The FT-IR spectra of the CaO–Al
2O
3 and CaO–Al
2O
3–CaF
2 slags were measured to understand the structural aspects of (fluoro-) aluminate slags. The infrared spectra of the CaO–Al
2O
3 slag was interpreted based on the relationship between bond length and force constant of Al–O bond. Thereafter, the role of F
- ions in the depolymerization of aluminate network was discussed. The wavenumbers of [AlO
4]-tetrahedra higher than that of [AlO
6]-octahedra would be originated from the Al–O bond length in tetrahedra shorter than that in octahedra. In the CaO
satd–Al
2O
3–CaF
2 system, the IR-transmitting bands of [AlO
nF
4-n]-complexes are observed through the entire composition of the fluoroaluminate system, while the bands of [AlO
4]-tetrahedra, [AlO
6]- and [AlF
6]-octahedra appear in the composition less than 24.0 (mol%) CaF
2. In the 26.2 (mol%) Al
2O
3-containing system, the bands of interlinked [AlO
4]-tetrahedra are shown at only 2-liquids boundary, while these bands are shown in the whole composition region in the 41.5 (mol%) Al
2O
3-containing system. In the 41.7 and 51.0(mol%)CaO–Al
2O
3–CaF
2 systems, the transmitting bands of [AlO
nF
4-n]-complexes are observed through the entire composition, while those of [AlO
4]-tetrahedra are shown in the composition of
XCaF2/
XAl2O3 [lt.gif] 1.0. Also, the relative intensity of the bands indicating [AlO
4]-tetrahedra in the 51.0 (mol%) CaO-containing slag is much less than that in the 41.7 (mol%) CaO-containing system.
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