Fly ash (FA) is becoming a common addition to ordinary portland cement (OPC) both in Europe and in the USA. Although a great deal of work has been carried out to characterize fly ashes from different sources, less work has been reported in the literature as to the exact nature of the reaction products when fly ash blends are hydrated. Microstructural studies carried out using scanning electron microscopy (SEM) of fracture surfaces have been summarized by Diamond . The results often show a shell of reaction product around the fly ash residues. Some authors have postulated the formation of a duplex film of calcium hydroxide (CH) and calcium silicate hydrate (C-S-H) around the particles. However SEM alone cannot reveal the composition of features in the microstructure and interpretation on the basis of the morphology of the features may be misleading. Some analytical electron microscopy has been carried out using crushed particles of blended pastes  but this technique results in a loss of information about the location of analyzed areas.