This study was performed to understand the influence of a critical amount of CA on the kinetics of C3S hydration. For this purpose, monoclinic C3S was blended with 15 wt.% CA and investigated by heat flow calorimetry and in situ XRD at 23 degrees C at a water to cement ratio of 0.5. The binary mixture shows 3 distinct heat flow maxima where the underlying C3S dissolution is proceeding stepwise. The C3S dissolution rates during the 3 steps are varying strongly, depending on the hydrate phase precipitated during the respective reaction step. Comparison of these dissolution rates with a pure C3S reference sample allows the conclusion that the dissolution rate of pure C3S after the heat flow maximum might be governed by either the remaining available reactive surface of C3S or a diffusion-controlled process, which would both be influenced by the respective hydrate phase precipitating on the C3S surface.