Crystallization of amorphous precursors in the calcia-alumina system: A differential scanning calorimetry study

Amorphous powders were synthesized by spray-drying aqueous solutions of calcium and aluminum nitrates, followed by calcination in order to completely decompose the salts. The formation of pure calcium aluminates-C(3)A, C(5)A(3), C(12)A(7), CA, CA(2), and CA(6) (C = CaO and A = Al2O3)-has been studied by DSC and XRD, A number of compositions first crystallized to metastable structures without partitioning, i.e., without phase separation. This is the case for orthorhombic C(5)A(3), orthorhombic CA, and gamma-Al2O3 solid solution which may incorporate up to about 25 mol% CaO, o-C(5)A(3) ultimately partitioned into a mixture of C(12)A(7) and CA according to the equilibrium dial:ram, o-CA transformed partitionless into the stable monoclinic m-CA, and the stable CA(6) compound was obtained pure at low temperature from gamma-Al2O3 solid solution through a second exotherm. Among all the calcium aluminates studied, C(3)A was the only one for which an intermediary amorphous precursor could not be obtained, CaO crystallizing as the nitrates were decomposed. Phase transitions involving metastable materials are often fast and release heat over a narrow temperature range leading to steep exothermal DSC signals even at a relatively low heating rate (5 degrees C/min), The present results :Ire consistent with current understanding of partitionless crystallization. They clearly show the ability of the synthesis used to achieve homogeneous mixing at the molecular level. Amorphous precursors formed in a large composition range may be advantageously used as glass models.