Magnesium refractory castables are made of electrofused magnesium sand or sintered magnesium sand aggregate and fine powder, the magnesium refractory castables combined with high magnesium cement have the advantages of high purity, small rate of change of post-firing line, and high loading softening temperature, but with the rise of the heat treatment temperature, especially between 400~1200 degree , due to the magnesium hydroxide generated by the hydration of the magnesium sand dehydrates and loses the cementing effect, and the combination of the refractory aggregates and powder is very loose and the solid phase reaction is weak, resulting in a sharp decline in the strength of refractory castables, which is prone to loosening and spalling. The combination of refractory aggregate and powder is very loose, and the solid-phase reaction is very weak, resulting in a sharp decline in the strength of refractory castables, easy to loose, flaking phenomenon. Water-glass bonded magnesium refractory castables have the advantages of high strength and strong resistance to alkaline and sodium salt melt erosion.
However, due to the use of water glass as a binding agent and bring in a large number of sodium oxide or potassium oxide and silica, thus making the magnesium refractory castable refractory, load softening temperature and so on a substantial reduction, thus limiting the water glass combined with the high use of temperature of the castables. Polyphosphate bonded magnesium refractory castables have high strength after firing, strength with heat treatment temperature increases the decline is small, what kind of softening temperature is high, thermal shock resistance is better, etc., but at high temperatures (more than 1400 degree C), due to the volatilization of a large number of phosphorus pentoxide so that the magnesium castable strength is reduced. Generally speaking, magnesium castable adopts sodium polyphosphate as binding agent more, commonly used are sodium tripolyphosphate, sodium hexametaphosphate and so on. In addition, in order to improve the high-temperature strength of magnesium refractory castables, add appropriate amount of calcium-containing materials (such as calcium carbonate, etc.), so as to generate a high bonding strength at high temperatures, high-temperature stabilization of the Na2O-2CaO-P2O5 phase. However, no matter what kind of binding agent is used, it is difficult to overcome the problem of easy hydration, and cracks are easy to be produced during the drying process. Adding appropriate amount of silicon oxide superfine powder can overcome the hydration problem.
Aluminum-magnesium refractory castables are made of fused magnesium sand, sintered magnesium sand, fused or sintered magnesium-aluminum spinel, fused corundum, alumina powder, high alumina powder and other configurations, and the combining agents are water glass, silicon oxide and alumina ultrafine powder, calcium aluminate cement, phosphates and so on. Aluminum-magnesium refractory castables have good thermal shock resistance due to the presence of magnesium-magnesium spinel or the generation of magnesium-aluminum spinel during the heating process to produce expansion and microcracks, buffering the thermal stress inside the material, and therefore have a good resistance to thermal shock, and at the same time, due to the presence of a large number of magnesium-aluminum spinel in the castables, and thus have a stronger effect on the furnace slag.
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