CHAPTER XLIV. 

 SLAG CEMENTS: COMPOSITION AND PROPERTIES. 



WHILE slag cements are sufficiently like Portland cements to be 

 usually marketed as Portland, certain interesting differences between 

 the two cements are shown on close examination. 



Identification of slag cement. Slag cements may usually be dis- 

 tinguished from Portland cements by their lighter color, inferior specific 

 gravity, and slower set. They show on analysis lower lime and higher 

 alumina percentages than Portlands and usually contain an appreciable 

 amount of calcium sulphide. Owing to the presence of this last named 

 constituent a briquette of slag cement left for some days in water will 

 show upon fracture a decided greenish tint; if it has been exposed to 

 salt water, this tint will be much more marked, and the odor of hydrogen 

 sulphide will be observed. Two things should be noted, however, in 

 this connection. The presence of sulphides though usual is not a neces- 

 sary occurrence in slag cements; and, on the other hand, sulphides are 

 occasionally present in Portland cements, being formed from the sul- 

 phates in case the flame of the kiln is not sufficiently oxidizing. Another 

 chemical difference between the two types of cement is in the high "loss 

 on ignition " shown by slag cements. This loss, which may range from 

 4 to 8 per cent, is due largely to the water carried by the slaked lime. 



Chemical Composition of Slag Cements. 



The ultimate composition of a sample of slag cement will, of course, 

 be brought out by chemical analysis; but the fact that the material 

 is not a chemical compound but merely a mechanical mixture will not 

 be shown in the ordinary 1 " report of such an analysis. The average 

 commercial chemist will, moreover, particularly, if he be accustomed 

 to analyzing Portland cements make careless and erroneous state- 

 ments concerning three important points. The three points noted are: 

 (a) the condition of the iron which is present, (6) the condition of the 

 sulphur which is present, and (c) the nature of the "loss on ignition". 



A discussion of analytical methods will not be undertaken, as that 

 subject does not properly belong in a treatise of this character. But 



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