Geopolymers cured at ambient temperature are of high interest since they provide promising environmentally friendly alternative to cement. This paper discusses the effect of the initial molar ratio of SiO2/Al2O3 and fly ash replacement with ground granulated blast furnace slag (GGBFS) on the properties of fly ash-based geopolymers. Geopolymer mixtures developed in this study are user-friendly (SiO2/Na2O larger than 1.4) and suitable for producing self-compacting geopolymer concrete at ambient temperature. Results showed that the increase in SiO2/Al2O3 ratio accelerated the setting of fly ash-slag geopolymer systems, which was quite different from that reported for metakaolin-based geopolymers. An increasing–decreasing trend in compressive strength was observed by increasing the SiO2/Al2O3 ratio. The maximum compressive strength achieved when the SiO2/Al2O3 ratio was 3.37 (Si/Al = 1.68). The higher compressive strength at this ratio could be related to the alumina-silica bondings in the amorphous area, which comprised more than 81%-84% of geopolymer microstructure. Furthermore, the study revealed that the decrease in fly ash replacement with GGBFS prolonged the setting of geopolymer mixtures and reduced their compressive strength significantly.