Comparison of Non-calcined and Calcined Red Muds from Different Sources as Potential Supplementary Cementitious Material

Red mud (RM) is a hazardous by-product of alumina refining processes. Due to its high alkalinity, large specific surface area, and complex and variable composition, RM is hard to treat or utilize on a large scale. To date, more than 4 billion tons of RM have been stockpiled globally and is still growing by more than 120 million tons annually. Many investigations have focused on the largescale utilization of RM as a construction material, either in its virgin form or after heat treatment. However, the huge differences among the chemical/mineralogical compositions of RMs due to differences among bauxite ores from different sources and/or different refining processes, it is difficult to prescribe a unique process for activating RM cementitious/pozzolanic properties. The present study aims to identify the key chemical and process-dependent factors that influence the cementitious/pozzolanic properties of RM. Two types of RM from different sources are investigated to determine the effects of the chemical/mineralogical composition, the alumina refining process, and calcination on the mineralogical phases and compressive strength of mortar cubes made with ordinary Portland cement (OPC)- RM blended cement. Mortar made with 15 wt.% OPC replaced by one type of virgin RM produced by the Bayer’s process was found to have better strength than a control mortar made with 15% OPC replaced by sand, which indicates that this RM had cementitious/pozzolanic property without requiring heat treatment. On the other hand, the RM produced by the bauxite calcination method needed heat treatment to improve its pozzolanicity, but, despite the improvement, mortar made with its optimally treated form had lower 91-day compressive strength than the companion control mortar made with extra sand as RM replacement. The compressive strength of mortars made with calcined RM was not only affected by the phase changes of the virgin RM properties brought about by calcination but also by changes to its physical.