The slag based geopolymers materials are synthesized at ambient temperature by using fly ash (FA) and ground granulated blast furnace slag (GGBFS) as the main starting materials, and NaOH and soluble glass together as activators. The influence of the mix proportion between GGBFS and FA on the strength and reaction products was investigated. The products obtained were characterized by means of XRD, FTIR spectroscopy and FE-SEM coupled with EDX. The results show that a great increase in compressive strength appears when the replacement percentage of GGBFS exceeds 30wt. %, the resultant products are mainly amorphous materials (amorphous aluminosilicate and/or C-S-H gel) and much more compact, homogeneous, and uniform when the amorphous aluminosilicate is increasingly turned into C-S-H gel with the increasing replacement percentage of GGBFS.