Metakaolin is a supplementary cementitious material with pozzolanic properties. Its precursor, kaolin, does not occur in natural conditions as a pure phase but is often mixed in various proportions with many secondary minerals. Two natural clays from the same deposit, mainly composed of kaolinite and quartz have been burnt at a suitable temperature and the resulting calcined products have been blended with laboratory normal portland cement for which the nature of added calcium sulfates as well as free lime content have been varied. Subsequent properties of cement pastes and mortars such as rheology, setting time and compressive strength have been compared. Concurrently, early hydration products have been characterized by means of DSC and combined water content has been evaluated by CO2-H2O analyzer. The difference of behavior that has been observed as a function of the artificial pozzolan mineralogy and cement chemical parameters is discussed in the frame of the mechanism of early hydration of metakaolin blended cement.