Backscattered electron imaging of cementitious microstructures: understanding and quantification

Abstract

During the last 20 years, backscattered electron imaging of polished surfaces has become well established as a method for the study of cement and concrete microstructures. The technique has many advantages, including the visualisation of representative cross-sections over a wide range of magnifications and reproducible contrast dependent on atomic number. Nevertheless the limitations of observing a two-dimensional section of a three-dimensional structure must be borne in mind. In this paper, the general microstructural features of hydrated cement pastes are described. Although the amount of aluminate phase (“C 3 A”) in cement is comparatively minor, it plays an important role in determining many of the microstructural features of cement paste microstructure, for example in the formation of “Hadley” grains. Despite the very heterogeneous nature of cement paste, it is important to be able to derive quantitative measures if the relationships between microstructure and properties are to be understood. The possibilities to quantify BSE images are described. The interface between paste and aggregates in concrete is particularly variable, but average features can be measured, which help to understand the processes of packing cement grains , which gives rise to this region. Finally an example of the potential for BSE images to study concrete durability is given.