Evidence from light and electron microscopy

The development of microstructure is considered in Section 7.4. The broad microstructures of pastes more than about 28 days old are similar to those of C3S pastes. Fig. 7.2 shows a typical backscattered electron image, obtained under conditions maximizing contrast from compositional differences. The following types of region are distinguishable by their differing grey levels and by their compositions determined by X-ray microanalysis. 

Transmission electron microscopy of ion-thinned sections provides data at higher resolution than can be obtained with polished sections. Rodger and Groves (R24) described regions which had probably formed in situ from the ferrite phase, and which consisted of C-S-H, a hydrotalcite-type phase and a poorly crystalline phase containing iron that could have been the precursor of a hydrogarnet. The particles of this last constituent were almost spherical and some 200 nm in diameter. The same investigation also showed that much of the product formed in situ from alite or belite was essentially pure calcium silicate hydrate. 

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