Incorporation of antimony-bearing mining wastes into clinker Portland raw feed: The difficulty of Sb analysis in calcium silicates

Within the global trend to valorise various mineral wastes as substituents in Portland cement clinker raw feed, mining wastes are promising candidates. However, they might still contain high levels of metallic elements. Their fate in the kiln is not always understood as well as their incorporation within the various clinker's phases. This is especially the case for antimony. Its in situ microanalysis by the means of energy-dispersive spectroscopy is tricky since several of its L lines (L & alpha;1 = 3604,72 eV, L & alpha;2 = 3595,32 eV) overlap with calcium lines (K & alpha;1 = 3691,68 eV, K & alpha;2 = 3688,09 eV). Hence, at low concentrations, it is not possible to visualise its characteristics peaks. Increasing the counting rate by increasing the acceleration current results in the generation of spurious sum peaks, rendering the analysis not viable. Wavelength dispersive spectroscopy (i.e. electron microprobe) allows a better spectral resolution and quantification of Sb in the clinker phases. In a Portland cement clinker doped with 1% of Sb-bearing mining waste, the Sb2O3 content in belite and alite is in the 0.2-0.4 wt.% range as well as for C3A; in C4AF the content is higher, from 1.4 wt.% to 2 wt.%. However, there are microstructural evidence that Sb forms blebs from less than 1 & mu;m in diameter up to 10 & mu;m, included in calcium silicates. Hence, its incorporation in the lattice of calcium silicates is dubious. However, for ferrites and aluminates there is no microstructural evidence for remaining Sb-bearing phases, suggesting a direct incorporation in crystal lattices. Energy-dispersive spectroscopy (EDS) is widely used in materials science in order to get microchemical information at a micronic scale. This powerful technique allows the quick estimation of almost the whole periodic table with a single detector. However, some troubles might arise when the X-ray emission lines of an element overlap with another. This is typically the case of Sb, which overlaps with Ca. The present article exemplifies this fact on the basis of the study of Portland cement clinker made with the incorporation of Sb-bearing wastes. It shows that for dubious analysis, especially in the case of low concentrations, wavelength dispersive spectroscopy (WDS) should be preferred.