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Studies of hardened cement paste and concrete microstructure were facilitated by the use of a scanning electron microscope (SEM). Backscattered electron (BE) imaging of polished sections provides high-magnification, high resolution (less than 1 micrometer) images of the cement paste microstructure [17].
Flat, polished samples are necessary for BE imaging. To maintain and support the microstructure during polishing all cement paste and concrete specimens are impregnated with a hardening resin. Cured concrete specimens are placed in dried ethanol to replace the pore solution and then in an ultra-low viscosity embedding resin to replace the ethanol [18]. This procedure results in the preparation of a polished section without drying thereby minimizing the occurrence of drying cracks in the microstructure. The potted samples are then ground to expose the cement paste and polished with a series of 6, 3, 1, and 0.25 micrometer diamond pastes using moderate hand pressure on a low-speed lap wheel for about 45 seconds with propylene glycol as a polishing lubricant. Evaporating a 100 nm thick coating of carbon on the polished surface eliminates specimen charging in the SEM.
The contrast in the BE image is dependent on the average atomic number (Z), with higher Z phases appearing brighter than lower Z phases. Thus, for hardened cement paste, anhydrous cement appears brightest followed by calcium hydroxide, and calcium silicate hydrate; porosity filled with embedding resin appears dark. Phase identification is made by examination of the phase shape, relative brightness, and chemical composition as determined by qualitative X-ray microanalysis. For example, an elongated, intermediate grey phase appearing to cleave along it's long axis and producing an X-ray spectra of calcium and oxygen is calcium hydroxide. With the fairly distinct grey-level separations between anhydrous cement, CH, C-S-H, and the resin-filled porosity, the BE image can be processed and analyzed to obtain quantitative phase abundance data.