Yield stresses and sedimentation in dense flyash slurries

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Dense slurries (of volume fraction up to 0.6) were made from ashes collected from three different power stations. The slurries were tested for yield strength by a vane technique when freshly mixed and after settling. A six-bladed vane attached to the sensing head of a rotational viscometer was inserted into the slurry and slowly rotated until shear failure occurred. The maximum volume fraction was measured by extrapolation of the yield stress. The maximum volume fraction and hence yield stress depends on the electrolyte concentration in the slurry. The effects can be explained in terms of the electrical double layer surrounding the particles. The yield stress of an ash containing leachable calcium increased with time due to the pozzolanic action of precipitated calcite.