We have previously shown that tartaric acid strongly retards the hydration of pure C 3 S at a concentration of 2 mmol L -1 which corresponds to a dosage 1 µmol g -1 by weight of C 3 S . It has been reported that tartaric acid preferably adsorbs onto C 3 A . Consequently, the low retardation observed in our experiments is due to the low dosage of tartaric acid that is most predominantly adsorbed onto the C 3 A phase or the early ettringite and, therefore, not available to retard the C 3 S hydration.