Combined shear and compression suspension rheology

© 2016 Sayuri Nadeeka RubasinghaThe behaviour of strongly-flocculated suspensions under the combined effect of shear and compression is of significant importance since many of industrial applications combine shear with compressional dewatering to enhance dewatering, such as in raked thickeners and belt filter presses. Determining the constitutive rheological behaviour is required to describe the response of such suspensions to superimposed shear and compressive stresses, however the fragile or soft nature of these suspensions means that multidimensional testing is not straight forward or obvious. The lack of experimental data means that the current understanding of yielding response of flocculated suspensions under combined loadings is unclear. This study focused on testing a model suspension (calcium carbonate flocculated at its isoelectric point) in uniaxial compression using sedimentation and filtration, pure shear using vane rheometry and superimposed shear and compression using a triaxial tester. This last technique is usually used for compacted soils and the application to the rheological analysis of fragile suspensions is novel. Pure shear and uniaxial compressive data across a range of particle sizes and concentrations along with triaxial data for one particle size and volume fraction at various compressive loads and shear rates are presented and analysed. Experimental data shows that the shear and compressive yield stresses exhibit pronounced dependency on particle size and volume fraction in line with expectations based on literature. A triaxial test protocol was developed along with an analysis method, but it was difficult to get good data from the triaxial tester due to slumping. Results showed that calcium carbonate suspension didn’t follow the traditional behaviour seen in soils. In most cases, the shear stress continued to increase with increasing strains and a well-defined peak or constant stress was not reached. The two failure criterions were used to determine the yielding points and results indicated that the failure may have occurred in the range of shear stress, 25-50 kPa and vertical strain, 1-7% according to the first failure criteria. Results indicated that the failure may have occurred in the range of shear stress, 40-90kPa and vertical strain, 5%-10% according to the second failure criteria. The outcome of this study is a test protocol that enables the measurement of suspension constitutive properties under multidimensional loadings