Impact of hematite in red mud on hydration characteristics and environmental performance of cementitious materials

Red mud (RM), a waste by-product of alumina production, is increasingly used in cementitious materials. However, the high iron content and alkaline substances in RM, in addition to heavy metal elements, hinder its utilisation in cementitious materials. To overcome these issues, a ternary system of RM, blast furnace slag (BFS), and cement was prepared to assess the effects of iron in RM on hydration and environmental performance. The results indicated that 97% of the iron in RM exists in the form of Hematite. As the Hematite content in the system increases, the cumulative hydration heat, heavy metal immobilisation rate, hydration-product content, degree of polymerisation, initial and final setting times, as well as early compressive strength gradually decrease. However, the later strength of the matrix initially increases and then decreases. In addition, the soluble alkali in RM was utilised. When the Hematite content in the system is between 2.47 % and 7.82 % (corresponding to 9.68 %- 31.28 % Hematite in RM), the mechanical properties of the matrix meet PO.42.5 standards, and the concentrations of hazardous elements such as Na in the leachate comply with China's drinking water standards (GB/T 5749-2022).