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

TL;DR

This study tackles the problem of particle settling in aging cement paste, a thixotropic yield-stress fluid, by combining X-ray tomography to track falling spheres with rheological measurements of the paste over time. It introduces a thixotropic framework that couples a structure parameter $\lambda = \lambda_g + \lambda_b$ to viscosity, and tests four cases that vary growth and breakdown dynamics. Empirical results show the average ball velocity decays logarithmically with paste age until stoppage, with static yield stress $\tau_s$ serving as a reliable predictor of arrest, while dynamic yield stress is not predictive. Model fits reveal that including paste breakdown (structure destruction) is essential to capture the observed behavior, with Case IV providing the best agreement ($R^2=0.91$). The findings have practical implications for predicting sedimentation in cement-based systems and underscore the role of aging-induced restructuring and destructuring in governing particle mobility.

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.

Figures (13)

• Figure 1: Rheology protocol used to study the cement paste.
• Figure 2: Cement paste viscosity as a function of shear rate from flow test
• Figure 3: Growth of storage modulus $G'$ with time (or paste age). The trials with amplitude sweep (AS) are shown with orange upward triangles and green downward triangles. The trial without any amplitude sweep is shown by purple diamonds.
• Figure 4: Growth of complex viscosity $\eta^*$ as a function of time $t$ (or paste age) represented by purple dots. The dashed line shows the fit of the equation $\eta^* = \eta_o^* + K\cdot t^m$.
• Figure 5: (a) Amplitude sweep at different aging times showing the drop of storage modulus $G'$ as a function of increasing strain amplitude $\gamma$. The gradient of color from light yellow to dark purple depicts increasing aging time from 120 to 600 s. (b) Growth of static yield stress $\tau_s$ as a function of age $t$ of the cement paste (green diamonds). The error bars represent the range (maximum-minimum) for two independent trials. The fitted green line shows a linear fit and returns $\tau_s = 0.026\cdot t + 3.55$.