Mechanical activation of calcium aluminate formation from CaCO3–Al2O3 mixtures

The effect of high-energy ball milling and subsequent annealing on a mixture of Al2O3 and CaCO3 has been investigated. DTA results show that the initiatory decomposition temperature and decomposition peak temperature of CaCO3 decreases with the increase of ball milling time duration. X-ray diffraction (XRD) measurement indicates that the nanocrystalline powder mixture is produced and no CaAl2O4 is formed during a high-energy ball milling process after milling for 30 h. Significant crystallization of CaAl2O4 from the nanocrystalline powder mixture milled for 30 h is observed after annealing at 900 °C for 2 h, while some amount of the intermediate phase Ca12Al14O33 exists at this temperature. With an increase of calcining temperature, the intensity of the diffraction peak of the dominating crystalline phase CaAl2O4 clearly increases, the intensity of the diffraction peak of the secondary crystalline phase Ca12Al14O33 obviously decreases. Single phase CaAl2O4 can be obtained for the sample milled for 30 h at 1300 °C, which is at least 100 °C lower than that used in the traditional solid-state method. While the secondary crystalline phases Ca12Al14O33 and CaAl4O7 exist for the physical mixture at 1300 °C. No significant grain growth is observed when the milled powders were annealed at 1300 °C with different holding time and the average size of the grain is about 1–3 μm.