Shear-induced particle migration in a cement slurry under pipe flow

Abstract

The behavior of a suspension made of white cement slurry and 0.3 vol. fraction (ϕ0) red glass beads is studied under pipe flow. Three flow distances, imposed by large amplitude oscillations, are investigated: 25 m, 48m and 200 m, below and above the development length predicted for a Newtonian suspension of identical bead size and pipe diameter. An extensive rheological characterization is performed and shows that the suspension behaves as a yield stress, shear thinning, thixotropic fluid. The radial distribution of beads over the pipe cross-section is measured after the hardening of the cement and shows a concentration gradient. Averaging all results, an increase in bead concentration by 35% above the nominal concentration (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, predicted to result in a plug flow zone of increasing radius with time. This work shows that the radial migration of particles in cement slurries may occur beyond the formation of a lubricating layer near the wall, discussed in literature, and over the whole cross-section of the pipe. This phenomenon may contribute to the formation of plugs (zone of high concentration of aggregates) at the forefront of concrete during pumping, explaining blockage.