Preparation and characterization of ultramarine blue pigments from fly ash by using the X-ray photoelectron spectroscopy (XPS) for the determination of chemical states of sulphur in chromophores

Natural ultramarine (UMB) pigments have been of broad use in the past and present in their natural form (lapis lazuli). They belong to the group of mineral pigments characterized by the sodalite structure and coloured sulphur species (chromophores) encapsulated inside. Nowadays, there is an urgent need for their cheaper analogues synthetized from other minerals or, if possible, from waste materials. At the same time, a better understanding of chemical interactions of the rather complex constituents of the UMB synthesis is needed with the help of sophisticated spectroscopic techniques of surface analysis. In this preliminary study, the novel X-ray photoelectron spectroscopy (XPS) is applied to determine the content and chemical state (oxidation and binding) of sulphur and other elements of a fly ash and other constituents used to prepare synthetic UMB pigments. The main aim was to elucidate a possible connection of these original parameters (especially sulphur-based chromophores) with a final colour of the UMB pigments (green, blue, reddish blue, pink, violet, red and greenish blue). The results were compared with these from more traditional ones: Raman and Infrared spectroscopy.