Effects of Calcination on the Cementitious Activity and Pozzolanic Reactivity of Bayer Red Mud from Different Sources

Red mud (RM) is a hazardous waste generated by aluminum production. It is difficult to utilize due to its high aluminum and iron oxide contents, high alkalinity, and large specific surface. Still, extensive research is underway to explore its potential as partial replacement for cement in concrete. Due to the differences in the physical, chemical, and mineralogical characteristics of bauxites from different sources, the associated RMs are also different. Some studies have reported that unless calcined, RM produced by the Bayer process has negligible pozzolanicity. However, the appropriate calcination temperature is not unique as it will depend on the RM mineralogical composition. Here, the calcination mineralogical composition nexus and its effect on RM pozzolanicity are investigated in three types of RM produced by Bayer's process. The RMs were calcined at 600, 800, and 1000°C for 2 hours, and were used as 15 wt.% replacement for Portland cement in mortar mixes. One of the RMs exhibited pozzolanicity without calcination while another showed increased reactivity after calcination at 800°C. The underlying mechanisms are discussed, and it is concluded that no specific calcination temperature(s) can be recommended to activate every RM. Contrary to the findings of previous studies, one of the investigated RMs, used in its virgin form at 15 wt.% replacement for cement, exhibited noticeable pozzolanic activity and achieved over 94% of the compressive strength of the control specimen at 91 days. The calcination of the same RM, irrespective of the calcination temperature, reduced its pozzolanicity.