Hydration and Mechanical Properties of Calcium Sulphoaluminate Cement Containing Calcium Carbonate and Gypsum under NaCl Solutions.

Hydration characteristics and mechanical properties of calcium sulphoaluminate (CSA) cement with different contents of CaCO and gypsum under NaCl solutions were studied, using the testing methods of isothermal calorimetry, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), linear shrinkage, and compressive strength. Results show that CaCO can promote hydration and reduce the hydration heat of CSA cement. The reaction between gypsum and CAS- releases a large quantity of heat in the initial hydration period; however, over 3 days of accumulation, the level of hydration heat is reduced. Under NaCl solutions, the aluminate phase has difficulty reacting with CaCO to form carbonate phase but combines with chloride ions to form Friedel's salt. On the contrary, gypsum reduces aluminate phase, and the content of Friedel's salt is also reduced. Furthermore, CaCO and gypsum both increase the total porosity of the CSA cement paste under NaCl solutions during the early curing phase, and over the long-term, pore structure is also optimized. CaCO and gypsum reduce the linear shrinkage of CSA cement paste under NaCl solutions. Overall, the compressive strength of CSA cement is reduced with the addition of CaCO, and the trend will be sharper with the increase in CaCO. However, when it comes to gypsum, the compressive strength is almost the same during early curing, but in the long-term, compressive strength improves. Essentially, the compressive strength of CSA cement mortar with CaCO and gypsum will improve under NaCl solutions.