Crystal Chemistry of Iron Containing Cementitious AFm Layered Hydrates

The crystal structure of the three main Fe-containing AFm phases (Al2O3-Fe2O3-mono: family of lamellar calcium alumina-ferrite hydrates) encountered in cement hydration process are characterized and compared with their Al-analogs. This includes AFm phases containing sulfate (which is present in Portland cement to regulate the hydration kinetic), carbonate (which is present in Portland cements, or originates from atmospheric carbon oxide) and chloride (either from the water used or from the environment). The results show that Fe-AFm and Al-AFm compounds are not (or rarely) isostructural. Iron in AFm phases does not simply substitute aluminium. Fe-carbonate has a rhombohedral symmetry whereas Al-carbonate has a triclinic symmetry, with carbonate anions located in different crystallographic sites in both compounds. Fe-Friedel’s salt corresponds to a 3R polytype whereas Al-Friedel’s salt corresponds to a 6R polytype. Both compounds have a temperature dependent transition with two different HT- and two different LT-polymorphs descriptions (HT: high-temperature, LT: low-temperature). Only Fesulfate and Al-sulfate are isostructural. Despite this isostructural feature, only limited solid solutions have been observed between both sulfate end-members. In a general way, this system (when considering sulfate, carbonate and chloride with aluminium and iron) leads to extremely complicated subsystems with limited solid solutions. The crystallographic studies and comparisons developed here have been fully completed by thermodynamic characterisations in order to make possible thermodynamic modelling of the hydrates assemblage during the hydration process and the aging of Portland concrete. Keywords: Cement hydrates, crystal structure, iron, solid solution, X-ray diffraction.