Influence of carbonation on the volume change of hardened cement pastes

Carbon dioxide uptake of hardened cement pastes and hardened cement pastes mixed with fly ash during carbonation cycles were investigated. The volume changes of that two series of hardened pastes on carbonation cycles were measured by the helium pycnometer. For further discussion, samples were analyzed by X-ray diffraction with RIR (Reference Intensity Ratio) method. The pH changes of those leaching solutions were also mentioned. Results showed the final trend of carbon dioxide uptake of two sample series was nearly the same, which was decreased to the identical uptake level. But there were differences in the volume changes of the two sample series. For the cement pastes, the volume change increased rapidly and then changed to gentle gradually. The volume change increased first, then reduced steadily as for the mixed pastes. However, the volume of hardened cement pastes was still expanded after 24-hour carbonation. Different trends of the volume changes might be due to the carbonation of not only the calcium hydroxide but the C-S-H (Calcium-Silicate-Hydrate).