Particle Size of Raw Materials and Molding of High Alumina Bricks

Practice shows that, an appropriate increase in the size and number of coarse particles can increase the bulk density of high alumina bricks and makes the molding easier. When fired, the secondary mullite reaction around the material is weak, so the molding is easier. 
high alumina bricks
But, there is a limit to the size. If the size is increased to 4-5mm, bricks may miss edges and angles and their structure homogeneity will be affected. So, the appropriate maximum size is 3mm. 
In the batch materials, fine powder is helpful to the sintering and can increase the density of products. Since the shrinkage of fine powder during sintering can reduce or eliminate the volume expansion caused the secondary mullite reaction, it is necessary to add fine powder. However, if too much fine powder is added, the molding will be difficult and the bricks are easy to crack. so, 40-50% fine powder is ideal. 
The middle-size particles cannot act as the skeleton of particles or be helpful to sintering like fine powder. So, the number of middle-size particles is limited at a low level. But it cannot be zero, or the bricks will be easy to crack and have bad appearance. In general, 10% is ideal. 
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