To determine the onset time according to ASTM C1679, the heat flow value, which corresponds to 50% of the maximum heat flow during the main hydration peak, is determined. Then, the time of the first occurrence of this value is determined, which ensures that the time on the ascending slope of the C 3 S peak is obtained. Again, the trends closely resemble the trend between t AST M and retarder dosage of the other methods (Figure ). As expected, the specific values are smaller than those obtained for t m .