The distortion of the diffuse double layer and its effect on flow properties of dense flyash slurries

Flyashes obtained from power stations burning pulverised Curragh and Tarong coals were air classified to obtain a fine fraction (−10 μm). When dense aqueous slurries of these fines were subjected to rheological measurement in a rotating viscometer, the two ashes exhibited different packing propensities. In interpreting the experimental flow data, a new volumetric packing factor Φ was defined to include the volume of the diffuse double layer. When the Krieger–Dougherty equation was fitted to the data, the maximum packing fraction (Φm) redesignated as the quasi-maximum packing fraction was found to vary at low shear rates. It is proposed that the differences in rheological behaviour and variation in Φm is due to the surface chemistry of fine particles. The two flyashes produce leachates of different ionic strength and hence generate different thicknesses of diffuse double layer. Shearing at close packing produces distortion in the diffuse double layer, especially in dilute leachates with thick diffuse double layers. A particle packing model based on the unit cell is proposed to quantify the distortion of the diffuse double layer, and to explain its influence on Φm and flow behaviour.