EFFECT OF BURNING CONDITIONS AND MINOR COMPONENTS ON THE COLOR OF PORTLAND-CEMENT CLINKER

The substitution of Me2+(Mg,Zn) for Al3+ and Fe3+ in the calcium alumino-ferrite phase, which introduces extrinsic oxygen vacancies in the structure, is essential to the change in color from yellowish brown to dark grey. However, an increase of the ratio Me4+(Si,Ti)/Me2+ in the ferrite phase, which, for example, is encouraged by slow cooling in nitrogen, reduces the number of extrinsic oxygen vacancies and prevents the above color change. With C6A2F solid solutions the color change is accompanied by a definite increase in electric conductivity, indicating the significance of electrons released by the oxidation of FeO in addition to the occurrence of oxygen vacancies.