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Summary
Proper specimen preparation is necessary for using the scanning electron
microscope for the study of cementitous materials and hardened Portland cement
concrete. Rough surfaces such as those prepared using only fracture, saw-cut,
or rough-lapped preparations are not suitable. These preparations may
actually create preparation-induced artifacts that are not representative of
the microstructure, create a surface that is difficult to describe, and not
being planar, are unsuitable for estimates of phase abundance. The
backscattered electron and X-ray imaging modes are particularly sensitive to
rough surfaces, which affect the definition of the constituents through loss
of contrast and signal shadowing. Procedures for epoxy-impregnated, polished
specimen preparations that have been refined in our laboratory over the past
ten years are presented here for materials ranging from clinker and powdered
cement to hardened Portland cement concrete.
Figure 5. A clinker surface after saw-cutting
and grinding using 600-grit silicon carbide (top image) exhibits no
discernible microstructure due to the rough surface. Increased polishing time
(bottom image) using 6 µm diamond paste progressively removes grinding
and cutting damage pits and begins to reveal the underlying microstructural
features.
Figure 6. Continued polishing removes additional
material of the damaged layer leaving fewer grinding pits as shown in
the upper image. The lower image shows a specimen where all grinding pits
have been removed and is now ready for the 3 micrometer and finer
polishing steps to remove any fine scratches.
Figure 7. Epoxy-impregnated, polished section of
concrete presents an optimum surface for backscattered electron and X-ray
imaging.
Figure 8. Backscattered electron (top) and X-ray
(bottom) images of hardened cement paste show good definition of constituents.
Regions of intermediate-intensity calcium (blue) and intermediate-intensity
aluminum (purple), and high-intensity sulfur (yellow) define locations of
monosulfoaluminate. Field width: 73 micrometers.
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