The hardening mechanism of refractory patching materials with a phosphate binder was investigated by means of
31P static and magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Ten refractory patching materials were made of refractory powders of SiO
2 and fused alumina and five mixed solutions of phosphoric acid and aluminum biphosphate as phosphate binders.
31P static and MAS NMR spectra were measured of five phosphate binders and twenty soft and hard refractory patching materials, respectively, to reveal the local structure around P atoms. The
31P static and MAS NMR spectra revealed that PO
4 tetrahedra in the phosphate binders have no P-O-P linkage between PO
4 tetrahedra and that as the patching materials become hard, the ratio of PO
4 tetrahedra with bridging oxygens increases, respectively. This result suggests that the hardening of the patching materials is caused by the condensation of the phosphate binder. But the ratio of PO
4 tetrahedra with bridging oxygens was not enough to form the network by P-O-P linkage between PO
4 tetrahedra. On the basis of these results, the hardening mechanism of refractory patching materials with a phosphate binder is proposed.
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