The initial set time is defined as the time when a needle with a weight of 300 g has a penetration of 25 mm into the paste. On the other hand, the final set time is defined as the time when the same needle has a penetration of 5 mm. We have determined the Vicat set times for the series of cement pastes containing sucrose or etidronic acid (Figure ). For the reference and low retarder dosages, the initial set time (the "+" sign in the curves) corresponds approximately to the maximum slope of the C 3 S acceleration phase. At higher dosages, the initial set time occurs after the maximum slope. The final set times (denoted by an "x" in Figure ) are found after the maximum heat flow of the C 3 S reaction. The difference between the final and initial set times is larger for sucrose than etidronic acid. The initial and final set times t V icat scale logarithmically with the retarder dosage for both etidronic acid and sucrose (Figure ). However, we note a deviation from the logarithmic trend for the two highest dosages of sucrose. It needs to be noted that the Vicat experiments were conducted at 23 °C and under a relative humidity of 65 %. At the same time, the isothermal calorimeter was set to 20 °C.