Crystallization of Amorphous Precursors in the Calcia–Alumina System: A Differential Scanning Calorimetry Study

Abstract

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 ‫؍‬ Al 2 O 3 )-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 ␥-Al 2 O 3 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 diagram. o-CA transformed partitionless into the stable monoclinic m-CA, and the stable CA 6 compound was obtained pure at low temperature from ␥-Al 2 O 3 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°C/min). The present results are 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.

Study by DTA/TG of the formation of calcium aluminate obtained from an aluminium hazardous waste

“…Finally, in the last zone, at temperature higher than 900°C a lower than 1100°C, the CA formation occurs according to DTA. The formation of the different aluminate phases agrees to the XRD results and the bibliographic data [13,14].…”

Crystallisation of amorphous spray-dried precursors in the Al2O3–SiO2 system

“…In the case of calcium up to almost 33 mol % CaO can be incorporated into the ␥-Al 2 O 3 solid solution. 16 In the present study, more than 20 mol % SiO 2 , possibly 25 mol % SiO 2 , can be incorporated into the ␥-Al 2 O 3 solid solution.…”

“…Under the same experimental conditions, aluminium nitrate alone is partially decomposed into an oxynitrate after spray drying. 16 On the TG trace the major part of the weight loss of the spray-dried sample corresponds to the decomposition of this oxynitrate in two steps (140 and 260 • C on the derivative curve DTG for the 90 mol % Al 2 O 3 , Fig. 2b).…”

Synthesis of CaAl4O7 via citric acid precursor

“…Therefore, increasing the molar ratio of citric acid to Williamson and Glasser [8] obtained pure CaAl 4 O 7 only for long heating treatments at 1380 • C. Tas [9] synthesized singlephase CaAl 4 O 7 at 850 • C for 48 h by self-propagating combustion synthesis technique. Using spray-drying technique, Douy et al [11] prepared phase pure CaAl 4 O 7 at 1050 • C. Stephan and Wilhelm [12] synthesized pure CaAl 4 O 7 using a sol-gel process at 1500 • C for 61 h and four intermediate grindings.…”

“…Chemical processing techniques have been used to synthesize calcium aluminate powders [8][9][10][11][12][13]. Williamson and Glasser [8] studied the behavior of co-precipitated hydroxide gels from calcium and aluminium nitrates; they obtained pure CaAl 4 O 7 from amorphous of corresponding composition only for long heating treatments at 1380 • C. Tas [9] persed mixed oxides or oxide solid solutions of perovskites, spinels and garnets [14][15][16][17][18][19][20][21][22].…”