Influence of Calcium Formate on the Hydration of Tricalcium Aluminate–Gypsum Systems

Calcium formate (CF) is a widely used, chloride-free organic admixture, and yet its effects on the hydration of tricalcium aluminate (C3A), the component in cement that hydrates the fastest, remain incompletely understood. This study investigates the mechanism by which CF influences the hydration products of the C3A-gypsum system with different gypsum contents, specifically focusing on ettringite (AFt), monosulfate (AFm), and hydrogarnet (C3AH6), through isothermal calorimetry, X-ray diffraction (XRD), temperature-modulated differential scanning calorimetry (TG-DSC), inductively coupled plasma (ICP), and thermodynamic phase composition calculation. The results demonstrate that CF accelerates the initial hydration of C3A and the formation of AFt. When the gypsum content is relatively high, the influence of CF on the conversion of AFt to AFm is not significant. However, with lower gypsum content (mass ratios of C3A to gypsum being 3∶1 and 2∶1), CF markedly inhibits the conversion of AFt to AFm while facilitating the generation of C3AH6. These findings enhance the theoretical understanding of additive interactions in cement and provide guidance for the development of noncorrosive concrete additives.