Hardening mechanism of refractory patching materials with phosphate binder

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.