Coal fly ash is attracting increasing attention as a raw material for processing into multiple products. Well designed processing equipment requires an understanding of how a suspension's rheology changes with increasing solid concentration and shear rate. Current rheological models applied to coal fly ash are insufficient to describe rheology behaviour over a wide range of concentrations. The current paper analyses the flow curves of two types of coal fly ash differing in particle size, shape, and distribution over a range 10-70 wt.%. It has been shown that both samples exhibit complex non-Newtonian behaviour including shear thinning and shear thickening. A new generalised modelling procedure has been proposed to predict the viscosity behaviour of solid suspensions. The resulting model was able to predict the relative viscosity as a function of both concentration and shear rate with a high degree of accuracy. It is envisaged that this will be extremely useful for the purpose of processing equipment design involving mixing operations. (C) 2013 Elsevier Ltd. All rights reserved.