Shear induced particle migration in cement mortars

Abstract

We show in this study that pumping mortars over long distances can lead to microstructural heterogeneities. This work follows numerous studies on particle migration of Newtonian-fluid-based suspensions under flow, where particles migrate from areas of higher shear rates (in a pipe, near the wall) to areas of lower shear rates (center of the pipe). Our fluid is here a mortar, made of a white cement slurry filled with 30 vol% red glass beads of approximate average size 500 um. After estimating that the distance needed to reach steady-state in mortars would be on the orders of hundred meters (Nott & Brady, 1994), we built a setup where the flow imposed on the suspension is a large-amplitude oscillatory flow (Butler et al. 1999, Snook et al. 2015). We performed experiments with three different mixtures, where the suspension yield stress and cover distance vary. As we could not measure particle concentration in-situ during flow, we let the mortar set and measured solid volume fractions in various cuts and polished sections of the pipe. Each image was treated with ImageJ to measure the local particle concentration as a function of the radial position. Quantification of the wall steric hindrance, as well as the stability (lack of settling of particles) of all three mixtures were controlled in reference samples (tube of same diameter of pumping pipe, but where no pumping was imposed). In almost all cases, particle migration is clearly evidenced, with a dependence on the cement yield stress.