Red mud as a substitute coloring agent for the hematite pigment

Red mud (RM), a highly alkaline iron oxide rich sludge obtained during the production of alumina, was treated to work as a coloring agent for ceramic glazes. The approach aims the valorization of this residue, adding environmental and economic value. RM was sintered at different temperatures (1100 °C, 1150 °C, 1200 °C, 1250 °C, 1300 °C and 1350 °C) to assess changes in its mineralogical composition. The obtained powders were characterized by XRD, UV-Vis and CIELab. The samples were added to glazes (transparent, opaque and matte) and their color stability analyzed by CIELab. RM sintered at 1300 °C was compared with commercial hematite producing lower coloring power but yielding better color stability on glazes for temperatures of 1100 °C ± 15 °C, thus presenting RM_1300C as a suitable substitute for the hematite pigment.