Calcined clays for low carbon cement: Rheological behaviour in fresh Portland cement pastes

In this paper the rheological behavior of fresh Portland cement pastes mixed with different calcined clays was analyzed. For this purpose, two samples of Portland cement with different mineral composition (one with low C3A and high C3S content and another with low C3S and high C3A) were used combined with different replacement percentages of two calcined clays (two different metakaolin). Grounded a quartz was used as control as well. Both mineral admixtures had different crystallinity and morphology: the a-quartz is fully crystalline while metakaolin is completely amorphous with a very small crystalline fraction giving very high pozzolanic properties in the vitreous state. All determinations were performed in the hydration latency period. The results show that the fresh Portland cement paste with low C3A(%) content and high C3S(%) content present great shear strength and the replacement with calcined clays affects the rheological behavior of the fresh pastes depending on the greater or lower pozzolanic reactivity of the mineral addition.