Rheology of fresh cement paste with superplasticizer and nanosilica admixtures studied by response surface methodology

Abstract

Thirteen cement pastes, keeping the water/cement ratio constant at w/c = 0.30, with different amounts of two admixtures were prepared. A superplasticizer (modified polycarboxylic ether polymer) was studied in the range of 0.14–1.00% (over cement weight), while a viscosity-modifying admixture (nanosilica aqueous dispersion) was tested at 0.50–3.00% range (over cement weight). Oscillatory and steady shear rheometry, as well as results of modified slump test and Marsh funnel flow time of 13 formulations were evaluated by response surface methodology, in order to find the optimum recipe that reduces the viscosity while keeping the paste workability. It was predicted that a formulation with 0.39% w/w of superplasticizer and 2.78% w/w of viscosity-modifying admixture would provide a paste with a fifth of the viscosity of control (cement paste without any admixture), but with an acceptable slump. This formulation was prepared and tested, and the plastic viscosity was 1.00 Pa-s, while the oscillatory yield stress was 330 Pa. The admixtures did not affect the compression strength (48 MPa average, after 28 days), compared with the control.