381 



CEMENTS, LIMES, AND PLASTERS. 



percentages of alumina and iron are increased. The strength and value 

 of the product, however, also decrease as the alumina and iron increase; 

 so that in actual practice it is necessary to strike a balance between 

 the advantage of : ow clinkering temperature and the disadvantage of 

 weak cement, and to thus determine how much alumina and iron should 

 be used in the mixture. This point will be further discussed on later 

 pages. 



It is generally considered that whatever alumina is present in the 

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

 2CaO,Al 2 O3, dicalcic aluminate. It is also held by 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 2CaO,Fe2O3. This 

 question of the action of the iron will be later referred to. For the 

 purposes of the present chapter 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 approximately equivalent to 

 alumina in its action. 



Influence of Normal Constituents on the Cement. 



Lime, silica, alumina, iron oxide, magnesia, sulphur, and alkalies 

 may be regarded as being normal constituents of any Portland-cement 

 mixture. The three first named are necessary ingredients, while the 

 last two, though undesirable, are rarely entirely absent from the raw 

 materials used. The influence exerted by greater or lesser propor- 

 tions of these seven constituents on the properties of both mixture 

 and finished cement will be discussed in the present chapter. 



Maximum lime content of mixture. On pages 392-393 New- 

 berry's method of proportioning cement mixtures will be described 

 and exemplified. It should be borne in mind, however, that the New- 

 berry formula there quoted will, if followed, give the maximum lime 

 content that the mixture could bear, providing that the grinding, mixing, 

 and calcination were performed with absolute perfection. As a matter 

 of fact, however, the lime content of the mixture should never be car- 

 ried quite as high as this formula would indicate, for in actual practice 

 the mixing, grinding, and calcination are never theoretically perfect, 

 and in consequence of a perfect combination of all the lime with all 

 the silica and alumina cannot be attained. There will always remain 

 a certain amount of uncombined material. If therefore, the lime in 

 the mixture is carried as high as is theoretically allowable, a certain 

 amount of free lime will occur in the cement. If, on the other hand, 

 the mixture carries less than its proper theoretical percentage of lime, 



