Calcium aluminate glasses are important materials where AlO4/2 is the only network former. Aluminum in crystals or glasses between CaO and Al2O3 can have different environments as a function of the CaO/Al2O3 ratio. Using X-ray absorption at the Al K- and L-edges, Raman and 27Al NMR spectroscopies, we have determined the structural surroundings of Al in glasses, crystals, and melts in this binary system. Aluminum is in octahedral coordination at high-Al2O3 content (>80 mol%) and essentially in fourfold coordination with 4 bridging O atoms (BOs) at Al2O3 contents between 30 and 75 mol%. At around 25 mol% Al2O3, Al is in tetrahedral coordination with two BOs. The presence of higher-coordinated species at high-Al2O3 contents and their absence at low Al2O3 imply different viscous flow mechanisms for high- and low-concentration Al2O3 networks.

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