Formation, Stability, and Crystallinity of Various Tricalcium Aluminate Polymorphs

Tricalcium aluminate is an important phase of Portland clinker. In this paper, three polymorphs of C₃A were prepared by means of the solid-state synthesis method using intensive milling of the raw material mixture which was doped with various amounts of Na₂O and sintered at a temperature of 1300 °C for 2 h. The final products were evaluated through X-ray diffraction using Rietveld analysis. The effect of the Na dopant content on the change in the crystalline structure of tricalcium aluminate was studied. It was proven that the given preparation procedure, which differed from other studies, was close to the real conditions of the formation of Portland clinker, and it was possible to prepare a mixture of different polymorphs of calcium aluminate. Fundamental changes in the crystal structure occurred in the range of 3-4% Na, when the cubic structure changes to orthorhombic. At a dosage of Na dopant above 4%, the orthorhombic structure changes to a monoclinic structure. There are no clearly defined boundaries for the existence of individual C₃A phases; these phases arise at the same time and overlap each other in the areas of their formation at different Na doses.