CHAPTER XVIII. 

 MANUFACTURE OF NATURAL CEMENTS. 



THE manufacture of natural cement is, compared to that of Portland 

 cement, a very simple proposition from a mechanical point of view. 

 Only two general processes are involved burning and grinding. In 

 the present chapter these processes will be taken up separately, after 

 which a brief statement will be given as to costs of manufacture. 



Burning Practice and Theory. 



Before taking up such practical questions as types of kiln used, 

 kinds of fuel, relation of fuel consumption to output, etc., it seems ad- 

 visable to discuss briefly certain more theoretical aspects of the process 

 of burning. On examination, however, it will be found that even these 

 apparently theoretical phases of the subject have a distinct practical 

 bearing. 



Chemical changes during burning. The rock as it is charged into 

 the kiln is a clayey limestone, consisting essentially of lime carbonate 

 and more or less magnesium carbonate, with clayey matter (silica, 

 alumina, and iron oxide). In addition to these essential ingredients 

 it will usually contain a few per cent each of alkalies (soda and potash), 

 sulphur or sulphur trioxide, and water. During calcination certain 

 chemical changes take place in about the following order. 



Any mechanically held water is driven off before the rock has reached 

 a temperature of much over 100 C. (212 F.). At about 400 C. mag- 

 nesium carbonate begins to be dissociated, its carbon dioxide being 

 driven off and the magnesia remaining in its caustic and active form. 

 When mixed with lime carbonate, as in natural-cement rock, it is probable 

 that this dissociation does not take place much below 700 C. When 

 the rock reaches 750 to 800 C. its lime carbonate is dissociated in like 

 manner. At a somewhat higher temperature its clay is decomposed 

 and combination between the alumina and iron oxide and the lime and 

 magnesia commences. This is aided by the presence of alkalies, which 

 here act as fluxes. 



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