Cause and mitigation of dilatancy in cement powder paste

Abstract Superplasticiser (SP) becomes essential in contemporary concrete manufacturing because it decreases the water demand for a prescribed set of concurrent strength and workability requirement. On the other hand, SP also creates unfavourable dilatancy (i.e. shear thickening) that decreases the uniformity of mixing and pumpable distance of concrete, as well as creates difficulty in in-situ handling. Although it is known that the dilatancy is created by clustering of free and adsorbed polymers of SP, the correlation of them is not fully understood. Herein, the dilatancy of cement powder paste was studied using a co-axial rheometer. It is evident that with the addition of SP, dilatancy of paste increases initially from zero up to certain dosage. Further addition of SP decreases dilatancy as it effectively disperses the cement particles. At a given SP dosage, replacing cement partially with pozzolanic filler such as fly ash or silica fume can effectively decrease dilatancy depending on SP dosage. It is because the fillers can fill up the interstitial void of the cement particles and free up more water for cement hydration to produce Ca2+ via ettringite formation. Hence, it attracts more negatively charged SP to adsorb to the cement surface, and decreases the free SP polymers that cause dilatancy.