Effect of calcination on the physical, chemical, morphological, and cementitious properties of red mud

Abstract

Red mud (RM) is a by-product of the Bayer process in aluminum industries, and its disposal leads to environmental imbalance. This study aimed to utilize RM as a cementing material in concrete to mitigate the negative environmental impact associated with its disposal. Initially, red mud was subjected to calcination at 600ºC for 0-6 hours with increments of 1 hour. Various characterization studies, including particle size analysis, BET analysis, XRF, XRD, TG-DTA, and SEM, were conducted to investigate the physical, chemical, and morphological changes in the RM resulting from calcination. Regarding the physical properties, specific gravity, and particle size values significantly decreased, while specific surface area and mass losses increased up to 2 hours of calcination due to moisture loss. Subsequently, contrasting results were observed. XRF analysis revealed abundant presence of iron oxide (Fe2O3), alumina (Al2O3), and silica (SiO2) in all the calcined red muds. Mineralogical phase changes, such as the transformation of goethite to hematite and gibbsite to alumina, were observed through XRD analysis. Morphological changes were observed using SEM analysis, showing a loose structure up to 2 hours of calcination, followed by a denser structure. Furthermore, all the calcined RMs were incorporated into cement at a 10% weight ratio, and their pozzolanic properties were investigated. Based on the results, the 2-hour calcined RM exhibited superior cementitious properties, including high compressive strength and strength activity index (46.27 MPa and 117.24%, respectively). Similarly, scanning electron microscope analysis was conducted to understand the behavior of the 2-hour calcined red mud in cement mortar, demonstrating better C-S-H gel formation in the corresponding mix. The present study concludes that 2-hour calcined red mud can be effectively used as a cementing material in concrete.