Application of TEM to characterize fly ash- and slag cements

A Portland fly ash cement containing 20% of a fine fly ash and a blast furnace slag cement of approximately 290 days old were examined with analytical transmission electron microscopy, in order to examine the (local) microstructure in these cements in detail. In the Portland fly ash cement the fly ash reacted with CH resulting from the cement hydration. Due to this pozzolanic reaction, initially a dense C-S-H reaction rim precipitates around the fly ash glass spheres. In course of time, a radially fibrillar and more porous zone of C-S-H is formed in between the dense C-S-H zone and the fly ash surface. The dense C-S-H notably contains aluminium and potassium, elements which are likely to originate from the fly ash. The microstructure of the blast furnace slag cement is also marked by a local chemical differentiation; a zoned structure is formed surrounding the original slag grain. Chemical analyses and element maps indicate that notably Si and Ca, and also some of the AI, is transported out of the slag grain interior. The driving force for this element transport is accounted for by a postulated gel-layer, creating a chemical potential gradient for water. Hydrophillic elements, such as Si and Ca will migrate out of the slag, and will cause a gradual paste densification, responsible for the low permeability in blast furnace slag cements. Within the gel-layer, a water poor region is formed in which a hydrotalcite-like phase precipitates, possibly together with minor amounts of C-S-H like material at its inner edge.Materials Science Engineering SectionCivil Engineering and Geoscience