  The behaviour in simple shear of two concentrated and strongly cohesive
mineral suspensions showing highly non-monotonic flow curves is described. Two
rheometric test modes were employed, controlled stress and controlled
shear-rate. In controlled stress mode the materials showed runaway flow above a
yield stress, which, for one of the suspensions, varied substantially in value
and seemingly at random from one run to the next, such that the up flow-curve
appeared to be quite irreproducible. The down-curve was not though, as neither
was the curve obtained in controlled rate mode, which turned out to be
triple-valued in the region where runaway flow was seen in controlled rising
stress. For this first suspension, the total stress could be decomposed into
three parts to a good approximation: a viscous component proportional to a
plastic viscosity, a constant isostatic contribution, and a third shear-rate
dependent contribution associated with the particulate network which decreased
with increasing shear-rate raised to the -7/10th power. In the case of the
second suspension, the stress could be decomposed along similar lines, although
the strain-rate softening of the solid-phase stress was found to be logarithmic
and the irreducible isostatic stress was small. The flow curves are discussed
in the light of recent simulations and they conform to a very simple but
general rule for non-monotonic behaviour in cohesive suspensions and emulsions,
namely that it is caused by strain-rate softening of the solid phase stress.
