Synergistic enhancement of water resistance in magnesium oxychloride cement by fly ash and carbonation curing

Developing high-performance, low-carbon cementitious materials is critical to addressing global climate change and the urgent need for carbon reduction in the construction industry. To overcome the poor water resistance of magnesium oxychloride cement (MOC) and reduce its carbon footprint, this study investigates the synergistic effect of incorporating fly ash (FA) as a partial replacement for MgO combined with carbonation curing. The workability, mechanical properties, water resistance and capillary water absorption of MOC-FA blends were systematically evaluated. Hydration heat was monitored using isothermal calorimetry (TAM Air), while phase composition and microstructure were characterized by XRD, TGA, MIP, and SEM to elucidate the underlying mechanisms. Results indicate that both FA incorporation and carbonation curing significantly improve water resistance, boosting the softening coefficient after 28 days of water immersion from 0.025 to 0.832. The addition of FA enhances workability, increasing fluidity by 1.3 %-11.4 %. Although FA leads to a reduction in early-age strength, the formation of M-S-H gel mitigates the dissolution of key hydration products, thereby markedly improving water resistance. Furthermore, carbonation curing not only facilitates CO₂ sequestration but also reduces total porosity by 2.6 %-29.1 % through the formation of stable carbonate phases and additional hydrates, leading to substantial gains in mechanical strength and impermeability. Moreover, carbonation curing effectively compensates for the early strength loss induced by FA (increasing strength by up to 26.9 %) and, through synergy with FA, enhances both the durability and overall performance of MOC. This approach demonstrates significant potential for CO₂ sequestration, resource utilization of FA, and development of high-performance MOC materials.