CHAPTER 2. 



PORTLAND CEMENT: DEFINITION, COMPOSITION 

 AND CONSTITUTION. 



In the following section various possible raw materials for 

 Portland cement manufacture will be taken up, and their rela- 

 tive suitability for such use will be discussed. In order that the 

 statements there made may be clearly understood, it will be 

 necessary to preface this discussion by a brief explanation re- 

 garding the composition and constitution of Portland cement. 



Use of term Portland. While there is a general agreement 

 of opinion as to what is understood by the term Portland ce- 

 ment, a few points of importance are still open questions. The 

 definitions of the term given in specifications are in consequence 

 often vague and unsatisfactory. 



It is agreed that the cement mixture must consist essentially 

 of lime, silica, and alumina in proportions which can vary but 

 slightly ; and that this mixture must be burned at a temperature 

 which will give a semi-fused product a "clinker." These 

 points must therefore be included in any satisfactory definition. 

 The point regarding which there is a difference of opinion is 

 wnether or not cements made by burning a natural rock can 

 be considered true Portlands. The question as to whether the 

 definition of Portland cement should be drawn so as to include 

 or exclude such products is evidently largely a matter of con- 

 vention ; but, unlike most conventional issues, the decision has 

 very important practical consequences. The question at issue 

 may be stated as follows : 



If we make artificial mixture of the raw materials and a very 

 high degree of burning the criteria on which to base our defini- 

 tion, we must in consequence of that decision exclude from the 

 class of Portland cements certain well known products, manu- 

 factured at several points in France and Belgium by burning a 

 natural rock, without artificial mixture, and at a considerably 

 lower temperature than is attained in ordinary Portland cement 

 practice. These "natural Portlands" of France and Belgium 



