Thermodynamic study on the mutual influence of CaSO4·2H2O and CaCO3 in the reactions with C3A

Based on the principle of thermodynamics, the mutual influences of molar ratios K1 = nCaCO3/nC3A and K2 = nCaSO4·2H2O/nC3A on the hydration reactions of CaSO4·2H2O and CaCO3 with C3A in cement-based materials at 25 °C and 101 kPa were studied. The results reveal that when K1 > 0, K2 < 3, and K1+K2/3 < 1, CaSO4·2H2O and CaCO3 can fully react with C3A. The reaction product of CaSO4·2H2O and C3A is ettringite, while the reaction product of CaCO3 and C3A is monocarboaluminate and hemicarboaluminate. Ettringite undergoes transformation into monosulfoaluminate under the action of the unreacted C3A. When K1 > 0, K2 < 3, and K1+K2/3 ≥ 1, C3A completely reacts with CaSO4·2H2O and CaCO3, preventing the transformation of ettringite, while the reaction between CaCO3 and C3A results in the formation of monocarboaluminate and tricarboaluminate. In this case, the hydration reactions of CaCO3 and CaSO4·2H2O with C3A are primarily influenced by K2 and have a minor dependence on K1. When K1 increases while K2 remains constant, there is almost no change in the generation of hydration products involving CaCO3 and CaSO4·2H2O with C3A. Conversely, when K2 increases while K1 remains constant, there is a decrease in the total production of monocarboaluminate and tricarboaluminate, accompanied by an increase in the formation of ettringite. When K1 > 0, K1+K2/3 ≥ 1, and 0.3 ≤ K2 ≤ 1.7, which closely aligns with the typical range of K2 values in ordinary Portland cement, the saturation reaction content of CaCO3 in cement is within the range of 1.30%–5.51%. At each K2 value, CaCO3 exhibits a specific range of saturation reaction content. As K2 increases, this range initially expands and then contracts, and the maximum and minimum values of saturation content corresponding to each K2 gradually decrease.