Properties and microstructure of light-weight wood-magnesium oxychloride cement composites

Abstract

This work developed a novel way to recycle timber waste in magnesium oxychloride cement (MOC). MOC has many superior properties compared to Ordinary Portland cement (OPC), but it shows poor water resistance due to the decomposition of hydration products (e.g. Phase 3 and Phase 5) In the first part, CO₂ curing and various supplementary cementitious materials (SCMs) were used to improve the water resistance of MOC paste. The experimental results demonstrated that the water resistance was enhanced when CO₂ curing method was adopted due to the formation of a certain amount of amorphous phase which was stable in water and protect the hydration product from decomposition. The addition of SCMs, pulverized fuel ash (PFA), glass powder (GP) and incinerated sewage sludge ash (ISSA), could significantly improve the water resistance and volume stability of MOC pastes in water. The active phases in PFA or ISSA could react with MgO and produce an amorphous phase (amorphous magnesium aluminosilicate gel), which was interspersed with Phase 5 and changed the morphology of Phase 5 to fibroid or lath-like phases. These fibroid or lath-like phases interlocked with each other and connected with the amorphous phase in the matrix to form a stable compact structure. In the second part, waste timber was added in MOC paste as fibre. The properties and microstructure of wood-MOC board was investigated. It was found that the flexural strength of wood-MOC paste was similar to that of MOC paste after 28 days of water immersion. And wood fibre could enhance the volume stability of MOC paste matrix after 28 days of water immersion due to the high porosity that accommodated the expansion of MOC paste. ATR­FTIR showed new bonding were formed between the wood fibre and the MOC paste matrix. XRD results showed the formation of dashkovaite (Mg(HCOO)₂ · ₂ H₂O), which was in agreement with the ATR-FTIR results. The flexural strength of the wood cement paste increased with the increasing fibre length due to the higher crack bridge function of the longer fibre. And the expansion in air decreased with the increasing fibre length. There is no clear trend about the effect of fibre length on the water resistance of wood-cement paste after 28 days of water immersion. Adding supplementary cementitious materials, especially incineration sewage sludge ash (ISSA), could improve the water resistance and volume stability of the wood-cement paste. In terms of fibre content, the result showed that the wood-MOC composites prepared with a higher content of wood fibre had a lower thermal conductivity, higher flexural strength, higher residual flexural strength after exposure to high temperatures and water immersion, and better noise reduction effect. Even though the water absorption was increased with the increase of wood fibre content, it can still be considered low. The wood ­MOC composites incorporating ISSA showed higher flexural strength, better high temperature resistance and better water resistance than other composites. Besides, greenhouse gases (GHGs) emission for the production of different types of composite boards was assessed and compared by using lifecycle assessment (LCA) technique. The 'cradle-to-gate'system boundary with 1 kg of board production was considered as the functional unit in this assessment. The results showed that the production of the wood MOC board induces lower GHGs emission than plywood and lower human toxicity than conventional resin-based particleboards.