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Carbonation of Expansive Concrete and Change of Hydration Products

Microstructure and mechanical properties of concrete with expansive additive are reported compared with ordinary concrete. Samples of long-term concrete (22 years) were collected from an actual building built in 1967 with calcium sulfoaluminate (CSA) used as expansive additive. Hydration products were separated from these samples by using heavy media and analyzed by means of DSC, XRD and FT-IR. The morphology of the mortar portion was observed by SEM. No differences were detected on the carbonation depth and the compressive strength between CSA concrete and ordinary concrete. Qualitative analysis shows that following carbonation of concretes, C-S-H was changed to silica gel or to C-S-H with low Ca/Si ratio and decomposed to CaCO (sub 3), whereas ettringite (AFt), presents in CSA concrete, was decomposed to CaCO (sub 3), Al (OH) (sub 3) gel and gypsum. Quantitatively, hydration products in carbonated CSA concrete are larger than in carbonated ordinary concrete. Therefore, decomposition rate of AFt by carbonation is slower than that of C-S-H.