Cementitious materials imaging

Appearance of common cementitious materials by backscattered electron imaging... 

Electron microscopy is one of the most powerful techniques for studying the microstructure of cementitious materials. Since the publication of the first fracture surface imaged via scanning electron microscopy (SEM) (Chatterji and Jeffery 1966), many advances have been made, perhaps, most notably, the imaging of polished sections by backscattered electrons (BSEs) (Scrivener 2004). Nowadays SEMs are widespread and even benchtop machines are available at relatively low cost. Transmission electron microscopy (TEM) has also made a significant contribution to understanding these materials, although this is much less widely used due to more limited availability of instrumentation and difficult sample preparation. 

These new methods, low-vacuum, cryomicroscopy and environmental microscopy, minimise damage to the delicate hydrate in cementitious materials (notably C-S-H). However, as illustrated in Section 8.3, the differences in morphology compared to carefully prepared conventional samples are not dramatic and support the view that conventional SEM can give images... 

BSE images reveal the different phases because of the atomic number contrast described earlier. Polished sections examined by BSE and characteristic X-rays can provide a wealth of information on length scales from millimetres to less than a micron. Nevertheless, the essential prerequisite is a well-polished sample. This is often challenging, particularly when the cementitious material contains aggregates (mortars and concretes). Aggregates are generally much harder than cement paste, so the paste component (which is the main subject of study) is frequently eroded between well-polished aggregates. 

The aim of this chapter is to illustrate the use of electron microscopy for the study of cementitious materials. Although imaging of fracture surfaces with SEs is perhaps the most commonly used technique, it is of limited use after the first day or so. The most useful technique is the study of polished sections by BSEs and characteristic X-rays. We have tried to illustrate some of the diverse applications of these methods, including the possibilities to obtain quantitative information by image analysis. However, the essential prerequisite is a well-polished sample, which is not easy to achieve. 

Scrivener, K. L. 1988. The Use of Backscattered Electron Microscopy and Image Analysis to Study the Porosity of Cement Paste . In Pore Structure and Permeability of Cementitious Materials, Proceedings of Materials Research Society Symposium 139 129-140. 

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