CHAPTER XXXVIII. 



CONSTITUTION, SETTING PROPERTIES, AND COMPOSITION OF 

 PORTLAND CEMENT. 



Limitations of chemical analyses. An ordinary chemical analysis 

 of a specimen of cement will determine what elements are present in 

 the cement, and in what percentages these various constituents are 

 represented. The comparison of a long series of such analyses, such as 

 is presented later in this chapter, will enable certain conclusions to be 

 drawn as to the probable limits of composition of good Portland cements; 

 and an analysis of a single cement may show that it contains undesirable 

 ingredients or that inert material is present in undesirable quantity. 

 But these methods of investigation fail to give the least information 

 concerning the real constitution of Portland cement as distinguished 

 from its composition; they give no information whatever as to the man- 

 ner in which the various elements are combined among themselves. 

 They fail, moreover, to give any clue to the reason why certain mixtures 

 give good cements, while others give weak or unsound products; and 

 they afford no explanation of the " hydraulic" or setting properties 

 which the powdered clinker possesses. It is evident, therefore, that 

 other methods of investigation must be adopted, since even the most- 

 careful chemical analysis fails to aid us in this line of research. 



Constitution and Setting Properties. 



Available methods of investigation. Two distinct methods of in- 

 vestigation are available microscopic and synthetic. 



The first has been applied with great success by geologists to the 

 study of the igneous rocks, and as cement clinker is practically an arti- 

 ficial (though very basic) igneous rock, the microscope can be used 

 successfully in its examination. By grinding normal clinker of known 

 analysis down to thin transparent slices, the microscope is able to detect 

 certain constituents common to all good clinkers. The next step, 

 of course, is to determine the composition of these different constitu- 

 ents, and here the synthetic method is applicable. 



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