Carbonation reaction kinetics of lime binders measured using XRD

Carbonation of lime binders has been studied using a carbonation set-up combined with in-situ XRD analysis in order to follow the real-time modification in portlandite and calcite phases while measuring the reaction kinetics. Accelerated carbonation was carried out in a 100% CO2 atmosphere at ~95% RH using lime pastes at different water contents. XRD results indicated that carbonation reaction initiated immediately with the uptake of CO2 by the water film and the precipitation of calcite crystals on the sample surface. At the initial stage, the reaction was limited by this water film on the sample surface, hindering the CO2 diffusion through the sample thickness. Upon drying, carbonation was accelerated through this diffusion process and resulted in a complete precipitation of calcite crystals on the sample surface while the reaction continued along the sample thickness at a reduced rate. Continuous measurement of RH during carbonation evidenced the acceleration of the reaction with the production of water. Higher reaction rates were recorded for the samples with a higher content of water due to an increase in the amount of dissolved CO2. Despite high CO2 concentration, 2.0-4.5% calcium hydroxide remained uncarbonated due to the heat generated during accelerated carbonation and precipitated calcite crystals hindering the diffusion of CO2.