The rheological properties of blended cement-based materials depend strongly on mixture proportions and the characteristics of the components, such as cement and fly ash. In this study, design of experiments is used to investigate the influence of three variables (cement particle size distribution (PSD), fly ash PSD, and ratio of fly ash to cement) at each of four levels on the yield stress and viscosity of blended cement pastes. To enable the fairest comparison among mixtures, all mixtures are prepared with a constant volume fraction of water. Both rheological parameters are seen to vary over several orders of magnitude for the design space evaluated in this study. Physical characteristics of the powders, such as cement and total particle densities and total particle surface area, are also computed for each mixture. A percolation-type relationship is observed between yield stress and cement particle (number) density, suggesting that the yield stress is regulated by the strong flocculation of the cement particles to each other, with little if any additional resistance provided via the addition of the fly ash particles, which function mainly as diluents. Conversely, while apparent viscosities were not particularly well described by the commonly employed Kreiger-Dougherty equation, plastic viscosities were found to be linear functions of either total (cement + fly ash) particle surface area or total particle density. The determined relationships could be useful in engineering rheological properties based on the particle characteristics of the blended cement and fly ash powders.
Citation: Cement and Concrete Research
Pub Type: Journals
Cement, fly ash, particle size distribution, rheology, surface area, viscosity, yield stress.