Influence mechanism of calcium nitrate tetrahydrate on phase composition and microstructure of silica refractories

In this article, silica bricks were prepared using calcium nitrate tetrahydrate (Ca(NO3)2·4H2O) as the mineralizer. Its reaction mechanism during the sintering of silica bricks was systematically elucidated via characterization techniques such as XRD, SEM, and X-ray CT. The results show that calcium nitrate tetrahydrate, with a low melting point (approximately 56°C ), can uniformly coat silica particles in liquid form during the mixing stage, and is successively converted into calcium nitrate and highly active CaO during the drying and sintering of the green body. The volume shrinkage associated with this transformation can precisely compensate for the expansion stress induced by the phase transformation of quartz to metastable cristobalite, thereby endowing the sintered bricks with a dense structure free of obvious cracks, together with fine and uniform intergranular pores. As a calcium-containing mineralizer, it reacts with SiO2 raw materials to form a calcium-silicate liquid phase at 1436°C , which effectively promotes the conversion of metastable cristobalite to tridymite. Compared with the traditional CaO-FeOx composite mineralizer, this mineralizer can significantly reduce the content of amorphous phases in the sintered bricks. Composition regulation and microstructure optimization directly enhance the high-temperature performance of silica bricks. The bricks doped with 5 wt% calcium nitrate tetrahydrate not only exhibit excellent high-temperature volume stability, but also achieve a thermal conductivity of 2.366 W/(m·K) while maintaining an apparent porosity of 21.4%.