Specimen preparation is important in any microscopical technique with proper preparation methods facilitating examination and interpretation of microstructural features. Improper preparation methods may obscure features, and even create artifacts that may be misinterpreted. Scanning electron microscope (SEM) analysis using backscattered electron and X-ray imaging requires a highly polished surface for optimum imaging. Rough-textured surfaces, such as those produced using only saw-cutting diminish the image quality by reducing contrast and loss of feature definition. Additionally, the lack of a polished specimen makes quantitative estimates arduous, as the surface is no longer planar. We have developed a series of preparation procedures in our laboratory that we feel provide simple, yet efficient specimen preparation, allowing clear definition of specimen features in SEM imaging [1, 2].
SEM is used for a variety of projects under the National Institute of Standards and Technology (NIST) Building Materials Laboratory's Partnership for High Performance Concrete Program. We examine Portland cement clinker, cement powder, cement pastes, mortars, and hardened portland cement concrete [3, 4, 5, 6]. Each of these preparations uses an epoxy resin to permeate the material's pore system or to encase powder particles. The specimens are then cut or ground to expose a fresh surface, and that surface is then polished using a series of successively finer grades of diamond paste. This polishing stage is necessary to remove cutting and grinding damage, and to expose an unaltered cross section of the material's microstructure.
Figures 1 and 2 show backscattered electron (BE) images of polished sections of portland cement particles and hardened portland cement paste. The BE image contrast is generated by the different phases' compositions relative to their average atomic number and is observed by the differential brightness in the image. The ferrite phase appears brightest in the cement image (Figure 1), followed by alite, belite, aluminate, and periclase. X-ray imaging facilitates identification of periclase, alkali sulfates, and calcium sulfates. Figure 2 shows that, for a 28-day hardened cement paste microstructure, demonstrates that polishing yields clear definition of the constituents: the black pore space filled with the cured epoxy; the bright grains of residual cement (C); the intermediate-gray calcium-silicate-hydrate (C-S-H); and somewhat brighter gray calcium hydroxide (CH).
In contrast, concrete that has been saw-cut using isopropyl alcohol as a cutting lubricant exhibits little constituent contrast and substantial cracking (Figure 3 ). The loss of contrast may be attributed to the roughness of the surface and the cracking to both the cutting and tearing action of the diamonds embedded in the saw blade, and drying shrinkage-related cracking of a damaged microstructure. A cross-section of this preparation, after epoxy impregnation and polishing, shows the extent of cracking resulting from the saw damage and drying shrinkage (Figure 4).
Epoxy impregnation of the pore system serves two purposes: A) it fills the voids and, upon curing, supports the microstructure serving to restrain it against shrinkage cracking, and B) it enhances contrast between the pores, hydration products, and cementitious material. With relatively high permeability materials or powders such as clinker or portland cement, an epoxy of low viscosity is necessary while for the less permeable cement pastes and concretes an ultra-low viscosity epoxy aids in rapid infiltration of the pore structure.
