Phase formation of CaAl2O4 from CaCO3-Al2O3 powder mixtures

The formation of CaAl2O4 from CaCO3-Al2O3 powder mixtures was studied, by varying holding times between 1 and 40 h and temperatures between 1300 and 1500 C. Phase formation was studied in samples both quenched from the holding temperatures and in samples slowly cooled. Samples were characterised by XRD, using Guinier-Hagg film data and the Rietveld method, SEM and TEM. Samples for TEM with very high site accuracy were produced using focused ion beam microscopy. In addition to CA (CaAl2O4), the samples contained major amounts of CA2 (CaAl4O7), C12A7 (Ca12Al14O33) and minor amounts of un-reacted A (Al2O3). Trace amounts of C3A (Ca3Al2O6) were observed only for samples heated to 1500 C. The amount of the Ca-rich phase C12A7 decreased with time as it reacted with A and, to a lesser degree, CA2 to form CA. In agreement with previous studies, the amount of CA2 formed decreased comparatively slowly with time. Its un-reactivity was due to its being concentrated in isolated porous regions of sizes up to 100 micron. The formation of the Ca aluminates was in response to local equilibria within small inhomogeneous regions, with no specific phase acting as an intermediate phase. Samples quenched from 1500 C contained smaller amounts of poorly crystallised phases. A reaction between C and A took place at 900 C, forming a metastable orthorhombic modification of CA. The orthorhombic unit cell with a = 8.732(2) Angstrom, b = 8.078(2) Angstrom, c = root3a = 15.124(4) Angstrom was verified by electron diffraction, revealing frequent twinning and disorder of the crystallites. 44 refs.