Dilatancy reversal in superplasticised cementitious mortar

By electrostatic repulsion and steric hindrance, polycarboxylate-based superplasticiser (SP) decreases the water demand for cement hydration in mortar/concrete production. Subsequently, less water is needed for a prescribed flowability, which improves its concurrent design limits of strength and flowability. Nonetheless, SP also introduces unfavourable dilatancy (or shear thickening) in mortar due to the clustering of mobile SP polymers in interstitial voids and adsorbed polymers on cement. Interestingly, the dilatancy does not increase monotonically with SP dosage. Reversal of dilatancy occurs when a threshold SP dosage is reached, and subsequent addition of SP decreases dilatancy because the fine particles in the mortar are better dispersed with adequate SP. Interstitial voids substantially decrease due to deflocculation of the fine particles, which improves the wet packing density (WPD) of mortar. Based on this, it is believed that by blending the cement with fly ash and/or using multi-sized sand, the dilatancy of mortar can be alleviated. In this research, using a coaxial viscometer, a rheology test programme of superplasticised mortar was conducted to determine the threshold SP dosage when dilatancy reversal occurs, to study the effect of using multi-sized powder and sand on mortar dilatancy and to correlate the dilatancy of mortar to its WPD.