24 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
Many engineers regard it as positively detrimental in even small 
amounts, and because of this feeling manufacturers prefer to carry 
it as low as possible. Newberry has stated that in amounts of less 
than 3J per cent it is harmless, and American Portlands from the 
Lehigh district usually reach well up toward that limit. In European 
practice it is carried somewhat lower. 
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 3CaO.Si0 2 — 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 3CaO.Si0 2 . The ideal Port- 
land cement, toward which cements as actually made tend in composi- 
tion, would consist exclusively of tricalcic silicate, and would be 
therefore composed entirely of lime and silica in the following pro-- 
portions: Lime (CaO), 73.6 per cent; silica (Si0 2 ), 26.4 per cent. 
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 hydrogen blowpipe; and the 
electrical furnace will also give clinker of this composition; but a pure 
lime-silica Portland is not possible under present conditions. 
In order to prepare Portland cement in actual practice, therefore, 
it is necessary that some other ingredient or ingredients 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 (A1 2 3 ) and iron oxide (Fe 2 3 ), when present in noticeable 
percentages, serve to reduce the temperature at which combination of 
the lime and silica (to form 3Ca0.8iO.) takes place; and this clink- 
ering temperature becomes further and further lowered as the per- 
centages 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 low clinkering temperature and the disad- 
vantage of weak cement, and thus to 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 
2CaO.Si0 2 — 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. 
Fe 2 3 . 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 equivalent 
to alumina in its action, and the two may be calculated together. 
