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 (Q
2) of SiO
4 tetrahedra branched with AlO
4 tetrahedra. AlO
4 tetrahedra are more polymerized than SiO
4 tetrahedra: Q
3 and/or Q
4. 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 Mg
21 and Ca
21 ions also occur in multiple sites, and the average coordination number is estimated to about 6 and 7, respectively. Especially, Ca
21 ions act as charge balancers of [AlO
4]
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 Mg
21 and Ca
21 ions rather than the framework of TO
4 (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 Q
n distributions of AlO
4 tetrahedra are also modified. The small amount but significant incorporation of Al
2O
3 influences the network connectivities, which should affect macroscopic properties such as viscosity.
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