PART III. MAGNESIA AND OXYCHLORIDE 

 CEMENTS. 



CHAPTER XI. 

 SOURCES AND PREPARATION OF MAGNESIA. 



MAGNESIA, or magnesium oxide (MgO), though possessing very 

 marked cementing properties, is at present too expensive to be used 

 as a cementing material for ordinary structural purposes. It merits 

 discussion in this volume, however, because (a) it is the basis of an 

 extensive magnesia brick industry; (b) under certain conditions it 

 posessses hydraulic properties; and (c) the facts brought out in a de- 

 scription of the manufacture of magnesia and magnesia brick may serve 

 to throw some light on the vexed question of the part played by mag- 

 nesia when present in hydraulic cements. 



Sources of magnesia. Magnesia may be obtained on a commercial 

 scale either by burning the mineral magnesite, a natural carbonate 

 of magnesium, or by chemical methods practiced on other natural sources 

 of magnesium salts such as highly magnesian limestones or even sea- 

 water. At present magnesite is by far the most important source of 

 magnesia, but the chemical methods of extraction may be of service 

 under certain commercial conditions. All the sources and methods 

 will therefore be considered in the present chapter, magnesite being 

 first discussed and then the chemical sources of supply. The chapter 

 following will be devoted to consideration of the properties and uses 

 of the magnesia, however obtained, and the manufacture and proper- 

 ties of magnesia bricks. 



Magnesite as a Source of Magnesia. 



Composition and character of magnesite. Magnesite occurs com- 

 monly as a fine-grained, compact mineral, varying from white to yellow- 



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