Thanks for the comments,
I have since been in touch with the company who produced the results. They have stated that the instruments used were not precise enough for the fluid and that rather than no data they put in zeros.
This would suggest a dilantant type fluid.
I am a mechanical engineer working for a company who will be undertaking slurry pipeline design.
We have basic rheology information on the slurry however i am unsure why the shear stress does not increase until a particular shear rate. Has anyone struck this before?
Research on the internet...