AbstractAmong various eco-friendly building products, geopolymers are emerging as a potential replacement material for ordinary portland cement due to its energy efficiency and environmental protection. Though various eco-friendly cementitious materials in combination with geopolymer concrete (GPC) have been studied by various researchers, their effect on the behavior of GPC made with ground granulated blast furnace slag (GGBS) and metakaolin (MK) is, however, not well established. This paper investigates the mechanical properties of GPC and flexural behavior of geopolymer-reinforced concrete beams, comprising various percentages of fly ash (FA), GGBS, and MK. The FA was partially replaced with various proportions of GGBS and MK. All the concrete specimens are cast for 10 molarity (10M) and are tested as per Indian standard code. The compressive, split tensile, and flexural strength of the F50G25M25 (50% fly ash + 25% GGBS + 25% MK) mix are increased by 147.75%, 57.21%, and 36.08% respectively, over the control specimen (F90G5M5) (90% fly ash + 5% GGBS + 5% MK). The ultimate load-carrying capacity of a geopolymer reinforced concrete beam was enhanced by up to 30.43% for F50G25M25 mix. The experimental ultimate loads are compared with theoretical predicted loads, and good agreement with the theoretical model is found. Replacement of FA by GGBS and MK at 25% each significantly increased the mechanical properties and flexural behavior of GPC. Other cementitious materials beyond those used in this paper could be used to investigate the mechanical properties of GPC.