Real-time investigation of reaction rate and mineral phase modifications of lime carbonation

Carbonation reaction of lime pastes has been studied with an on-line carbonation set-up combining the measurement of the amount of CO2 consumed by Ca(OH)2 with an in situ XRD analysis monitoring the real-time modification of portlandite and calcite phases. The reaction rates obtained from the net CO2 uptake by Ca(OH)2 and that obtained from the calcite precipitation were compared for lime putty and lime hydrate pastes carbonated in CO2 atmosphere (7, 17 and 33 mol/m3). The results indicate that the carbonation reaction is initially rapid and chemically controlled at the exposed surface with a strong limitation of CO2 diffusion into the sample by the pore water. The reaction proceeds with a chemical-reaction controlled regime at a much higher rate followed by a transition to a CO2-diffusion controlled regime by the carbonated sample depth. Drying and formation of amorphous CaCO3 on the portlandite faces has been found to create a dormant effect for the calcite precipitation at the first stage. Lime putty indicates faster carbonation than lime hydrate due to its small particle size and morphology properties, both at a rate which is independent of the CO2 gas concentration. These results will contribute to the understanding of lime carbonation in lime-based mortars for applications in heritage conservation, in renovation and in modern brick and stone masonry.