Effect of composite activators on mechanical properties, hydration activity and microstructure of red mud-based geopolymer

Portland cement is the most commonly used cementing material in cemented paste backfill. However, the production of it is limited by high energy consumption and environmental pollution. The development of cementless solid waste filling material has been attracting the attention of researchers. In this study, the potential application of CaO/CaSO4 composite activator in red mud based geopolymer was explored. The mechanical properties, hydration activity and microscopic structure of samples were analyzed by uniaxial compressive strength (UCS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric (TG), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). The results showed that the combination of CaO and CaSO4 can significantly increase the strength of red mud-based geopolymer at early ages. The ratio of CaO to CaSO4 played a crucial role in enhancing the mechanical properties of red mud-based geopolymer. The samples containing 5% CaO and 10% CaSO4 showed the highest UCS, which was 1.6 MPa after curing for 28 days. The simultaneous existence of CaO and CaSO4 can improve the polycondensation degree of red mud-based geopolymer. Under the action of composite activator, the chemical bonds of Si−O and Al−O in silicon oxygen tetrahedron [SiO4]4− and aluminum oxygen tetrahedron [AlO4]5− continue to depolymerize and recombine, so as to form Si−O−Al−O long-chain red mud-based geopolymer. This approach can help reduce the filling cost and promote the simultaneous utilization of red mud, fly ash, and gangue to protect the environment. •The composite activators can increase the properties of red mud-based geopolymer.•The ratio of CaO to CaSO4 played a vital role in enhancing the mechanical properties.•The optimal red mud-based geopolymer contained 5% CaO and 10% CaSO4.