To understand the relationship between thermodynamic properties and structures in the CaO–SiO
2–BO
1.5 slag system, the thermodynamic properties of BO
1.5 and SiO
2 were measured by a chemical equilibrium technique and the local structures of boron(B) and silicon(Si) were investigated by
11B and
29Si magic angle spinning–nuclear magnetic resonance (MAS–NMR) measurements.Activity coefficients of BO
1.5 increased with an increase in the BO
1.5 content of the slag system. Activity coefficients of SiO
2 increased with increasing BO
1.5 content and decreased with increasing CaO/SiO
2 ratio.By using
11B MAS–NMR, the existence of two types of B sites was confirmed—three- and four-coordinated B sites. The relative fraction of four-coordinated B increased with an increase in the BO
1.5 content and decreased with an increase in the CaO/SiO
2 ratio.
29Si MAS–NMR results revealed that bridging oxygen bonding to Si atoms increased with an increase in the BO
1.5 content and decreased with decreasing CaO/SiO
2 ratio. The number of non-bridging oxygen atoms bonded to the tetrahedrally coordinated Si atom (NBO/T) was calculated using
29Si and
11B MAS–NMR under specific assumptions. By comparing NBO/Ts calculated from these two methods, the number of non-bridging oxygen atoms bonded to three-coordinated B in the studied composition region was estimated to be one.Finally, by comparing the thermodynamic properties with the results of MAS–NMR measurements, it was found that the composition dependences on BO
1.5 and SiO
2 activity coefficients were dependent on changes in the local structure of B and Si with composition.
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