The behavior of a suspension of spherical particles in a yield stress, shear thinning, thixotropic fluid is studied under pipe flow at low Reynolds number. Three flow distances, imposed by large amplitude oscillations, are investigated: 25 m, 48 m and 200 m, below and above the development length predicted for a Newtonian suspension of identical bead size and pipe diameter. The base fluid is a cement slurry and the particles are red glass beads, added at 0.3 vol. fraction (ϕ0). The radial distribution of beads over the pipe cross-section is measured after the hardening of the cement and shows a concentration gradient. No clear influence of travel distance is observed. Averaging all results, an increase in bead concentration by 35% above ϕ0 (reaching an average ϕ = 0.41) is measured in the pipe center region, and a decrease by 20% (reaching an average ϕ = 0.24) near the pipe wall. The absence of further densification of beads in the pipe center contrasts with results obtained in Newtonian fluids. This is attributed to the paste increase in yield stress with time at rest, predicted to result in a plug flow zone of increasing radius with time. The radial migration of particles evidenced in this work for a low yield stress cement slurry may contribute to the formation of plugs (zone of high concentration of aggregates) observed at the forefront of concrete during pumping, explaining blockage.
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