This study demonstrated a low-carbon and value-added approach for upcycling waste wood formwork into reactive magnesia cement-bonded particleboards, in which green alternatives and carbon dioxide curing were employed to improve mechanical properties and reduce carbon footprint. Spectroscopic and thermogravimetric analyses showed that the replacement of pulverised fly ash and incinerated sewage sludge ash promoted the generation of magnesium silicate hydrates gel via pozzolanic-like reaction. The incorporation of mussel shell densified the microstructure as illustrated from scanning electron microscopy. Therefore, these modified particleboards presented satisfying flexural strength (>9 MPa) and thickness swelling (<2%). However, the spontaneous hydration of blast furnace slag manifested a competition against magnesia cement hydration and resulted in strength reduction. Furthermore, carbon dioxide curing promoted the transformation from magnesium hydroxide to additional magnesium carbonate and hydrated magnesium carbonates, which significantly enhanced early strength. However, the carbonation process consumed magnesium hydroxide, unavoidably suppressing the pozzolanic reaction. By contrast, blast furnace slag-incorporated samples showed synergetic hydration and carbonation. In summary, pulverised fly ash materials are promising substitutes in air curing condition, whereas blast furnace slag is suitable for accelerated carbonation of magnesia cement-based particleboards.