Al–Si spinel phase formation in diphasic mullite gels

Diphasic aluminosilicate gels of varying Al2O3:SiO2 ratios were synthesized out of TEOS and ANN in basic condition, heated to different temperatures and finally analyzed for phases. All gels precalcined to ∼1000 °C were further calcined after mixing with CaCO3 and analyzed by XRD. XRD intensity of (440) of intermediate spinel phase formed during mullite formation was recorded. It has been shown that the XRD intensity of spinel phase to calcium fluoride is in excess of what is possible if the spinel phase is simply γ-Al2O3. This substantiates the view of silicon-incorporation in spinel lattice. This view is verified and extended by the further study of phase development on heating spinel-generated samples with CaCO3. Gehlenite forms in Si-rich gels up to approximately 3:2 mullite composition whereas in more Al-rich gels additional calcium aluminate phases are formed.