Nine concrete mixes, with and without replacement of cement by Class C fly ash, were exposed to the freeze-thaw cycling at ages in the range of 102 to 222 days. Prior to the cycling, the specimens were examined by a low-vacuum scanning electron microscope (SEM) for their microstructure. Infilling of ettringite in entrained air voids seems to reduce freeze-thaw resistance of the concretes tested. The extent of the infilling depends on the length of moist curing prior to the start of the cycling as well as on the usage of the Class C fly ash.
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