  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.
