Pozzolanic efficiency of calcined clays in blended cements with a focus on early hydration

Fundamental knowledge about the reaction mechanism of calcined clays in cement and their mutual interaction is important for their assessment as supplementary cementitious materials and the resulting concrete properties. In this study, the hydration of two cements differing in alkali content and with the addition of a highly reactive, aluminium-rich metakaolin and a calcined common clay with low kaolinite content was investigated during the first 48 h. For this purpose, four established methods that describe the early hydration were used: isothermal calorimetry, thermogravimetric analysis, in situ X-ray diffraction and chemical analysis of pore solution. This as yet unique combination of methods led to greater understanding of the hydration behaviour of the complex binder (cement–calcined clay). The results showed considerable differences depending on the type of calcined clay, its chemical–mineralogical composition and fineness, especially with regards its reaction mechanism with aluminate clinker phases controlled by the composition of the pore solution. The impact of calcined clay on early clinker hydration extends far beyond well known physical effects.