As a kind of alkaline refractory castable, magnesia refractory castable is prepared from fused magnesia or sintered magnesia aggregate and its fine powder. Small size, high load softening temperature, excellent slag corrosion resistance and no pollution to molten steel have attracted people’s attention and are widely used in ladles, tundishes and other parts.
According to different binders, it can be divided into periclase cement bonded magnesia castable, water glass bonded magnesia castable, polyphosphate bonded magnesia castable, etc.
It is combined with periclase cement has the advantages of high purity, small line change after burning, and high load softening temperature. However, as the heat treatment temperature increases (especially between 400 and 1200 °C), the magnesium hydroxide generated by magnesia hydration dehydrates and loses its cementation effect, and the combination of refractory aggregate and powder begins to loosen, and solid-state reaction Weak, resulting in a sharp drop in the strength of refractory castables, prone to loose, peeling phenomenon.
It is using water glass as a binder have the advantages of high strength, strong corrosion resistance to alkali and sodium salt melts, etc. However, due to the use of water glass as a binder, a large amount of Na2O (or K2O) and SiO2 are brought in, resulting in a significant decrease in the refractoriness and load softening temperature of magnesia refractory castables, thus limiting the maximum use of water glass bonded castables temperature.
The magnesia refractory castable combined with polyphosphate has the advantages of high strength after firing, a small decrease in strength with the increase of heat treatment temperature, high softening temperature under load, and good thermal shock stability. However, at high temperature (above 1400°C), the strength of magnesium castables decreases due to the large amount of volatilization of P₂O₅.
In general, magnesium castables use sodium polyphosphate as a binder, such as sodium tripolyphosphate, sodium hexametaphosphate or sodium polyphosphate. The degree of polymerization of polyphosphate and the content of P₂O₅ have a great influence on the strength of castables. When the average degree of polymerization of polyphosphate is 24, the castable has the greatest strength.