The hydration of alite in the presence of concrete admixtures

Abstract

The role of three concrete admixtures on the hydration of alite clinker is studied. The effects of the three admixtures are (i) acts as a superplasticizer; (2) acts as a retarder; and (3) acts as an accelerator. The hydration products of alite with and without admixtures, added at different dosages, is examined by DTA and TG methods. The kinetics of hydration were followed by the estimation of free Ca(OH) 2 liberated during hydration as well as the chemically-combined water contents of the hydration products. The results reveal that the superplasticizer did not alter the rate of hydration of alite to any considerable extent, though the workability of the paste was greatly increased. The addition of 2–4% retarder delays the hydration of alite by prolongation of the dormant period from 6 h to about 7 days. The chemically-combined water contents suggest that the suitable dosage of this retarding agent is 1.0–2.0%. When 0.15–0.25% accelerator was used, the reaction rate of alite slowed down until the end of the first 24 h and was then enhanced. The chemically-combined water content was found to go parallel with the reaction rate.