Local structure around phosphorus and silicon in the CaO–SiO2–PO2.5 system

Abstract

The local structure of phosphorus and silicon in the molten CaO–SiO2–PO2.5 slag system was investigated by magic angle spinning nuclear magnetic resonance (MAS-NMR). The 31P MAS-NMR spectra revealed that phosphorus was present primarily as the monophosphate complex ion PO43−, with a small amount of diphosphate ion also present. Their relative ratio to total phosphorus was independent of the phosphate concentration of the sample. In the case of the 29Si MAS-NMR, the mean number of the non-bridging oxygen atoms associated with tetrahedrally coordinated silicon decreased as the phosphate concentration increased at a fixed CaO/SiO2 ratio. This indicates that the nonbridging oxygen atoms around the silicon were replaced by bridging oxygen atoms around the phosphorus as the phosphate concentration in the samples increased.To elucidate the basicity dependence of the structure of slag, the relationship between the structure and optical basicity was also investigated. The relative ratio of Qn (Qn means the silicon atoms tetrahedrally bonded with “n” number of bridging oxygen atoms) strongly depends on the optical basicity. These optical basicity dependencies of the structures of phosphorus and silicon can be explained clearly by the basicity equalization concept (Duffy and Ingram, 1976) [12].