In this study, the effects of ground granulated blast furnace slag (GGBFS) content, alkaline solution to binder (Al/Bi) mass ratio, sodium silicate solution to sodium hydroxide solution (SS/SH) mass ratio, and additional water to binder (Aw/Bi) mass ratio on the compressive strength, setting time and workability of geopolymer pastes were studied. A series of mini-size specimen compression tests, setting time tests and mini-slump tests were conducted at ambient condition (23 ± 2 °C). The GGBFS and Class F fly ash (FA) were used as aluminosilicate source. The alkaline activator was a blend of sodium silicate solution and sodium hydroxide solution. Additional water was added to improve the workability and prolong the setting time. Based on the test results of compressive strength, setting time and workability, the optimum mix design was found to have GGBFS content of 40%, Al/Bi ratio of 0.5, SS/SH ratio of 2.0, and Aw/Bi ratio of 0.15. It was found that the properties of the geopolymer paste under optimum mix design were better than those of ordinary Portland cement (OPC) pastes. After that, the geopolymer concrete tests based on optimum mix design of geopolymer paste were conducted, in comparison with OPC concrete tests. It was found that the properties of geopolymer concrete were also better than the properties of OPC concrete. It is worth noting that this relatively simple and fast test methodology to obtain the optimum mix design of geopolymer concrete can help engineers save time and labour. Lastly, new mathematical models were proposed to predict the properties of geopolymer pastes, which showed high accuracy.