Agathe Robisson | Searcharxiv

arXiv2026

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

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.

cond-mat.soft↗

arXiv2024

Discrepancies in dynamic yield stress measurements of cement pastes

The dynamic yield stress associated with the flow cessation of cement pastes is measured using a rheometer equipped with various shear geometries such as vane, helical, sandblasted co-axial cylinders, and serrated parallel plates, as well as with the mini-cone spread test. Discrepancies in yield stress values are observed for cement pastes at various volume fractions, with one to two orders of magnitude difference between vane, helical and mini-cone spread measurements on the one hand, and co-axial cylinder and parallel plate measurements on the other hand. To understand this discrepancy, the flow profile of a cement paste in the parallel-plate geometry is investigated with a high-speed camera, revealing the rapid formation of an un-sheared band near the static bottom plate. The width of this band depends upon the rotational velocity of the top plate, and upon the shear time. Recalculation of shear stress shows that the reduced sheared gap alone cannot explain the low measured yield stress. Further exploration suggests the formation of zones with lower particle content, possibly linked to cement particle sedimentation. Here, we argue that the complex nature of cement pastes, composed of negatively buoyant non-Brownian particles with attractive interactions due to highly charged nano-size hydration products, accounts for their complex rheological behavior.

cond-mat.soft↗