Setting of cement with controlled superplasticizer addition monitored by ultrasonic measurements and calorimetry

The use of ultrasonic measurements to monitor the early hydration of cement based systems is extended during the last decade. Superplasticizers (SPs) affect the rheology just as the hydration process of cementitious systems. With delayed addition, a minor consumption and intercalation of SPs can take place, which enhances their impact.This study presents investigations on cement pastes and mortars with additions of different SPs in varying dosages and addition times. Effects on hydration process, setting and rheological behavior were monitored.The critical as well as the saturation dosage can be reduced significantly with a delayed addition of the SPs. By the application of highly charged SPs, the onset of the acceleration is significantly retarded by delayed addition whereas low charged SPs influence the onset by its concentration. Calorimetric and ultrasonic data determine significant influences on the renewed AFt formation and transformation of AFt to AFm during the deceleration period. A new calculation method was applied which allows a very good location of the initial setting directly from the calorimetric data. A nearly direct correlation was found between the calculated onset of the acceleration and the time of the first inflection point of the ultrasonic velocity.