The Microstructure of Anhydrous Cement and its Effect on Hydration
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THE MICROSTRUCTURE OF ANHYDROUS CEMENT AND ITS EFFECT ON HYDRATION KAREN L. SCRIVENER Dept. of Metallurgy & Materials Science, Rd, London SW7 2BP, UK
Imperial College,
Prince Consort
ABSTRACT The hydration and microstructural development of cement are affected by the arrangement of anhydrous phases within the cement grains. Backscattered electron images and X-ray microanalysis can be used to characterize the microstructure of anhydrous cement. An image analyzer has been used to measure the overall proportions of silicate and interstitial phases in several cements and to measure the proportions of these phases that are exposed on the surface. These results are compared with the rates of heat evolution during the hydration of the cements.
INTRODUCTION In some respects, the hydration and microstructural development of Ordinary Portland Cement (OPC) differs significantly from that of C3 S (and alite) and from that of a mixture of alite, C3 A and gypsum [1,2]. In cement paste, C-S-H forms as shells, around the reacting grains, but separated from them by gaps of up to 1 pm after about 24 hours hydration. This phenomenon is not observed in C3 S pastes, where the hydrate forms directly on the surface of the grains. For a mixture of separate particles of alite, C3 A and gypsum, large gaps (up to 10 pm) were observed around the C3 A grains but not around the alite grains [1,2]. This difference in behavior between cement and a mixture of individual constituents indicates that the hydration process may be affected by the distribution of the anhydrous phases within the cement grains. At the firing temperature of clinker, solid crystals of alite (impure C3 S) and belite (impure C 2 S) exist in a liquid matrix. On cooling, the ferrite phase (usually approximated as C4 AF) and aluminate phase (C 3 A) solidify from the liquid, together constituting the interstitial material. The clinker is then ground with gypsum to give cement. The manner in which the clinker fractures during grinding determines the proportions and distribution of the anhydrous phases within, and on the surfaces of, the cement grains. Hornain and Regourd [3] pointed out that the ease with which a phase fractures depends predominantly upon its brittleness rather than its hardness. They found alite to be much the most brittle of the anhydrous phases. In addition cracking of alite, which often occurred during cooling, facilitates fracture through this phase. Beke and Opoczky [4] found that a sample of alite was the most easily, and one of belite the least easily ground of the anhydrous phases. Gutteridge [5], using quantitative X-ray diffraction analysis (QXDA),-found that there was a higher proportion of belite in the larger particle size fractions. Most investigators rely on the Bogue calculation for an indication of the phase content of a cement; but, due to the assumptions involved, the Bogue composition often deviates significantly from the true composition of a cement. The relative proportions of phases in cement can be determined more directly using QXDA [
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