Heterogenous flows in sheared cement suspensions

Abstract

Fresh cement pastes are complex suspensions, with a yield stress that depends upon the material recipe (cement-to-water ratio and additives) and increases with time in the paste at rest. The yield stress is linked to the formation of a multiscale network composed by the equally highly charged ground clinker particles containing calcium silicates and the precipitating calcium silicate nano-hydrates (C-S-H). High yield stress concretes typically need to be vibrated to flow and be placed, while low yield stress ones flow easily under their own weight. The measurement of the yield stress is therefore crucial both for practice and to estimate the early cohesion between hydrating cement particles. In this study, we measure the yield stress of various cement pastes using a stress-controlled rheometer with different geometries, namely serrated parallel plates, vane, concentric cylinders, and helix. Results show discrepancies, with a yield stress lower by a factor of 5 to 18 in the parallel plate geometry compared to the vane/helix one, depending on the cement recipe. To explain this inconsistency, we image the flow in the parallel plate geometry along the external diameter, using a fast rate camera, and treat the data using particle image velocimetry (PIV). Results show shear banding, with a stationary band near the static plate and a sheared band near the upper rotating plate. Using the width of the stationary band, the shear stress is recalculated from the applied torque, but the moderate value increase cannot explain the above discrepancy. We hypothesize that material heterogeneities develop during flow, and a band of less concentrated cement paste is formed.