Heavy metals in cement phases: on the solubility of Mg, Cd, Pb and Ba in Ca3Al2O6

The compounds formed when the divalent cations Mg2+, Cd2+, Ba2+ and Pb2+ are present during the preparation of Ca3Al2O6 have been studied using X-ray microanalysis and diffraction methods. The smaller Mg cations are found to partially substitute for Ca2+, and structural refinements show that Mg preferentially occupies the smaller six-coordinate sites in Ca3-xMgxAl2O6. When Ba is present, it preferentially occupies the larger eight- and nine-coordinate sites. X-ray microanalysis suggests that Pb and Cd are lost from the samples during the preparation process. The diffraction patterns show a small decrease in the lattice parameters, suggesting that a defect structure of the type Ca3-x(vac)xAl2O6 is formed. The distribution of products formed on hydration of the doped Ca3-xMxAl2O6 is found to be very different than that observed for the undoped material