Kaolin, synthetic, from silica

Experiments thus far have shown that the addition of CaO, silica gel, or mixtures of both will give a harder product. These additions were made to the reaction mixture (10.0 g kaolin 2 plus 10 ml standard synthetic waste), and the samples kept at 75°C for 48 hr. The cooled, damp products were then measured for hardness with penetrometer (Soiltest model CL-700 penetrometer—Soiltest, Inc.). The results are given in Table IX. The greatest increase in hardness results from the addition of a mixture of CaO and Si02 x H2O. 

Many refractories are derived directly from natural minerals, but synthetic materials are also widely use. Clays, espeeially those having inherent temperature resistance, are the oldest and most eommon of the naturally occurring refractory minerals. Major natural refi actoiy materials are kaolin, chromite, bauxite, zirconia, and magnesite. These are often marketed under specific trade names. Refractory materials may be acid, such as silica, or basic, such as magnesite or bauxite, for use in aeid- or basic-process steel furnaces. Graphite and chromite are generally considered neutral refractories. 

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