Incorporation and substitution mechanism of cadmium in cement clinker

Cement clinkers doped with CdO (0-2.0% by weight) were prepared to investigate cadmium distribution and lattice substitution during cement kiln co-processing of municipal solid and/or hazardous wastes. The contents of incorporated CdO were investigated by Energy Dispersive Spectrometer (EDS), detecting the samples packaged by epoxy resin, and Atomic Fluorescence Spectrometry (AFS), detecting the silicate and interstitial phases separated by extraction. EDS revealed that high concentrations cadmium was incorporated in silicate phases, which resulted in lower Ca/Si ratio. AFS also presented more than 90 wt.% CdO were taken by silicate phases; however, calculated partition coefficient (Kf) revealed cadmium ions had equal opportunities to be incorporated in silicate and interstitial phases. Furthermore, X-ray Diffraction showed no new phases were formed, while X-ray photoelectron spectroscopy proved the existence of Cd-O bond, which might be attributed to the lattice substitution of Cd2+ to Ca2+. This substitution resulted in shortened average chain length of C-S-H and postponed hydration of doped clinkers.