Microstructure of entrained air voids in concrete, Part I

The microstructure of air voids in both air-entrained and non air-entrained paste, mortar, and concrete has been studied at different ages (5 min to 60 days) in order to understand how air-entrained voids form in portland-cement systems. Scanning electron micrographs of air voids are presented for different ages. The solidification process of portland-cement paste and mortar was frozen at different ages using a low temperature scanning electron microscope and freeze drying. At very early ages the air voids show thin shells made of very fine particles. The packing of the cement grains behind the air void varies, depending on the water-to-cement ratio. Air voids appear to have the same interface with the cement paste matrix as aggregates. No readily visible difference was found between entrapped and entrained air voids.