Effect of activator on rheological properties of alkali-activated slag-fly ash pastes

The time-dependent rheological behaviors of alkali-activated cement (AAC) are expected to be precisely controlled, in order to meet the requirements of modern engineering practices. In this paper, the effects of activator, including the Na2O concentration and SiO2/Na2O (S/N) molar ratio, on the rheological behavior of alkali-activated slag-fly ash pastes were investigated. The small amplitude oscillatory shear (SAOS) and shear test were used to evaluate the structural build-up and flowability of pastes. Besides, zeta potential measurement, calorimetric test and thermogravimetric analysis (TGA) were carried out to reveal the physico-chemical mechanisms behind the rheological evolution of fresh pastes. It was found that high Na2O concentration and low S/N molar ratio improved the flowability and structural build-up rate of paste. Moreover, the structural build-up of alkali-activated slag-fly ash pastes consists of two stages, which is controlled by the dissolution of solid reactants and formation of C-(A)-S-H gels, respectively.