Effect of Different Retarders and their Combination with Superplasticizer on the Properties of CSA

Abstract

The aim of this work was to investigate the effect of polycarboxylate type superplasticizers (PCE), and three different retarders (citric acid, tartaric acid, potassium gluconate) and their combination on hydration process and rheological properties of CSA at early age. The hydration process was described through isothermal calorimetry, while the rheological properties were characterized through yield stress of cement pastes determined by rheometry. The results show that gluconate is the most powerful retarder, followed by tartaric acid, then citric acid. PCE strongly improves the fluidity of the pastes but a rapid loss of workability is noticed. On the contrary, retarders slightly decrease the yield stress of the pastes but slowdown the loss of fluidity. The combination of the both admixtures allows to maintain high fluidity for longer time. However, for citric acid, and even more for tartaric acid, a rheological instability, characterized by a decrease followed by an increase in the yield stress over time, is noticed. Potassium gluconate seems to be the best retarder to combine with PCE since no instability and longer maintenance of workability were observed. An optimum dosage, leading to the best initial fluidity to the pastes, was also determined for the three retarders. The rheological results are related to the delay induced by the retarder on the hydration products formation for the first hours of hydration and to competitive adsorption between retarder and PCE.