Properties of Flue Gas Desulphurization Gypsum–Activated Steel Slag Fine Aggregate Red Mud–Based Concrete

The large amount of discharge and accumulation of red mud and steel slag has led to the waste of land resources and serious environmental pollution problems. Therefore, the utilization of a large amount of red mud and steel slag in reasonable fields is an urgent problem. At present, the main practical methods include using red mud as a substitute for cement or raw material in cement clinker production and grinding steel slag as a mineral admixture or directly replacing fine aggregates. However, experimental studies on the simultaneous use of large amounts of red mud and steel slag in concrete are extremely limited. In this paper, red mud was used to replace cement (40%–60%) and steel slag to replace fine aggregate (50%–100%) to prepare steel slag fine aggregate red mud–based concrete (SFA-RM concrete). In addition, flue gas desulphurization gypsum (FGDG) was added as a sulfate activator in the concrete to stimulate the potential cementitious activity of red mud and steel slag to prepare the FGDG-activated SFA-RM concrete with improved strength. A total of 18 concrete mixes with different proportions of red mud, steel slag, and gypsum were designed and the mechanical properties, workability, hydration products, and microstructure of these concretes were studied. The results showed that the mechanical properties of FGDG-activated SFA-RM concretes were excellent, the 60 day compressive strength of the SFA-RM concrete reached 43.8 MPa, and the compressive strength of FGDG-activated SFA-RM with 15% gypsum and 100% replacement rate of steel slag improved to 47.1 MPa, which were both better than ordinary concrete. Moreover, the results of X-ray diffraction (XRD) and scanning electron microscope (SEM) tests indicated that the hydration products of concrete mainly included Ca(OH)2, C-S-H gel, ettringite, and zeolite, with small and few micropores as well as dense microstructure.