We’ve been exploring how to use different electron imaging techniques with the scanning electron microscope (SEM) to illustrate aspects of microstructure that are relevant to the performance of various materials. Most SEM studies of cement-based materials such as mortar and concrete use the backscatter electron (BSE) mode. This is an excellent way to image cement-based materials, because heavy elements, or those with a high atomic number, have stronger backscatter signals than light elements, or those with a low atomic number. Therefore, there are marked contrasts between areas of different chemical compositions, and this shows up very well in composite materials such as concrete and mortar. The secondary electron (SE) mode is probably the most commonly used mode of detection in the SEM. This imaging mode is suited more strongly for variations in the topography of a surface. Recently we began using a combined BSE/SE imaging mode. We use a false color capability in the Quanta to produce these images, which in many cases enhances resolution. Below are three images of a deposit of alkali-silica gel that was removed from an airfield pavement core and placed on carbon tape for SEM/EDS analysis. All three images are of the same area but are done in, from left to right, BSE mode, SE mode, and combined BSE/SE mode.
25 September 2014