Influence of Calcined Clays on Workability of Low Carbon Composite Cements

This work highlights that the CO2 footprint of cement can be reduced significantly by blending Portland cement clinker with thermally activated (calcined) clay (CC). Investigations on pure meta phases obtained via calcination of native kaolin, montmorillonite, illite and muscovite reveal that they noticeably increase the water demand and decrease workability of the cement. The effect depends on the fineness and internal porosity of the calcined clay and the chemical composition of the native clay. A comparison of three industrial calcined samples of mixed layer clays originating from natural deposits in Germany, India and China confirmed the increased water demand of composite cements holding up to 40 wt% of these calcined clays. The increase in water demand correlates with the amorphous part and the meta kaolin content. Also, the particle size and morphology of the calcined clay impact water demand. For one sample holding ~50% meta kaolin, an increase in superplasticizer dosage of ~400% as compared to neat OPC was recorded. Whereas, a high content of meta kaolin proved to be favorable with respect to rapid early strength development as a result of its high pozzolanic reactivity. It can be concluded that calcined clays offer the potential of significant CO2 reduction in cement manufacture, however higher superplasticizer dosages need to be used. Still, because of the low CO2 footprint of superplasticizers a substantial savings in CO2 emission can be realized, and the cement industry can progress into an era of more eco-friendly binders.