Spherical Balls Settling Through a Quiescent Cement Paste Measured by X-ray Tomography: Influence of the Paste Thixotropy

Abstract

The settling of spherical balls in quiescent cement pastes of increasing age is studied. Metallic spheres with radii of 2, 2.5 and 3mm are dropped into the paste and allowed to settle, while their position is tracked using X-ray tomography. The instantaneous velocity of the spheres, calculated from their movement, is observed to be quasi-constant during their fall, and an average is estimated. The results show that the average velocity of the balls decreases logarithmically with paste age until ball stoppage, for all three ball sizes. In parallel, the rheological properties of the cement paste are measured using a rheometer with a vane geometry. The evolution of the paste static yield stress over time is evaluated, and proves to be a reliable predictor for ball stoppage. Finally, thixotropic models of increasing complexity are evaluated. These models consider four forms of structural growth and breakdown parameters, and their ability to capture the ball settling velocity as a function of paste age is compared. This emphasizes the importance of considering paste breakdown in relation to shearing of the paste when the ball passes through it.