for which the moisture content ranging from 16.7% to 17.9% was considered optimal. Moreover, the addition of grid basalt fibre (0.5% by wood volume) enhanced the flexural strength and fracture energy of the particleboards by 68% and 6.5 times, respectively. The particleboards also manifested outstanding structure-borne noise reduction (at 32-100 Hz) and low thermal conductivity (0.29 W m⁻¹ K⁻¹), suggesting potential application as acoustic and thermal insulating materials. This study also developed an innovative integrated technology by using magnesia cement and CO2 curing to transform contaminated wood waste into eco-friendly particleboards, which demonstrated excellent compatibility and value-added properties. An integration of 2-h CO₂ curing facilitated carbonation at early stage and reduced the volume of mesopores and air pores, which contributed to strength development and carbon sequestration in the particleboards (8.78% by weight) helping to combat global warming. A subsequent 7-d air curing further enhanced the strength, because rehydrated formation filled in capillary pores. Moreover, fire resistance and thermal stability were improved by the chemistry of magnesia cement and accelerated carbonation. The carbonated particleboards retained high strength and stable dimension after 1-h heating up to 200 °C. Based on the above findings, potential use of magnesia-phosphate cement (MPC) was exploited for upcycling wood waste into rapid-shaping particleboards. The results showed that the magnesia-to-phosphate (M/P) ratio controlled the formation of magnesium potassium phosphate hexahydrate (MgKPO₄·6H₂O, MKP) for strength development. Low M/P ratios gave ill-formed MKP, while high M/P ratios produced unreacted magnesia. The optimal M/P ratio of 7 presented much shorter setting time and greater compatibility with wood waste than ordinary Portland cement. Wood waste may provide a platform for cement hydration and porosity for harbouring crystalline MKP, as well as regulate water release to maintain moderate MPC reaction. We highlighted the importance of reaction sequence for promoting matrix homogeneity and MKP crystallinity. Since the MPC hydrates is unstable in water, alumina and red mud were used to improve water resistance of MPC particleboards. Addition of alumina or red mud (Mg/Al or Mg/Fe at optimal molar ratio of 10:1) facilitated formation of amorphous Mg-Al or Mg-Al-Fe phosphate gel, respectively, which enhanced compressive strength. Alumina improved short-term water resistance, whereas red mud provided better long-term water resistance. Red mud-MPC binder enhanced strength retention (by 22.8%) and reduced water absorption (by 26.4%) of particleboards after 72-h water immersion. In summary, technological innovation is crucial for delivering an eco-friendly solution to waste wood recycling for the construction industry. Before large-scale application, the functional properties of cement-bonded particleboards should be further validated, such as fire rating, flame spread, toxicant emission, acoustic reduction. The production technology should be optimized by using continuous production and flow-through CO₂ curing. Technical-economic assessment and life cycle assessment of particleboards should be performed for future scale-up production.