In this paper, using red mud, fly ash and calcium carbide slag as the main raw materials, the composite cementitious test blocks of red mud-fly ash, red mud-calcium carbide slag, fly ash-calcium carbide slag binary systems, and red mud-fly ash-calcium carbide slag ternary system were prepared respectively. The size of gelling effect of red mud, fly ash and calcium carbide slag was compared with the compressive strength of the cementitious test blocks at different curing ages. The phase composition, molecular bond structure, microstructure and thermogravimetric characteristics of the cementitious test blocks were analyzed by XRD, FT-IR, SEM and TGA to study the hydration mechanism of different cementitious systems. The results show that the synergistic gelling effect of the ternary system of red mud, fly ash and calcium carbide slag is the largest. When the content of red mud, fly ash and calcium carbide slag is 35%, 30% and 35% (mass fraction) respectively, the 28 d compressive strength of the test block reaches 25.4 MPa. Compared with the binary cementitious system, the ternary cementitious system has higher hydration degree and more complex products. SEM analysis shows that the ternary cementitious system test block of sodium aluminate hydrate (N-A-S-H) and calcium aluminate hydrate (C-A-S-H) are interlinked with the raw materials to form a three-dimensional network structure, which makes the structure more dense and has better mechanical properties.