Modelling the yield stress of ternary cement–slag–fly ash pastes based on particle size distribution

Abstract

Mineral admixtures are blended with Portland cement to improve strength and durability, and/or to reduce the cost and environmental footprint of concrete mixes. Some mineral admixtures such as fly ash have also been reported to enhance workability of blended cements. This study shows that the key parameter controlling the yield stress of ternary Portland cement–blast furnace slag–fly ash pastes, in the absence of an added rheology-modifying admixture, is the width of the particle size distribution (PSD) at a given water/solid mass ratio. A small addition of fly ash has a significant effect on workability because of its broad PSD, and any other precursors with a similar PSD could possibly have a comparable effect. A model to predict the yield stress is therefore derived based on the Rosin–Rammler PSD width parameter, n, and the water/solid mass ratio, which provides a good description of the experimental yield stress data. It is believed that a broader PSD provides a higher packing density of particles which reduces the volume of water volume required to fill the voids. The excess water is then used to disperse particles and reduces inter-particle forces, hence yield stress. Therefore, at a given water/solid ratio, PSD width is able to be used as a sole parameter to correlate the yield stress of such concentrated suspensions. Also, a very low yield stress can be approached if the PSD is broad enough, in the absence of any rheology-modifying admixtures.