Local Structure of Crystalline and Amorphous Blast Furnace Slags by Solid-state NMR and MD Simulation
Koji Kanehashi, Keiji Shimoda, Koji Saito
Local environments of main constituent elements (Si, Al, O, Mg, and Ca) in slow-cooled (crystalline) and rapid-quenched (amorphous) blast furnace slags have been investigated using multi-nuclear solid-state NMR spectroscopy. The framework of the amorphous slag has a depolymerized, chain-like network (Q2) of SiO4 tetrahedra branched with AlO4 tetrahedra. AlO4 tetrahedra are more polymerized than SiO4 tetrahedra: Q3 and/or Q4. A 17O MQMAS spectrum demonstrates that oxygens occupy structurally inequivalent multiple sites due to a difference in their connectivities. 25Mg and 43Ca MQMAS spectra show that Mg21 and Ca21 ions also occur in multiple sites, and the average coordination number is estimated to about 6 and 7, respectively. Especially, Ca21 ions act as charge balancers of [AlO4]2 as well as network modifiers. The corresponding spectra of the crystalline slag prove the existence of melilite and a small amount of merwinite. A difference in chemical structure between the amorphous and crystalline slags is the coordination state of Mg21 and Ca21 ions rather than the framework of TO4 (T5Si or Al).
Chemical structure of the amorphous slag has been also studied by means of MD simulation. Our calculation manifests that the structural properties at 300K agree well with the NMR results. At 1873K, the coordination number of the tetrahedral cations almost remains unchanged, while 5-fold Al species slightly increase. The Qn distributions of AlO4 tetrahedra are also modified. The small amount but significant incorporation of Al2O3 influences the network connectivities, which should affect macroscopic properties such as viscosity.
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