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 SiO2 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 PO4 tetrahedra in the phosphate binders have no P-O-P linkage between PO4 tetrahedra and that as the patching materials become hard, the ratio of PO4 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 PO4 tetrahedra with bridging oxygens was not enough to form the network by P-O-P linkage between PO4 tetrahedra. On the basis of these results, the hardening mechanism of refractory patching materials with a phosphate binder is proposed.
