Geopolymer concrete (GPC), usually produced via the activation of the cementitious nature of industrial by-products (IBPs) such as ground granulated blast furnace slag (GGBFS) and fly ash (FA), has potential to be used as a replacement for conventional portland concretes (CPCs). In this study, the effect of the concentration of sodium hydroxide (NaOH) and substitution of GGBFS with pozzolans such as natural zeolite (NZ) and silica fume (SF) on the mechanical properties of GPC was investigated experimentally. For this purpose, the compressive, flexural, and tensile strengths of various GPC mixes were measured. GPC mixes were prepared with various concentrations (i.e., 4, 6, and 8 M) of sodium hydroxide as well as GGBFS substitution (i.e., 5, 10, 15, 20, 25, and 30 wt%) with NZ and SF. Furthermore, the response surface method (RSM) was employed to achieve the optimum values of the design variables to maximize the compressive, flexural, and tensile strengths of pozzolanic GGBFS-based GPC. Overall, the results showed that with increasing NaOH concentration, the compressive strength was decreased while the maximum flexural and tensile strengths were obtained when the concentration of NaOH was 6 M compared with 4 and 8 M. In addition, the utilization of NZ improved the compressive, flexural, and tensile strengths of GGBFS-based GPC by about 4%, 6%, and 20%, respectively, at 10 wt% replacement. Moreover, the substitution of GGBFS with SF could improve the compressive, flexural, and tensile strengths of GPC up to 30%, 20%, and 25%, respectively, at 30 wt% substitution. Besides, the optimization results demonstrated that by substituting GGBFS with 60.29 kg (i.e., 15.9%) of NZ and using 5.28 M NaOH the optimal trinary conditions in terms of the compressive, flexural, and tensile strength values could be achieved. The optimal conditions could also be obtained by substituting GGBFS with 113.79 kg (i.e., 30.0%) of SF and using 6.19 M NaOH.
