One-part Geopolymer Concrete (GPC) is advantageous to conventional two-part GPC due to use of solid activator in its design mix and improved strength properties at ambient curing temperature. The major limitation associated with the use of one-part GPC is non-availability of standards for its strength-based design mix. The present study addressed the problem and proposed a model for design mix of normal-strength one-part GPC using Response Surface Methodology (RSM). The input parameters of the study include content of Ground Granulated Blast Furnace Slag (GGBS), anhydrous sodium metasilicate, water, coarse aggregates and fine aggregates whereas the output parameter was compressive strength. The model has successfully designed the desired grade of concrete which was further validated experimentally in the laboratory. The designed concrete was studied for its compressive strength, flexural strength, split-tensile strength and microstructural properties. Measured values of compressive strength, flexural strength and split-tensile strength were 39.25, 4.76 and 3.30 MPa, respectively, at 28 days. The SEM and EDS analysis of the concrete confirmed the presence of C/N–A–S–H binding gel contributing to its strength development and dense microstructure. Further, the FTIR analysis of the concrete microstructure revealed presence of GGBS particles, unreacted GGBS, Si–O–Si, Si–O–Al bonds and alkali hydroxide bonds.