Local Structure of Crystalline and Amorphous Blast Furnace Slags by Solid-state NMR and MD Simulation

Abstract

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.