Iron in carbonate containing AFm phases

One of the AFm phases in hydrated Portland cement is Ca-3(AlxFe2-x)O-6 center dot CaCO3 center dot nH(2)O. It is based on hexagonal and platey structural elements and the interlayer structure incorporates CO32-. The solid phases were experimentally synthesized and characterized by different techniques including X-ray techniques (XRD and EXAFS) and vibrational spectroscopy techniques (IR, Raman). Fe-monocarbonate (Fe-Mc) and Al-monocarbonate (Al-Mc) were found to be stable up to 50 degrees C, while Fe-hemicarbonate (Fe-Hc) was unstable with respect to Fe-Mc in the presence of calcite. Fe-Mc has a rhombohedral R (3) over barc symmetry which is different from the triclinic of the Al analogue. Both XRD and thermodynamic modelling of the liquid compositions indicated that Al-Mc and the Fe-Mc phases do not form solid solution. The solubility products were calculated experimentally at 20 degrees C and 50 degrees C. Under standards condition the solubility products and other thermodynamic parameters were estimated using temperature-solubility product extrapolation and found to be logK(so) (Fe-Mc) = -34.59 +/- 0.50, logK(so) (Fe-Hc) = -30.83 +/- 0.50 and logK(so)(Al-Mc) = -31.32 +/- 0.50. (C) 2010 Elsevier Ltd. All rights reserved.