The nanoscale dissolution flux of tricalcium aluminate (C3A) in flowing water is characterized in situ by Digital Holographic Microscopy (DHM). The pure dissolution of C3A in flowing water with a flow rate of 34 ml·min-1 before the precipitation of hydrated phases only lasted about one quarter of a second, and the pure dissolution rate mostly falls in the range of 500 ~ 1500 μmol·m-2·s-1. Subsequently, foil shape hydration product formed and rapidly covered the surface of C3A, leading to the dramatic decrease of the overall dissolution rate to 55 ± 10 μmol·m-2·s-1.