Mechanisms of set acceleration of Portland cement through CAC addition

The use of calcium aluminate cement as an efficient way of accelerating the setting time of Portland cements has been known for decades. In the absence of other additives, the poor strength development of such mixtures limits their use. However, additives can enable improved strength development for certain applications. This paper discusses the hydration mechanisms occurring in these systems. The precipitation and growth of ettringite are discussed as related to simple calcium aluminate cement/calcium sulphate mixtures, followed by discussion of the early hydration of pure Portland cements and the mechanism whereby the C 3 A reaction is controlled by calcium sulphate additions to avoid rapid setting. Studies of OPC/CAC systems indicate that the CAC additions destabilise the balance of the reaction of calcium sulphate with C 3 A. In these binary systems, set may be brought about by ettringite formation even when only low amounts of this phase are present. The reaction of CAC with different forms of calcium sulphate (gypsum, hemihydrate or anhydrite) can be used to understand why different Portland Cements require different levels of CAC addition.