Proper sample preparation is critical to the successful imaging of fine cement particles. Difficulties encountered in preparing polished sections of cement include the elimination of scratches, edge rounding, surface relief, and grain plucking.
To prepare a specimen for viewing in the SEM, about 25 grams of the cement powder of interest are blended with an epoxy resin 2 ) to form an almost dry paste. The epoxy resin has a viscosity close to that of water so that this specimen will be similar to a suspension of cement particles in water. The paste is pressed into a sample mold and cured at 60 ºC for 24 hours. The cured specimen is cut using a low-speed diamond wafering saw, cutting first a layer from the outer surface and then about 10 mm into the sample. This two-cut procedure produces parallel faces, minimizing the need to refocus as one traverses a specimen.
Saw marks are easily removed by dry grinding with 400 grit followed by 600 grit sandpaper. Final polishing is done on a lap wheel with 6, 3, 1, and 0.25 µm diamond paste for about 30 seconds each. We have been able to produce low relief, fine polishes using a lint-free polyester SEM cleaning cloth 3 and diamond paste on top of Texmet paper. The specimen is cleaned after each polishing stage by gently wiping on a clean cloth; residual polishing compound is removed with ethanol after the final polishing stage. Repeated cleaning with ethanol is avoided because it may soften the epoxy and increase the chance of grain plucking. The specimen is then coated with carbon to provide a conductive surface for viewing in the SEM.
3 Lint-fee cleaning cloths are available from most SEM suppliers.