Shear thickening of cement powder paste – why and how to mitigate?

ABSTRACT

Superplasticiser (SP) is indispensable in the production of high-performance concrete to satisfy multi-dimensional performance criteria such as strength, workability and durability. The use of SP can decrease flocculation of fine powder and enhance the workability of concrete at a given compressive strength. However, it will also promote shear thickening, which unfavourably reduces pumping distance, causing less uniform mixing and more difficult on-site manual handling, e.g. shovelling or surface trowelling. Herein, the causes of shear thickening of the cement powder paste are studied. To mitigate shear thickening of the cement powder paste, limestone filler is recommended to partially replace the cement. Tests of cement powder paste using co-axial rheometer in this study, showed that shear thickening decreased with limestone replacement up to 25% by volume, after which it increased again. As shear thickening is due to the clustering of free polymer chains of SP in the interstitial voids, adding finer limestone decreases the void with a better filling effect, which can effectively decrease the tendency of polymer chain clustering and in turn shear thickening.

KEYWORDS:

• Cement powder paste
• limestone
• plastic viscosity
• shear thickening
• wet packing density

Acknowledgements

Raw materials provided with compliments by the following suppliers are gratefully acknowledged: BASF; Boral Concrete; Cement Australia; Sibelco; Sika; Southern Pacific Sands; Sunstate Cement; Wagner Cement and XYPEX.

Additional information

Funding

The support provided by a research grant from Australian Research Council (LP140100747) is also gratefully acknowledged.

Notes on contributors

J Maybury

Mr J Maybury obtained his BEng (Hons) degree from the School of Civil Engineering, The University of Queensland in 2016. In the final year of his undergraduate study, Mr Maybury participated in a research thesis on “shear thickening of cement paste” and successfully studied the correlation between shear thickening and wet packing density of cement paste. After graduation, he joined Cornell Engineers as a Graduate Structural Engineer working on various building and infrastructure projects in Australia.

J C M Ho

Ir Dr J C M Ho is a Senior Lecturer in the School of Civil Engineering, The University of Queensland. Before joining the university in 2013, he worked as an Assistant Professor in The University of Hong Kong from 2007 to 2013. Practically, Ir Dr Ho worked in both Hong Kong and Brisbane offices of Arup on some large-scale infrastructure projects such as the Stonecutters Bridge in Hong Kong and the Ipswich Motorway Upgrade (Wacol to Darra) in Brisbane. Ir Dr Ho’s research interests are on mix design of HPC with multi-sized fillers, rheology of cement paste and mortar, as well as their application in concrete-filled-steel-tube columns.