In this paper, the Taguchi method has been used to design optimum mix proportions for geopolymer concrete with ground granulated blast furnace slag (GGBFS) as aluminosilicate source at ambient curing condition. The influences of binder content, alkaline activator to binder content (Al/Bi) ratio, sodium silicate to sodium hydroxide (SS/SH) ratio, and sodium hydroxide (SH) concentration on the geopolymer concrete were investigated. A total of nine mix designs were evaluated. It was found that specimens with a binder content of 450 kg/m3, Al/Bi ratio of 0.35, SS/SH ratio of 2.5, and SH concentration of 14 M produced the highest 7-day compressive strength (60.4 MPa). However, the setting time was found to be short. Hence, fly ash (FA), metakaolin (MK), and silica fume (SF) were used as partial replacement of GGBFS in different proportions to increase the setting time. It was found that the setting time improved for the partial replacement of GGBFS with FA, MK, and SF.