Application of μ-XAFS for the determination of the crystallization ratio in a series of vitro-ceramic materials containing industrial waste

The distribution and microstructure of Fe in a series of vitrified Pb- and Fe-rich industrial waste samples is studied by means of X-ray fluorescence mapping (XRF), conventional and micro-X-ray absorption fine structure (μ-XAFS and XAFS) spectroscopies. The under study samples contain toxic ash, that is vitrified with appropriate quantities of vitrifying (SiO2) and flux (Na2O) agents and are studied in the as-casted state as well as after annealing. XRF mapping in combination with μ-XAFS applied to samples containing 50% and 60% ash, demonstrated that annealing at temperatures above 600 °C induces loss of homogeneity and formation of Fe-rich microcrystalline islands. By appropriate fitting of the EXAFS spectra it is found that X% (where X depends on the sample composition and annealing temperature) of the Fe atoms form Fe-rich islands where Fe is octahedrally coordinated in FeO6 polyhedra (RFe–O = 0.193 nm). Contrary to that, (100 − X)% of the Fe atoms are incorporated in the vitreous matrix (which has a lower Fe content), and are coordinated in FeO4 tetrahedra (RFe–O = 0.188 nm). The crystallization ratio X/(100 − X) is found to increase with the annealing temperature and the loss of homogeneity becomes more extensive.