Rheological properties and microstructure of binary waste red brick powder/metakaolin geopolymer

•Addition of waste red brick powder causes a decrease in plastic viscosity of metakaolin-based geopolymer.•Thixotropic behavior of metakaolin diminished when more than 75% of waste red brick powder was used in geopolymer mixture.•Part of the crystalline phase in waste red brick takes part in the geopolymerization process.•Waste red brick powder and metakaolin show a synergic effect on the compressive strength of blended geopolymer.