58 



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. 



The chemical analyses will determine what ingredients are 

 present, and in what percentages, but other methods of investi- 

 gation are necessary to ascertain in what manner these various 

 ingredients are combined. A summary of the more important 

 results brought out by these investigations on the constitution 

 of Portland cement is here given. 



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

 Portland cement, much of the lime is combined with most of 

 the silica to form the compound 3 CaO, SiO2, tricalcic silicate. 

 To this compound 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 3 CaO, SiO2. The ideal Portland cement, toward which ce- 

 ments as actually made tend in composition, would consist ex- 

 clusively of tricalcic silicate, and would be therefore composed 

 entirely of lime and silica in the following proportions : 



Lime (CaO) 73.6 



Silica (SiO2) 26.4 



Such an ideal cement, however, can not be manufactured 

 under present commercial conditions, for the heat required to 

 clinker such a mixture can not be attained in any working kiln. 

 Newberry has prepared such mixtures by using the oxy-hydro- 

 gen blowpipe; and the electrical furnace will also give clinker 

 of this composition ; but a pure lime-silica Portland is not pos- 

 sible under present-day conditions. 



In order to prepare Portland cement in actual practice, there- 

 fore, it is necessary that some other ingredient or ingredients 

 should be present 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 (A12O3) and iron oxide (Fe2O3), when present in 

 noticeable percentages, serve to reduce the temperature at which 

 combination of the lime and silica (to form 3 CaO, SiO2) takes 

 place ; and this clinkering temperature becomes further and fur- 

 ther lowered as the percentages of alumina and iron are in- 

 creased. The strength and value of the product, however, also 

 decrease as the alumina and iron increase ; so that in actual 



