In backscattered electron (bse) images of polished cement sections, anhydrous material, calcium hydroxide, other hydration products (mainly C-S-H) and porosity can be distinguished on the basis of their grey level in the image. Using an image analyzer connected directly to the SEM, it is possible to resolve these four components and so measure their relative proportions and distributions. The effects of magnification and the number of fields measured on the accuracy of bse image analysis are examined. The volume fractions of anhydrous material, porosity and calcium hydroxide derived from bse image analysis are compared with those obtained by other techniques and good correlation was found for the measurement of anhydrous material and porosity.
Carbonation was measured on cement pastes preconditioned and maintained at selected relative humidities. Specimens with a range of water/cement ratios were tested, and the effect of replacing Portland cement with pulverized fuel ash or ground granulated blastfurnace slag was investigated. Carbonation was characterized using a variety of techniques in parallel. The various techniques provided descriptions of carbonation that were generally, but not always, self-consistent. Carbonation under normal environmental conditions of carbon dioxide concentration and relative humidity was largely controlled by gas diffusion through the empty pores in the exposed surface layer. The relative carbonation performance for mixes with different pore structures was strongly dependent upon moisture conditions during carbonation; the more porous mixes carbonated rapidly at the higher relative humidities where reinforcing steel in carbonated concrete would be likely to corrode.
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