There is limited literature available on the rheological behavior of slag-based geopolymer mortar and fly-ash-based geopolymer paste. This study will attempt to examine the rheological behavior of fly-ash-based geopolymer concrete with a mixture of sodium hydroxide and sodium silicate solutions. It was observed that thixotropy is present in fly-ash-based geopolymer concrete, and the degree of thixotropy depends on the molar strength of the sodium hydroxide solution and the ratio of silicate to hydroxide solutions. It was also observed that fly-ash-based geopolymer concrete can be assumed to behave in the Bingham model with a very good accuracy. Bingham parameters, such as yield stress and plastic viscosity, are affected by the molar strength of the sodium hydroxide solution and the ratio of silicate to hydroxide solutions.