Cationic substitution in tricalcium aluminate

Cubic and orthorhombic crystals of tricalcium aluminate doped with Na2O, Fe2O3 and SiO2 were prepared and examined using an electron probe microanalyzer. The cationic ratios based on six oxygen atoms were derived from the oxide compositions. These data, together with those in previous studies for clinker aluminates containing Mg2+ and K+, provided excellent correlations between Al+Fe and Si (Al+Fe=2.001-1.03Si) and Ca+Mg and Na+K+Si [Ca+Mg=3.006-0.51(Na+K+Si)]. The chemical variation that is constrained by these equations is well accounted for by the general formula (Na,K)2x(Ca,Mg)3-x-y[(Al,Fe)1-ySiy]2O6, where x is the amount of Ca substituted by Na and K (0≤x<0.158), and y is the amount of Al substituted by Si (0≤y<0.136). The crystal system changed from cubic to orthorhombic with increasing x value. The substitution of Si and Fe for Al extended the solid solution range of the orthorhombic phase to lower values of x, while its effect on the solid solution range of the cubic phase was reversed