Effects of carbonation on micro structures of hardened cement paste

Abstract

In order to investigate the effects of carbonation on the microstructure of cement concrete, the carbonation depth and microstructure of cement paste with 0.3, 0.4 and 0.5 water/cement ratio after 7, 14, 21 and 28 d accelerated carbonation were studied respectively. The results showed that with the increase of waterto- cement ratio and carbonation age, the carbonation depth was deepened with faster early carbonation speed and slower later carbonation rate. Carbonation densified the structure of hardened cement stone with refinement of pore structure and reduced porosity. Then, during the carbonation process from the surface to the inside of carbonation area, it was prone to form micro-cracks extending to the interior specimen, resulting in cement paste carbonation depth uneven. It is further illustrated that the color reaction method using phenolphthalein solution combined with X-CT and X-ray diffraction analysis is much more reasonable to evaluate the cement concrete carbonation degree. Moreover, during carbonation process sulfur element in cement paste migrated to the area un-carbonated and the concentrated shape of sulfur element is consistent with the coloring region in carbonation interface. Finally it was identified that carbonation not only reduced the pH value in cement concrete but also made prone to crack in carbonation zone, which increased the probability of reinforcement corrosion.

Keywords

hardened cement paste / carbonation / sulfur / microstructure / X-CT analysis

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