Iron in carbonate containing AFm phases

One of the AFm phases in hydrated Portland cement is Ca3(AlxFe2 − x)O6.CaCO3.nH2O. 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 °C, while Fe-hemicarbonate (Fe-Hc) was unstable with respect to Fe-Mc in the presence of calcite. Fe-Mc has a rhombohedral R3¯c 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 °C and 50 °C. Under standards condition the solubility products and other thermodynamic parameters were estimated using temperature-solubility product extrapolation and found to be logKS0 (Fe-Mc) = −34.59 ± 0.50, logKS0 (Fe-Hc) = −30.83 ± 0.50 and logKS0(Al-Mc) = −31.32 ± 0.50.