Carbonate binders by “cold sintering” of calcium carbonate

The solubility of calcium carbonate (limestone) particles depends on the amount of CO₂ dissolved in the water, which is a function of temperature and the pressure of CO₂ that is in equilibrium with water. At a constant temperature, increasing CO₂ pressure increases the solubility of CaCO₃, and decreasing CO₂ pressure favours the crystallisation of CaCO₃. This dissolution–crystallisation behaviour of CaCO₃ can be used in the development of carbonate binders—a process called “cold sintering”—of limestone. This paper examines the effect of a range of parameters on the cold sintering process of limestone powder. The parameters studied are CO₂ gas pressure (atmospheric, 10 atm and 35 atm), exposure time (0–90 min), post-compaction pressure (10 and 15 MPa) and compact pressing time (10–60 min). The water/limestone powder ratio was kept constant at 0.2 (by weight). The compressive strength of the limestone compacts was used as a measure of the efficiency of the carbonate binder formation process, and scanning electron microscopy was used to study the microstructural developments. The results show that carbonate binders can be produced by cold sintering of limestone powder. Exposure of limestone compacts to high CO₂ pressure followed by post-compaction at high mechanical pressure, enhances the strength of the compact. From the microstructural data, it is evident that newly formed calcium carbonate crystal growths are responsible for the strength improvements observed. The amount of water that is used in the limestone powder mixture is one factor that controls the quantity of the cementing phase. Future work has to be focussed on the application of methods to further increase the solubility of calcium carbonate, as the amount of calcium carbonate available for recrystallisation is important in producing a strong binder.