Dilatancy reversal in superplasticised cementitious mortar

Abstract

By electrostatic repulsion and steric hindrance, polycarboxylate based superplasticizer (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 caused by clustering of mobile SP polymers in the interstitial void and adsorbed polymers on cement. Interestingly, the dilatancy does not increase monotonically with SP. Reversal of dilatancy occurs when a threshold of SP dosage is reached, and subsequent addition of SP decreases dilatancy because the fine particles in mortar are better dispersed with adequate SP. The interstitial void substantially decreases due to deflocculation of fine particles that improves wet packing density 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. Herein, a rheology test programme of superplasticized mortar was conducted via a co-axial viscometer to: (1) find out the threshold SP dosage when dilatancy reversal occurs; (2) study the effect on dilatancy of mortar by using multi-sized powder and sand; (3) correlate the dilatancy of mortar to its wet packing density.