Display omitted •One-part geopolymer was synthesized by using Bayer red mud as main raw material.•Long-term strength of binder was significantly improved with addition of 20–30wt% SF.•Lower water/solid ratio contributed to increasing the strength.•The compressive strength of geopolymer cured for 28d reached 31.5MPa.•Geopolymerization of dissolved aluminosilicate and silica formed dense matrices. One-part geopolymer was synthesized from alkali–thermal activated Bayer red mud (RM) with addition of silica to optimize its composition. The RM was pretreated through alkali–thermal activation and turned to geopolymer precursor, which could be used by only adding water in blending process. However the long-term strength of the binder with only RM was poor because of the unstable polymerization due to the low SiO2/Al2O3 molar ratio (1.41). Silica fume (SF) was chosen to increase the SiO2/Al2O3 molar ratio of the geopolymer formulation. By adding 25wt% of SF, the 28d compressive strength of the geopolymer with a SiO2/Al2O3 molar ratio of 3.45 could reach 31.5MPa at a water/solid ratio of 0.45. Sodium aluminosilicate in the activated RM dissolved in water and formed an alkaline environment to dissolve SF. The dissolved silica participated in geopolymerization, leading to a satisfactory geopolymer composition. Typical amorphous geopolymer matrices were formed in the binder completely cured.