Effect of silica fume on rheology of slag-fly ash-silica fume-based geopolymer pastes with different activators

This study examined the effect of silica fume contents on the rheological behavior of slag-fly ash-silica fume-based geopolymer pastes by packing density measurement, low-field NMR, zeta potential, dynamic light scattering and FTIR analyses. Incorporating silica fume into sodium hydroxide (SH)-activated geopolymer pastes resulted in the decrease of packing density and the increases of yield stress, plastic viscosity and hysteresis loop curve area. The contrary results were found in the pastes activated by sodium silicate (SS). The water states of geopolymer pastes were influenced by SiO₂/Na₂O ratio of activator and silica fume dosage. The mobile water content in the sample increased as the silica fume dosage increased, regardless of the SiO₂/Na₂O ratio of activator. The particle size of the silica species in the activator and interstitial fluids is close to that of silica fume. The incorporation of silica fume resulted in the variation in the silicate structural characteristics of the interstitial fluids. The influence of silica fume on the rheological behaviors of geopolymer systems is strongly governed by the interparticle repulsion or attractiveness and activator nature, opposed to the filler effect and dissolution of silica fume. In SS-activated systems, the repulsion between the silanol groups on the silica fume surface and silicate species adsorbed on the surface of precursors particles contributed to the particle separations, thereby enhancing rheological behaviors. In SH-activated system, the attractiveness between divalent metal cations and silanol groups on the surfaces of precursor and silica fume particles was increased and the particle space was restrained. This study would provide new insights into the role of silica fume in the particle interaction of fresh ultra-high performance geopolymer concrete.