298 CEMENTS, LIMES, AND PLASTERS. 



differ in composition very markedly from this. Alumina is always 

 present in considerable quantity, forming with part of the lime the 

 dicalcic aluminate (2CaO, A1 2 3 ). This would give, as stated by New- 

 berry, for the general formula of a pure Portland, 



X(3CaO,Si0 2 ), F(2CaO,Al 2 3 ). 



But the composition is still further complicated by the presence 

 of accidental impurities or intentionally added ingredients. These 

 last may be simply adulterants, or they may be added to serve some 

 useful purpose. Calcium sulphate is a type of the latter class. It 

 serves to retard the set of the cement and in small quantities appears 

 to have no injurious effect which would prohibit its use for this pur- 

 pose. In dome kilns, sufficient sulphur trioxide is generally taken up 

 by the cement from the fuel gases to obviate the necessity for the latter 

 addition of calcium sulphate, but in the rotary kiln its addition to the 

 ground cement, in the form of either powdered crude gypsum or plaster 

 of Paris, is a necessity. 



Iron oxide, within reasonable limits, seems to act as a substitute 

 for alumina, and the two may be calculated together. Magnesium 

 carbonate is rarely entirely absent from limestones or clays, and mag- 

 nesia is, therefore, almost invariably present in the finished cement 

 but in small percentage. Though magnesia, when magnesium car- 

 bonate is burned at low temperature, is an active hydraulic material 

 (see Chapter XII) it does not normally combine with silica or alumina 

 at the clinkering heat employed in Portland-cement manufacture. At 

 the best it is an inert and valueless constituent in the normal Portland * 

 cement ; many regard it as positively detrimental in even small amounts,. 

 and because of this feeling manufacturers prefer to carry it as low as 

 possible. In amounts of less than 3^ per cent to 5 per cent it is cer- 

 tainly harmless and American Portlands from the Lehigh district 

 usually reach well up toward that limit. In European practice it is 

 carried somewhat lower. 



Cementation Index. In discussing the hydraulic limes and natural 

 cements, use has been made of the Cementation Index, a device which 

 affords an easy means of comparing the hydraulic and other proper- 

 ties of various cements. In dealing with Portland cement, this device 



* This statement should not be construed to mean that it is impossible to 

 make a good cement of the Portland type, but containing high percentages of 

 magnesia, for this very possibility will be discussed on a later page (p. 348). But 

 such a magnesia Portland will, of necessity, differ quite markedly both in prep- 

 aration and properties from the lime Portlands now in use. 



