The influence of two different types of polycarboxylic acid-based superplasticizers (homo- [HP] and copolymer [CP]) on the early hydration of Portland cement clinker is investigated. Polished clinker surfaces are hydrated in aqueous phase extracted from cement paste with and without superplasticizers as well as in saturated Ca(OH)(2)/CaSO4 solution and in deionized water. Scanning electron microscopy (SEM) is used to characterize the early hydration products after 30 min and after 24 h of hydration. Contrary to the hydration reaction with deionized water, in the presence of ionic solutions, the texture and morphology of the clinker surface are significantly altered, and amorphous and crystalline hydration products, mainly ettringite, are formed. The amount and type of ettringite formed are strongly related to solution,p chemistry and type of superplasticizer. In the presence of superplasticizers, the formation of hydration products and ettringite is reduced and the hydration of the surface is suppressed. Furthermore, the rate of hydration in mortar is studied in the presence of the two admixtures by measuring the temperature evolution as a function of time. The concentration-dependent delay in temperature evolution correlates well with the microscopic observations of different amounts of ettringite formation. (C) 2002 Elsevier Science Ltd. All rights reserved.
