Influence of clay impurities on the performance of calcined clay‐limestone cements

The use of blended cements including novel supplementary cementitious materials is the most powerful lever to cut down the carbon footprint of cementitious materials. This has led to an increased interest in research on calcined clay limestone cement (CCLC) over the last years. In this study, we investigate the fresh and early hydration properties of CCLC using kaolinitic clays with a wide range of chemical and mineralogical compositions, which are all of industrial origin. Results show that early performance of calcined clays is not only dependent on kaolinite content but that the total specific surface area of the raw and calcined clays plays a key role in the onset and evolution of rheological properties as well as early hydration and reactivity. Further analysis indicated that among secondary mineral impurities, iron phases affect the physiochemical properties of calcined clays. This is shown by the high specific surface area of these clays. It appears possible that by causing higher kaolinite structure disorder, iron phases also impact hydration.