Phase formation of CaAl2O4 from CaCO3-Al2O3 powder mixtures

Calcium aluminate is the main constituent in calcium aluminate cements, used in a wide range of applications in construction and mining industries and recently also as biomedical implant. In applications that demand very precise reaction features, such as the biomedical ones, the phase purity is of very high importance. In this paper the formation of CaAl2O4 from CaCO3-Al2O3 powder mixtures has been studied, varying holding times between 1 and 40 h and temperatures between 1300 and 1500 degrees C. Phase formation was studied in samples both quenched from the holding temperatures and in samples slowly cooled. Samples were characterized by X-ray powder diffraction (XRPD), using Guinier-Hagg film data and the Rietveld method, and scanning (SEM) and transmission (TEM) electron microscopy. 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 CA(2) (CaAl4O7), C(12)A(7) (Ca12Al14O33) and minor amounts of un-reacted A (Al2O3). Trace amounts of C(3)A (Ca3Al2O6) were observed only for samples heated to 1500 degrees C. The amount of the Ca-rich phase C(12)A(7) was found to decrease with time as it reacts with A and, to a less degree, CA(2) to form CA. In agreement with previous studies the amount of CA(2) formed decreases comparatively slowly with time. Its un-reactivity is due to that it is concentrated in isolated porous regions of sizes up to 100 mu m. The formation of the Ca aluminates is found to be in response to local equilibriums within small inhomogeneous regions, with no specific phase acting as an intermediate phase. Samples quenched from 1500 degrees C were found to contain smaller amounts of poorly crystallized phases. A reaction between C and A takes place already at 900 degrees C, forming a meta-stable orthorhombic modification of CA. The orthorhombic unit cell with a = 8.732(2) b = 8.078(2) angstrom, c = root 3 center dot a = 15.124(4) angstrom was verified by electron diffraction, revealing frequent twinning and disorder of the crystallites. (c) 2007 Elsevier Ltd. All rights reserved.