In addition to the well-established relation between compressive strength development and the heat of hydration, the heat flow signal can be used to determine the setting time of the cement paste. The relationship between the Vicat set and isothermal heat flow calorimetry has been studied with different approaches . One approach is the determination of the intersection of the tangent to the minimum of the heat flow curve during the dormant period and the tangent to the acceleration slope of the C 3 S reaction. It was found that the intersection of these two tangents shows a good correlation to the initial set time . A second approach is to determine the time of the largest acceleration of the heat flow signal, i.e., the steepest slope of the C 3 S reaction. Ge and coworkers have reported that the timing of the maximum slope corresponds to the initial set . Another method to determine the initial set is described in ASTM C1679.