The influence of low calcium sulfate contents on early calcium aluminate cement-calcite hydration kinetics and pore solution chemistry

A binder mix consisting of CAC, calcite and H2O was investigated to clarify the influence of the addition of low amounts of calcium sulfate in the form of gypsum, hemihydrate or anhydrite on the hydration at 23 degrees C. Based on experimental data, a model for the hydration of CAC + calcite in the presence of sulfate ions could be developed. Using heat flow calorimetry at 23 degrees C, it was shown that hydration is accelerated depending on the solubility rates of the different sulfates. In addition, early hydration is characterized by a sequential reaction in which the calcium sulfate must be consumed completely from the pore solution by the precipitation of ettringite before the already known course of CAC + Cc hydration can occur. Monocarbonate and AH3 precipitate as the stable dominant hydrate phases during the main reaction, but conversion-sensitive hydrate phases such as CAH10 are not stable.