Carbonate binders: Reaction kinetics, strength and microstructure

This study focussed on the synthesis of calcium carbonate binders, in situ, from the reaction between hydrated lime and carbon dioxide (CO2). The aim was to establish the characteristics of the calcium carbonate binders that are associated with its strength, which was considered as an indicator of binder performance. The role of the parameters that are known to play an important part in the kinetics of hydrated lime carbonation processes, in changing the strength of a binder was examined in detail.The parameters identified were CO2 gas pressure, exposure time and the initial degree of compaction of raw material. All hydrated lime mixtures were prepared at a constant water/solid ratio of 0.25. The hydrated lime compacts made at a range of compaction water/solid ratio (W/S) of 0.25. The hydrated lime compacts made at a range of compaction pressures (0.65–6.0 MPa) were exposed to different CO2 gas pressures (ambient to 2 MPa) for different periods of time. The resulting products were tested for the amount of Ca(OH)2 that had converted to carbonate, and for compressive strength. A microstructural analysis of the products was carried out using scanning electron microscopy.The rate of Ca(OH)2 conversion to carbonate seemed to be enhanced with increasing gas pressure, but it decreased with increasing compaction of the initial mixture. It was revealed that the crystalline state and the morphology of the carbonate formed, rather than the degree of conversion of calcium hydroxide into carbonate, is highly critical to the strength of the binder. The study concluded that in the development of calcium carbonate binder, it is important to meet the experimental conditions that favour the crystallisation of calcium carbonate.