Polishing removes the damage imparted by the sawing and grinding operations. This stage involves use of a sequence of successively finer particle size diamond polishing pastes ranging from 6 µm to 0.25 µm, and a lap wheel covered with a low-relief polishing cloth. This may be performed manually or, for greater sample throughput, using a semi-automated polisher.
Figures 5 and 6 illustrate the increased clarity of a clinker microstructure as the grinding damage is progressively removed with initial polishing stages using a 6 µm diamond paste. While illustrated using optical imaging, this improvement may be realized with any cementitous material for any microscopical method. Subsequent polishing stages of 3 µm, 1 µm, and 0.25 µm pastes remove fine scratches from the 6 µm polish, further improving constituent definition.
A comparison of the epoxy-impregnated, polished section BE images (Figure 7) with the saw-cut preparation (Figure 3) for the sections of the same concrete core illustrates the marked difference in feature clarity and artifacts. In Figure 3, the saw-cut surface exhibits cracking that resulted from drying shrinkage of a surface-damaged specimen. The surface, being rough and partly covered with particulate matter, exhibits little phase-related contrast. The image shows an aggregate at the base but, aside from the uniform hardened paste, no distinct hydration products. The epoxy-impregnated, polished specimen (Figure 7) shows microstructural feature clarity not seen in Figure 3. Here, the aggregate is not only clearly seen, but one can distinguish between silicious and carbonate aggregate by gray level. The residual cement grains appear bright and large voids within both the paste and aggregate are dark. In the higher-magnification image, the hardened cement paste / aggregate interfacial zone is shown. The residual cement grains are the brightest feature followed by calcium hydroxide, the carbonate aggregate, the C-S-H, and finally the black pores. The highly polished surface also aids in X-ray imaging. Figure 8 illustrates the use of X-ray imaging of a region shown in the BE image (top) to examine element spatial distribution. In this example, visual assessment of calcium, aluminum, and sulfur images allow one to locate monosulfate within the hardened cement paste. Referring back to the BE image shows the monosulfate as uniform-gray that is slightly darker than C-S-H, with a platy parting.