Cement Paste Microstructure in Concrete
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CEMENT PASTE MICROSTRUCTURE IN CONCRETE
SIDNEY DIAMOND School of Civil Engineering Purdue University,
West Lafayette,
IN 47907
ABSTRACT An understanding of the microstructure of cement paste in actual field concrete is considered to be the primary objective of cement microstructural studies. Cement paste in concrete is shown to consist of narrow ribbons of the order of 80 to 150 pm separating sand and coarse aggregate grains, and interrupted by frequent air voids. Individual cement grains form shells around themselves early in the hydration process. In subsequent hydration they either deposit dense layers of internal hydration product or empty out in a hollow-shell mode and precipitate hydration products in the empty spaces between nearby shells. Analogous shells are similarly formed around fly ash particles, which then react by progressive etching from within the shells. Interfacial zones around aggregates are pictured as having a complex characteristic structure including at least four different forms of calcium hydroxide particles with various preferred orientations. It is suggested that the conventional picture of the "aureole de transition" derived from successive x-ray diffraction measurements may have been misinterpreted.
INTRODUCTION This paper is an interpretation of current understanding of the microstructure and microstructural development of portland cement paste in concrete, based on results of the writer and others. It derives in part from a principal paper recently presented by the writer at the recent Congress on the Chemistry of Cement in Rio [1], and shares the philosophy expressed therein. This philosophy can be summarized briefly in the following statements: 1.
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While cement pastes of varying microstructures can be produced in a variety of ways in the laboratory, the important cement paste microstructure is that which develops between sand and aggregate grains in real concrete. The continuing study of the hydration of C3S and other individual cement constituents under laboratory conditions chemically and physically different from the concrete environment has limited further applicability; most of what can be learned from such studies has either been learned or is not applicable to microstructure of cement paste in concrete. Since most current concretes contain fly ash, slag, silica fume, or other blending component that affects the microstructure of the cement paste, the study of pastes containing such components is no longer peripheral to the field, but an important central consideration in the study of microstructure. The object of the study of cement paste microstructure is an entirely practical one; that of understanding and modifying concrete behavior. Since concrete behavior may be dominated by microstructure at special locations (e.g. interfacial zones near aggregates, fibers, etc.) the study of such special microstructures may be more important than the study of the 'bulk' microstructure of the major part of the cement paste.
Mat. Res. Soc. Symp. Proc. Vo. 85. , 1987 Ma
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