Influence of iron content on the color of the C3A–Fe2O3 system synthesized under different conditions of temperature, atmosphere and cooling

Abstract

The 3CaO·Al2O3–Fe2O3 (C3A–Fe2O3) system is important for the production of white clinker. In the present study this system was examined from the perspective of improving the sustainability of the production process. Microstructural evaluation was employed to explain the changes in color caused by variation of: iron content; temperature; type of atmosphere; and cooling conditions. It was found that color was more significantly affected by the iron content, temperature and type of atmosphere than by the type of cooling used. It was also observed that the utility of iron-rich raw materials could be maximized by understanding and enhancing the solubility of Fe2O3 in C3A. It was found that a 2wt.% Fe2O3 solid solution was stable only under kiln open to atmospheric conditions and remained clear at temperatures up to 1370°C. However, the same 2wt.% Fe2O3 solid solution suffered a significant change in color when the temperature rose to 1400°C. Mössbauer spectroscopy showed that the oxidation state of Fe was Fe3+, which did not change between 1370 and 1400°C; however, a structural change in the C3A–Fe2O3 solid solution was detected as a result of the alteration of the thermal treatment. The distinction between the structures at these two temperatures was that at 1370°C, all of the Fe3+ had a tetrahedral coordination, while at 1400°C, 19wt.% of the Fe3+ appeared in octahedral sites, a result that was corroborated by Rietveld analysis.