Spherical Balls Settling Through a Quiescent Cement Paste

Abstract

Fresh cement pastes are complex suspensions of micron-size cement particles (ground clinker) and nano-size C-S-H particles that result from a dissolution-precipitation process. They exhibit a colloidal behavior due to interactions between charged particles through an interstitial solution of high ionic strength, giving rise to a macroscopic yield stress, that increases with time at rest. Upon flow, this multiscale network is broken down (fluidized). In this study, we investigate the evolving rheology properties of a cement paste in quiescent conditions by measuring the velocity of metallic balls settling in the paste using high speed X-ray CT imaging. Once the cement is mixed with water, it is allowed to rest to develop the yields stress for a given amount of time in a cylinder of diameter 7 cm, time during which it is building up its microstructure and yield stress. We look at the influence of resting time on the velocity of steel balls falling through it. We vary the parameters of rest time, ball size (diameter 5 and 10 mm), ball weight and time interval between two successive ball drops. In parallel, an extensive rheological characterization of the paste, capturing the rheological properties "at rest" and under flow, focusing on the effect of time and shear history, is performed. Compiled results are then compared with literature of non-aging and aging yield stress fluids.