Preparation of durable magnesium oxysulfate cement with the incorporation of mineral admixtures and sequestration of carbon dioxide

To reduce the consumption of energy and raw materials caused by the production of Portland cement and enhance the carbon dioxide sequestration of building materials, this paper aims to manufacture durable and green magnesium oxysulfate cement based on incorporating mineral admixtures (fly ash (FA) or ground granulated blast-furnace slag (GGBFS)) and CO₂ curing treatment. Compressive strength, flexural strength, resistance to water and wetting-drying cycles of magnesium oxysulfate (MOS) were evaluated. Phase compositions and microstructures of typical samples were measured by X-ray diffraction (XRD), differential scanning calorimetry (DSC-TG), and scanning electron microscope (SEM) techniques. The results showed that mechanical strength and strength retention after wetting-drying treatment of MOS cement was increased by the sequestration of carbon dioxide. Both FA and GGBFS could improve the water resistance due to restrainting the phase conversion of MgO into Mg(OH)₂. However, the addition of FA or GGBFS deteriorated the compressive strength of MOS cement samples after wetting-drying treatment, owing to the formation of more magnesium hydroxide crystals and decomposition of 5 Mg(OH)₂·MgSO₄·7H₂O (5·1·7 phase). Furthermore, about 5% carbon dioxide can be captured by MOS cement paste during 24 h accelerated carbonation treatment. Therefore, the incorporation of mineral admixtures and sequestration of carbon dioxide was suggested as an effective method in manufacturing the highly durable and cleaner magnesium oxysulfate cement.