CHAPTER XII. 

 MAGNESIA BRICKS AND OXYCHLORIDE CEMENTS. 



AFTER magnesia (MgO) has been obtained by any of the methods 

 described in the preceding chapter, it is put to use in two quite different 

 ways. As the products differ greatly in both composition and use 

 they will here be discussed separately under the headings of "Magnesia 

 Bricks " and " Oxychloride Cements ". 



Magnesia Bricks. 



Magnesia bricks, which are commonly but very erroneously called 

 magnesite br'cks in the trade, are largely used as furnace linings, etc., 

 and have also been used to a small extent as linings for Portland-cement 

 kilns. 



Manufacture of magnesia bricks. In discussing the methods and 

 effects of calcining magnesite it was stated that two different forms 

 of magnesia could be obtained, according to the temperature at 

 which the calcination is carried on. If the magnesite be burned 

 at a light-red heat, the resulting magnesia will have a low specific 

 gravity (3.00 to 3.07), will possess sufficient plasticity to be capable 

 of being molded into shapes, and will gradually absorb water and 

 carbon dioxide from the atmosphere, just as quicklime would do. The 

 result of this absorption is that this form of magnesia will finally become 

 partly recarbonated. 



If the calcination takes place at a higher temperature, however, 

 the resulting magnesia will be heavy, with a specific gravity of 3.61 

 to 3.80; it will be absolutely devoid of plasticity; and it will not recar- 

 bonate on exposure to the atmosphere. 



These differences in the physical and chemical properties of the 

 two forms of magnesia are taken advantage of in the manufacture of 

 magnesia bricks. Each contributes certain good qualities to the brick. 



Magnesia bricks are made of a mixture of the two forms of mag- 

 nesia, in the proportions of four to six parts heavy magnesia to one 



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