CALCULATION AND CONTROL OF THE MIX. 383. 



Theoretical Composition of Portland Cement. 



During recent years much attention has been paid by various inves- 

 tigators to the constitution of Portland cement. The chemical com- 

 position of any particular sample can, of course, be readily determined 

 by analysis, and by comparison of a number of such analyses, general 

 statements can be framed as to the range in composition of good Portland 

 cements. This subject is discussed further in Chapter XXXVIII, 

 where a large number of analyses are presented. 



Chemical analyses will determine what ingredients are present, and 

 in what percentages, but other methods of investigation are necessary 

 to ascertain in what manner these ingredients are combined. A summary 

 of the more important practical results brought out by these investiga- 

 tions on the constitution of Portland cement will be given in the 

 present chapter, while in Chapter XXXVIII a more detailed discussion 

 of the problem will be presented, as well as references to the principal 

 papers on the subject. 



It would seem to be firmly established that in a well-burned Port- 

 land cement much of the lime is combined with most of the silica 

 to form the compound 3CaCO,Si02, tricalcic silicate. To this com- 

 pound is ascribed, in large measure, the hydraulic properties of the 

 cement; and in general it may be said that the value of a Portland 

 cement increases directly as the proportion of 3CaO,SiO2- The ideal Port- 

 land cement, toward which cements as actually made tend in compo- 

 sition, would consist exclusively of tricalcic silicate, and would be there- 

 fore composed entirely of lime and silica in the following proportions: 



Lime (CaO) 73 . 6 



Silica (Si0 2 ) 26.4 



Such an ideal cement, however, cannot be manufactured under 

 present commercial conditions, for the heat required to clinker such a 

 mixture cannot be attained in any working kiln. The oxy hydrogen 

 blowpipe and the electrical furnace will give clinker of this composition; 

 but a pure lime-silica Portland is not possible under present conditions 

 as to burning and grinding on a commercial scale. 



In order to prepare Portland cement in actual practice, therefore, 

 it is necessary that some other ingredient or ingredients should be pres- 

 ent to serve as a flux in aiding the combination of the lime and silica, 

 and such aid is afforded by the presence of alumina and iron oxide. 



Alumina (A^Oa) and iron oxide (Fe2Oa) when present in notice- 

 able percentages serve to reduce the temperature at which combina- 

 tion of the lirne and silica (to form 3CaO,Si02) takes place; and this 

 clinkering temperature becomes further and further lowered as the 



