118 



CEMENTS, LIMES, AND PLASTERS. 



TABLE 39. 

 ANALYSES OF LEAN LIMES. 



The last of the above analyses is of a lime made by burning oyster- 

 shells in a mixed-feed kiln. In this case the impurities shown in the 

 lime are largely the result of coal-ash taken up by the lime during manu- 

 facture. (See pp. 94, 95.) 



Composition of commercial magnesian limes. In discussing the 

 classification of limes, pp. 97, 98, it was stated that limes carrying over 

 5 per cent of magnesia were to be grouped commercially as mag- 

 nesian limes. In the present place this statement will bear discussing 

 in somewhat more detail. 



In theory a limestone can carry anywhere from to 45.65 per cent 

 of magnesium carbonate (MgCO 3 ). In the first case it would have the 

 composition of the mineral calcite ( = CaC0 3 ), in the second that of 

 the mineral dolomite (CaCO 3 + MgC0 3 ) , while the intermediate stages 

 falling between these two limits (0 and 45.65 per cent MgC0 3 ) would be 

 known simply as magnesian limestones. A further theoretical con- 

 clusion is that such intermediate stages would occur approximately as 

 commonly as the extreme stages. 



In practice, however, we find that this last theoretical deduction 

 does not hold. If a large series of limestone analyses be compared, 

 it will be found that by far the great majority of them are of the two 

 extremes of the series. That is to say, actual limestones are either 

 very low in magnesia or very high in it. This condition causes the 

 product made by burning limestone to fall usually in one of the two 

 following classes: 



A. High-calcium limes: magnesia less than 5 per cent. 



B. High-magnesium limes: magnesia over 30 per cent. 

 Intermediate limes are of course burned, but they are compara- 

 tively rare. Analyses 10, 11, 12, and 13 of Table 40 are of such inter- 

 mediate types, carrying respectively 7.41, 7.52, 13.42, and 6.08 percent 

 of magnesia. 



The composition of the typical magnesian limes is well shown by 

 analyses 1-9, inclusive, of Table 40. 



Lime-slaking. The subject of lime-slaking has always been a mat- 

 ter of importance in connection with the ordinary use of lime in making 



