Kinetics and mechanism of formation of tricalcium aluminate, Ca3Al2O6

The kinetics of formation of tricalcium aluminate, Ca2Al2O6 or C3A, have been studied in the range 1150–1350 °C using quantitative X-ray diffraction (XRD) analysis of pellets made from powder mixtures (3:1 molar ratio) of calcite and α-alumina (α-Al2O3)(the alumina was replaced by gibbsite in some experiments). Dodecacalcium heptaaluminate, C12A7, and monocalcium aluminate, CA, were formed as intermediate phases, which persisted until the reaction to form tricalcium aluminate was nearly complete. No calcium dialuminate or calcium hexaaluminate was observed. The kinetic mechanism was identified with the aid of reduced time plots to be based on a diffusion-controlled step, described by the Ginstling–Brounshtein equation. There was a good fit to this equation up to a fraction reacted of at least 0.6. The activation energy for the reaction between calcite and α-Al2O3 was determined to be 216±10 kJ mol−1, while that between calcite and gibbsite was 224±10 kJ mol−1. It is proposed that the rate-determining step is the diffusion of Ca2+ ions through the layers of reaction products, while the Al3+ ions remain relatively immobile. Formation of C12A7 may be the first step.