Composite alkali activator was prepared by using sodium carbonate instead of sodium hydroxide to adjust the modulus of water glass. The effects of different alkali content and sodium carbonate replacement ratio on fluidity, setting time, and compressive strength of geopolymer cementitious materials were studied. The phase composition and microstructure of hydration products of geopolymer cementitious materials were analyzed through FT-IR, XRD, and SEM experiments. The results show that the combined effects of sodium hydroxide and sodium carbonate combined with composite water glass activators are superior to the effects of their individual combined with water glass activators. When alkali content is 6% (mass fraction) and the replacement ratio of sodium carbonate is 40%(mass fraction), the fluidity of geopolymer cementitious materials reaches 185 mm, and 28 d compressive strength reaches 94.4 MPa. The increase of replacement ratio of sodium carbonate can prolong the setting time of geopolymer cementitious materials. When the replacement ratio reaches 100%, the initial setting time and final setting time of geopolymer cementitious materials reach 372 and 420 min. When different alkali components are used as activators, similar hydration products are observed in geopolymer cementitious materials, mainly consist of amorphous aluminosilicate C-(A)-S-H gel.