2025 Volume 78 Issue 1 Pages 119-127
Carbonation of concrete can lead to not only a decline in the pH of pore solution due to the formation of calcium carbonate, but also the volume change associated with decalcification and decomposition during the carbonation of calcium silicate hydrate（C-S-H）. This study investigated the effect of the decalcification of C-S-H and silica gel formation on the carbonation shrinkage of hardened cement paste. Cement pastes samples with three types of water/cement ratio（w/c＝45％, 55％ and 65％）were carbonated under different relative humidity levels（40％RH, 52％RH, 70％RH and 85％RH）in accelerated conditions（5％ CO2 concentration and 25℃）. Carbonation shrinkage, CO2 uptake, IR spectrum and the quantity of silica gel formation were measured over carbonation time. Results show that carbonation shrinkage increases over carbonation time, and the shrinkage strain was highest at 52％RH and/or 70％RH after 182 days of carbonation time for all three w/c types. In the case of same CO2 uptake, carbonation shrinkage strain was found to increase at lower relative humidity during carbonation. The change in carbonation shrinkage due to relative humidity was attributed to micro-structural alteration of carbonated C-S-H, based on FT-IR results of Si-O stretching vibrations. The relation between carbonation shrinkage strain and silica gel content appears to be independent of w/c types of sample and relative humidity during carbonation. From these results, it was indicated that the decalcification of Ca2＋ between the calcium silicate layer and silica gel formation due to decomposition of calcium silicate layer progress simultaneously at a certain Ca/Si ratio.
