Dilatancy swerve in superplasticized cement powder paste

Abstract

Superplasticizer (SP), particularly poly-carboxylate based, can improve substantially the strength-workability limits of cement powder paste by decreasing the water demand for hydration. It however brings unfavourable effect of dilatancy which is the increased viscosity at high shear flow that impairs the pumpable distance and mixing uniformity. The dilatancy in cement powder paste was caused by the clustering of mobile and adsorbed polymers in SP that forms dead zone of flow and increase flow impedance. Previous research revealed that the dilatancy increased as SP dosage increased. Nevertheless, this study found that it does not increase monotonically as the SP dosage increases. It increases initially due to the introduction of SP but decreases after reaching the “critical SP dosage”. It is because adequate SP disperses effectively the cement particles for better hydration which decrease the interstitial void hence mobile SP polymers. This phenomenon is referred to “dilatancy swerve”, the understanding of which is essential in designing practical mortar or concrete mix to mitigate the impact of increased viscosity in pumping and mixing. To investigate the factors affecting the critical SP dosage, this study tested the dilatancy of various types of cement powder paste that consist of cement, fly ash (FA), silica fume (SF) and/or limestone (LS). It was found that the critical SP dosage decreased as FA was used to substitute an equal volume of cement but increased as SF or LS was used. Lastly, the general trend of decreasing dilatancy as WPD increased was observed.