Effect of gypsum content on the carbonation of sulfoaluminate cement

The carbonation behavior of sulfoaluminate cement (SAC) with different contents (0 %, 5 %, 10 %, 15 %, and 20 %, by mass) of gypsum was investigated via the accelerating method. During 28 days of carbonation, the mineral phase evolution, micromorphology, and pore structure were monitored using X-ray diffraction (XRD), thermal gravimetric analysis (TGA), backscattered scanning electron microscope (BSEM), and mercury intrusion porosimetry (MIP). Additionally, the alkalinities of hardened cement pastes and the compressive strengths of mortars that underwent carbonation were examined. In the carbonation process, monosulfoaluminate was first converted to hemicarbonate and finally to calcium carbonate, whereas monocarbonate was not found. The alkalinity of the hardened cement paste decreased significantly after carbonation, and the decrease was alleviated by introducing gypsum, especially in the early stage of carbonation. The porosity in the carbonated area of the cement paste was lower than the uncarbonated zone. The compressive strengths of the SAC mortars exhibited a slight decrease after 28 days of carbonation. It should be noted that the increase of gypsum content delayed the carbonation process of SAC; however, the relationship between the gypsum content and the carbonation resistance of SAC was not a simple positive correlation, but an optimal content (10 %) of gypsum existed.