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• The relationship between GGBFS:FA ratio and geopolymer strength in the whole humid-heat-treating temperature range presents a mountain-shaped respond-surface containing "positive-side" and "negative-side". • Appropriate humid-heat-treating makes the strength of GFS geopolymer with 50% SS dosage reach to more than 70 MPa, which realizes the "win–win" of high SS utilization and high strength. • The micro-hydration process on "positive-side" and "negative-side" are distinct for the crystallization of massive matrix gels causing by distinct Ca/(Si + Al) value. In order to improve the comprehensive utilization of steel slag in China, the ternary geopolymer with ground granulated blast furnace slag (GGBFS), fly ash (FA), and steel slag (SS) was prepared under the fixed addition of 50% SS in this research. The GGBFS:FA ratio was optimized to improve the mechanical performance of ternary groups. Humid-heat-treating ranging from 60 °C to 195 °C was adopted as the accelerated method to try to further strengthen the geopolymers compared with the standard curing. The strength development and hydration mechanism with different GGBFS:FA ratios were studied. The result revealed that appropriate humid-heat-treating significantly improved the geopolymers' strength, which built a mountain-shaped response surface with the variations of the GGBFS:FA ratio and the corresponding treating parameters. The reasons for this phenomenon were the different activities of GGBFS, FA, and SS under different treating conditions, as well as the change of Ca/(Si + Al) ratio in hydration products that cause massive matrix gels with low Ca/(Si + Al) crystallized to crystal calcium silicate hydrated with high Ca/(Si + Al). The crystallization of massive matrix gels made the initial dense structure break up, resulting in a loss of strength in geopolymers. The suggested Ca/(Si + Al) ratio value in whole raw material mixtures of GFS should be lower than 1.0, which reflects the GGBFS:FA = 4:1 in the system with 50% SS. Hence, the treating parameters needed to be controlled and selected in practical application, respectively. [ABSTRACT FROM AUTHOR]