59 



practice it is necessary to strike a balance between the advantage 

 of low clinkering temperature and the disadvantage of weak 

 cement, and to thus determine how much alumina and iron 

 should be used in the mixture. 



It is generally considered that whatever alumina is present 

 in the cement is combined with part of the lime to form the 

 compound 2 CaA, SiO2, dicalcic aluminate. It is also held 

 hy some, but this fact is somewhat less firmly established than 

 the last, that the iron present is combined with the lime to 

 form the compound 2 CaO, Fe2O3. For the purposes of the 

 present paper, it will be sufficient to say that, in the relatively 

 small percentages in which iron occurs in Portland cement, it 

 may for convenience be considered as almost equivalent to 

 alumina and its action, and the two may be calculated together. 



