The Development of Microstructure in Calcium Sulphoaluminate Expansive Cement
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THE DEVELOPMENT OF MICROSTRUCTURE IN CALCIUM SULPHOALUMINATE EXPANSIVE CEMENT
John Herrick, Karen L. Scrivener & Peter L. Pratt Department of Materials, Imperial College of Science, London, SW7 2BP, United Kingdom.
Technology & Medicine,
ABSTRACT Using backscattered electron images, it is shown that an expansive clinker is polymineralic in nature, with individual particles containing the constituents (lime, anhydrite and anhydrous calcium sulphoaluminate) to form ettringite on hydration. The hydration reaction with time of a simple paste of a mixture of this expansive clinker with Ordinary Portland Cement (OPC) has been monitored using backscattered electron images, and the formation of ettringite and a change in the morphology of the calcium hydroxide formed from the added free lime from the expansive clinker is observed. Thermal analysis studies on pastes of the pure expansive clinker have confirmed this apparent change in morphology.
INTRODUCTION The quest for a method of overcoming drying shrinkage in concretes has led to the use of expansive additives, and in particular, mixtures of materials that form ettringite on hydration. Thus, expanding cements based on a mixture of OPC and an expansive component, consisting of anhydrous calcium sulphoaluminate (C 4 A3 ?), lime (C) and a source of calcium sulphate, either anhydrite (CS) or gypsum (CSH 2 ) are of particular interest. These are known as Type K expansive cements and in this study, a commercially available expansive additive, made as a single clinker, containing all three materials required to form ettringite on hydration (i.e. anhydrous calcium sulphoaluminate (C 4 A3 S), lime (C) and anhydrite (CS)) has been used. This clinker is ground and mixed with OPC to give the expansive cement. The microstructure of this expansive cement additive and the hydration products of a simple paste of its mixture with OPC has been investigated using backscattered electron images coupled with EDX in the SEM, thermal analysis and XRD. The stoichiometric equation for the hydration of this expansive clinker to form ettringite is as follows:-
C4 A3S + 8 CS + 6 C + 96 H ------- > 3 C6 AS3 H3 2 All previous workers have reported the formation of ettringite from this type of expansive clinker, and various theories for the mechanism of this ettringite formation have also been reported by MehtaI and Cohen 2 .
EXPERIMENTAL A sample of the anhydrous expansive additive, as supplied, was set in resin, lapped and polished (to ¼ ym diamond) to give a freshly cut flat, polished surface which was carbon coated for examination in the SEM. Paste samples (12% expansive additive in OPC, w/c = 0-62 ) were mixed Mat. Res. Soc. Symp. Proc. Vol. 245. ©1992 Materials Research Society
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at room temperature and cast in small plastic tubes. The tubes were sealed 0 and the samples held at 20 C for 24 hours, after which they were demoulded to allow unrestrained expansion. These paste samples were then held in a 0 sealed environment, maintained at 20 C and 65 % RH. The reaction was stopped in in
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