Preparation and coloring mechanism of MAl2O4/CoAl2O4/quartz sand (M = Ca or Ba) composite pigments

The doping of divalent metal ions is considered as a feasible strategy for reducing the consumption of expensive cobalt source and regulating the color performance of CoAl2O4 pigments. However, it may be controversial for formation of solid solution in the system of MAl2O4/CoAl2O4 (M = Ca or Ba) due to the bigger differences in ionic radius and coordination number between Ba2+ (or Ca2+) and Co2+, and thus the coloring mechanism of CoAl2O4 pigments is vague after incorporation of Ba2+ (or Ca2+). In this study, MAl2O4/CoAl2O4/quartz sand (M = Ca or Ba) composite pigments are fabricated using the quartz sand waste discarded from mineral processing as substrate. The effect of the added amount of Ba2+ (or Ca2+) on the color properties of composite pigments are investigated, and the optimum b* values of CaAl2O4/CoAl2O4/quartz sand and BaAl2O4/CoAl2O4/quartz sand reach -47.92 and -48.06, respectively. Furthermore, it reveals that the obtained pigments are composed of quartz, hexagonal CaAl2O4 or BaAl2O4, and cubic spinel CoAl2O4, and thus CaAl2O4 or BaAl2O4 cannot form solid solution with CoAl2O4 by replacing Co2+ with Ba2+ or Ca2+ owing to the significant differences in ionic radius and coordination number between Ba2+ (or Ca2+) and Co2+. The improvement in the color performance of CoAl2O4 pigments may be related to the intrinsic oxygen vacancy defect of CaAl2O4 or BaAl2O4, which promote CoAl2O4 to reflect more blue visible light at 430 nm.