Modulating red mud for fabrication of cementitious material by analyzing the thermal evolution of hydrogarnets

This work aims to develop a modulation strategy for converting red mud (RM) into cementitious material based on elucidating the phase transformation of hydrogarnet. The results show that cementitious minerals 2CaO⋅SiO 2 (C 2 S), 12CaO⋅7Al 2 O 3 (C 12 A 7 ), and 4CaO⋅Al 2 O 3 ⋅Fe 2 O 3 (C 4 AF), as well as the free iron minerals Fe and FeO, are formed by integrating calcification dealkalization and reduction roasting treatment of RM. During the reduction roasting process, CaO is preferentially combined with SiO 2 and Al 2 O 3 to form cementitious minerals, and the Fe(III) compounds in hydrogarnet and hematite can be directly reduced to free iron minerals without intermediate ferrites. By optimizing the reduction roasting parameters and eliminating the useless minerals 2CaO⋅Al 2 O 3 ⋅SiO 2 (C 2 AS) and FeO, the reduction roasting product is mainly composed of C 2 S, C 12 A 7 , C 4 AF, and Fe. Therefore, cementitious material is obtained after the magnetic separation of Fe, which possesses both early and late hydration properties. In addition, 75% Fe in RM can be recovered, and the reduced iron powder (RIP) is also useful in the cement clinker production or steel smelting process. The findings in this work lay the foundations for understanding the phase transformation of RM-derived hydrogarnet in the reduction roasting process and also provide a new reference for the modulation and utilization of RM in the cement and concrete field.