Cement Paste Microstructure in Concrete

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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 μm 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.

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