Microstructure of entrained air voids in concrete, Part II

The microstructure of air voids in both air-entrained and non air-entrained paste, mortar, and concrete containing silica fume has been studied at different ages (5 min to 60 days) in order to understand how air-entrained voids form in portland cement–silica fume systems. Scanning electron micrographs of air voids are presented for many different ages. The solidification process of portland cement–paste and mortar was frozen at different ages using both a low-temperature scanning electron microscope and freeze drying. During the course of these experiments, it was discovered that air voids can be considered “windows” in the microstructure in which one can observe early hydration features that give insight into the nature of portland-cement systems. A well-developed sheaf of wheat morphology has been observed in concretes incorporating silica fume, and a new interpretation of the microstructure of silica fume concrete is presented.