Fillers to lessen shear thickening of cement powder paste

Superplasticiser (SP) is an indispensable ingredient in the production of contemporary concrete, which is designed to satisfy multi-concurrent performance criteria such as strength, flowability, passing ability, segregation and dimensional stability. Use of SP can decrease flocculation of fine powder and enhance the workability of concrete. However, at the same time it increases the shear thickening of concrete resulting in reduced pumping distance, less uniform mixing and more difficult on-site manual handling, e.g. shovelling or surface trowelling. Therefore, the SP dosage for concrete mix design should be limited, or more practically, fillers should be added to reduce the shear thickening. This paper proposes to decrease the shear thickening of cement powder paste by replacing part of the cement with an equal volume of filler. From the test results, it was evident that when fly ash, silica fume or limestone is added to replace cement, the shear thickening decreases significantly. Also, it was found that the shear thickening of cement powder paste can be correlated negatively to the wet packing density. As the wet packing density increases, the tendency of having clustering of particles or free polymer chains from the SP decreases and thus shear thickening.