Development of microstructural heterogeneities in cementitious slurries and mortars at rest and under flow: a hypothesis for pumping blockage and material defects

Abstract

Cement pastes are negatively buoyant suspensions of non-Brownian particles (μm-size range), with attractive interactions due to highly charged hydration products (nm-size range). In mortar and concrete, sand (mm-size range) and aggregates (cm-size range) are added. These particles, depending on their size and density, may sediment under gravity, or migrate under flow, leading to the creation of heterogeneities, with zones less rich in cement particles, or zones more rich in sand and aggregates. In this talk, we discuss the physical processes that lead to these heterogeneities by studying both model systems (oil and beads) and real systems (slurries, i.e. only cement particles, and mortars, i.e., with sand). By studying these systems under shear using a rheometer, under flow using a pumping system, or at rest using optical tools, we highlight the formation of heterogeneities. Result analysis is enabled by extensive rheological characterization and previous works on Newtonian based suspensions (Acrivos et al., Nott & Brady, Guazzelli et al.). While shear-induced migration of aggregates is held responsible for concrete pipe blockage, sedimentation of cement particles at rest may explain defects after cement hardening.