Concentration effects in the rheology of cement pastes: Krieger-Dougherty revisited

Cement pastes are concentrated suspensions of granular particles in water and their rheology strongly affects the behaviour of all concretes and other cementitious materials. While the rheology of cement pastes has been extensively studied over the last 60 years, leading to the general conclusion that cement particle shape, size and concentration are
key variables, the overwhelming majority of the results to date have been expressed in terms of the effect of water/cement ratio on the measured rheological parameters. While this has been helpful in making empirical progress, a more fundamental approach requires that the concentration be expressed in volumetric terms. A suitable relationship is the Krieger-Dougherty equation but while originally formulated for viscosity it can be used for other rheological parameters such as yield stress. This paper uses a comprehensive series of datasets relating rheology and concentration, which have been collected from the literature over the past 60 years. Each dataset has been converted from the original water/cement ratio form to volume concentration and then fitted to the logarithmic transformation of the Krieger-Dougherty equation by linear regression. The logarithmic form makes it possible to use a linear fit, whereas the untransformed equation diverges to infinity at the maximum packing fraction which makes it difficult to assess the best fit of the data. The paper draws conclusions on the appropriate values of the suspension parameters (maximum packing fraction and intrinsic viscosity) for the different datasets and discusses the implications of the findings in the light of what we know about the properties of cement. It considers the validity of the equation for modelling the rheology of cement pastes and
other cementitious materials.