The rheological behaviour of alkali-activated low-calcium fly ash (AAF) pastes is evaluated. The sodium-based, alkali-silicate activating solutions are proportioned for specific reactive oxide ratios in the paste considering the reactive content of the fly ash. Alkali-silicate activating solutions with different silica contents, with an MS value (the mass ratio silica to sodium oxide (SiO2 : Na2O)) of less than 1.80 are used. The activating solutions exhibit a Newtonian behaviour with higher viscosities at larger silica contents or lower MS. The yield stress and the viscosity of the AAF pastes increase with the viscosity of the activating solution. The zeta potential of the fly ash particles in the alkaline solution with dissolved silica is lowered on decreasing its MS. The yield stress of the AAF paste increases sensitively on decreasing the MS of the activating solution due to the combined influences of reducing the zeta potential of fly ash particles and increasing the viscosity of the activating solution. There are counteracting effects of increasing the viscosity of the activating solution and raising the zeta potential of fly ash, produced by increasing the silica content in the activating solution. The yield stress of AAF paste is less sensitive to the silica content in the activating solution. © 2022 ICE Publishing. All rights reserved.