Effect of Portland Cement Blending with Calcium Sulfoaluminate Belite Cement and Calcium Sulfate on Carbonation Resistance

Calcium sulfoaluminate belite cement (CSAB) is an environment-friendly alternative to portland cement (PC). CSAB cement has been shown to have up to a 30% reduction in carbon dioxide emissions. Carbonation is one of the significant factors that can cause durability concerns in the reinforced concrete system. Carbonation of a cementitious system reduces the pH, destabilizing the passive layer around reinforcing steel and causing corrosion. In CSAB system, the primary hydration products are ettringite, monosulfate and aluminium hydroxide. The CSAB system with additional gypsum will have ettringite as the major hydration product. The surplus gypsum will promote ettringite formation at the expense of monosulfate, limiting its capacity to bind carbon dioxide. The microstructure of PC system will also be influenced by blending it with CSA admixture or CSAB cement and gypsum. This study aims to evaluate the carbonation behaviour of two expansive binders: 1) blend of PC and expansive CSA admixture, and 2) blend of PC, non-expansive CSAB cement and gypsum. The specimens were exposed to a 3% concentration of CO2 at 65% relative humidity to accelerate the carbonation rate. Additionally, the pore structure and microstructure changes were monitored by characterising non-carbonated and carbonated samples. The results indicate that PC blended with CSAB cement and gypsum carbonated faster than PC blended with CSA admixture. The additional ettringite present in PC blended CSAB cement resulted in increased carbonation rate.