PART Y. NATURAL CEMENTS. 



CHAPTER XVI. 

 DEFINITION AND RELATIONS OF NATURAL CEMENTS. 



BEFORE taking up a detailed description of the materials, man- 

 ufacture, and properties of natural cements it will be useful to 

 make some brief general statements concerning the group. In the 

 present chapter, therefore, an attempt will be made to discuss the 

 natural cements as a class, laying emphasis upon the points of resem- 

 blance of the various brands and disregarding for a time their many 

 points of difference. 



The difficulties which are encountered in such an attempt are greater 

 than the reader, at first sight, may imagine; for few engineers realize 

 what a heterogeneous collection of products is included under the well- 

 known name of " natural cement ". The cause of this lack of knowl- 

 edge is not far to seek. Natural cements are too low in value to be 

 shipped, under ordinary circumstances, far from their point of pro- 

 duction. The natural cement made at any given locality has usually, 

 therefore, a well-defined market area within which it is well known 

 and subject to little competition. The engineer practicing within 

 such an area naturally forms his idea of natural cements in general 

 from what he knows of the brands encountered in his work, and as all 

 the brands from one cement-producing locality are apt to resemble 

 one another quite closely, he is likely to conclude that natural cements 

 are quite a homogeneous class, with many points of resemblance and 

 few of difference. The truth is, on the contrary, that there may be 

 far greater differences in strength, rate of set, chemical composition, 

 etc., between the natural cements made in two different localities 

 than between any given brand of natural cement and a Portland cement. 

 This will be brought out clearly in a later chapter, where the compo- 



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