Synergistic effect of red mud, desulfurized gypsum and fly ash in cementitious materials: Mechanical performances and microstructure

The use of industrial solid waste to prepare cementitious materials can not only solve the pollution problem of ecological environment, but also help carbon emission reduction and sustainable development. In this work, typical alkaline solid waste - red mud (RM), sulfate solid waste - desulfurization gypsum (DG) and silicon-aluminum character solid waste - fly ash (FA) were used to prepare cementitious materials. The influence of synergistic effect of RM-DG-FA on the mechanical performances and hydration products of cementitious materials were systematically investigated. The hydration products were characterized and analyzed by XRD, TG, FTIR, MIP and SEM-EDS. It was found that the mechanical performances of cementitious materials were significantly improved under the synergistic effect of three kinds of solid wastes. When the content of the three solid wastes was 50%, the compressive strength of the cementitious materials at 28 d was 50.60 MPa, and the mechanical performances at each age were higher than the # 42.5 Portland cement standard. The microstructure results showed that under the synergistic action of alkali and sulfate, the active silicon and aluminum substances in RM and FA can quickly participate in the reaction, which makes the hydration degree of cementitious materials can move to the reaction direction with higher polymerization degree. The hydration products mainly included ettringite, C-S-H and C(N)-A-S-H gel. A large amount of gel and ettringite grew alternately, which reduced the porosity and formed a dense structure of the cementitious materials matrix, and its mechanical performances were improved. The leaching results showed that the synergistic effect also improved the solidification ability of the cementitious materials to Na+. In this work, the synergistic effect of RM-DG-FA significantly increased the utilization of solid wastes and the performances of cementitious materials. The research results provided a scientific basis for improving the utilization rate of alkaline solid wastes, sulfate solid wastes and silicon-aluminum character solid wastes and their collaborative utilization as building materials.