Effect of ageing on the hydration and carbonation reactivity of ordinary Portland cement

Accelerated carbonation endows the ordinary Portland cement with high mechanical property and large amount of carbon dioxide sequestration. However, the moisture and carbon dioxide in the air lead to the pre-hydration and pre-carbonation of ordinary Portland cement during production, transportation and storage, which is known as ageing. It is still unknown whether the carbonation reactivity of the aged cement is reduced compared to the normal cement. This study investigates the carbonation behavior variation of the aged and normal cements, and compares it with their hydration property difference. The results indicate that the pre-hydration and pre-carbonation products cover the surface of clinkers of the aged cement. The compressive strength of aged cement pastes after hydration for 3 d is much lower than that of the normal cement pastes, but increases fast and becomes comparable to that of normal cement after hydration for 7 d and longer. The compressive strength of the aged cement compacted pastes during accelerated carbonation is much lower than that of the carbonated normal cement compacted pastes, no matter at the early age or the later age. It is deduced that the slightly detached ageing products act as seedings which lead to the concentrated hydration heat release and is conducive to the further dissolution and hydration of aged cement. However, as the water film is thin in cement compacted pastes for carbonation, the detached and un-detached products still cover the aged cement particles and hinder the entire carbonation reaction.