Effect of silica fume and slump-retaining polycarboxylate-based dispersant on the development of properties of portland cement paste

Abstract

This paper investigates the effect of silica fume (SF), a slump-retaining polycarboxylate ether (PCE) dispersant, and their combinations, on the fresh and hardened properties of ordinary portland cement paste. The investigated properties include hydration kinetics, rheological properties (i.e., plastic viscosity and yield stress), setting time, and compressive strength. Cement was partially replaced by SF at mass substitution levels of 0%, 10%, 20%, and 30%. The PCE dosages included 0%, 0.6%, 1.2%, 1.8%, and 2.4%, by mass of the binder. The results indicate that SF accelerates cement hydration rates and reduces the time of initial/final setting in relation to its content in the paste. This is due to the provision of additional C-S-H nucleation sites on SF surfaces. However, when PCE is present, such enhancement induced by SF is suppressed. In addition to enhancing cement hydration rate, SF reduces inter-particle spacing, thus amplifying rheological properties. In contrast, PCE reduces viscosity and yield stress. Both SF and PCE reduce 1-day compressive strength due to the effects of cement dilution (caused by SF) and the suppression of early-age hydration kinetics of cement (caused by PCE). The 7-day compressive strength, however, are broadly the same among the binary pastes; this is because the acceleratory and retarding effects of SF and PCE, respectively, are less pronounced at later ages. Finally, robust correlations between hydration kinetics, rheological properties, and compressive strength evolution are established. Those correlations can provide a basis for optimizing binder formulation and proportioning of high/ultra-high performance concrete.