Impact of metakaolin characteristics on the rheological properties of mortar in the fresh state

Abstract This study investigated the flow properties (slump, flow time, apparent viscosity at different shear rates) of cement/metakaolin-based mortars. The mix proportion that was used per weight was 3:1:0.5 (sand:binder:water), and the binder was composed of either 100% cement (OPC) or combinations of cement and metakaolins (MKi). Replacement rates of cement with MK were 12.5% and 25% per weight. Four metakaolins produced in two areas with two processes of calcination (fluidized bed and flash calcination) were employed. Their chemical and mineralogical compositions, physical properties, size distribution and morphology were fully characterized. Relationships between various MK properties and the flow properties of mortars were studied. Results show that metakaolins can produce significant differences in the flow of mortars, depending on their physical and chemical properties. In particular, the nature and content of impurities are key factors acting directly on the morphology of the distribution of the constituting particles and the water demand of MK. All these parameters control the rheological properties of the resulting cementitious suspensions. The mode of calcination is a second order factor influencing the shape of particles, since the flash process can smooth the surface of the smallest particles. More specifically, the high quartz content present in flash MK as rounded particles is minimally or not at all affected by the calcination process, and maintains flow properties more successfully than mortar incorporating cement only.