7 to 14%, the lime from
60 to 67%. All such variations are permissible provided that the
quantity of silica and alumina is sufficient to saturate the whole of
the lime and to leave none of it in a "free" condition, likely to
cause the cement to expand after setting. Other things being equal,
the higher the percentage of lime within the limits indicated above
the stronger is the cement, but such highly limed cement is less easy
to burn than cement containing about 62% of lime; and unless the
burning is thorough and the raw materials are intimately mixed, the
cement is apt to be unsound. Although the ultimate composition of
cement, that is, the percentage of each base and acid present, can be
accurately determined by analysis, its proximate composition, i.e. the
nature and amount of the compounds formed from these acids and bases,
can only be ascertained indirectly and with difficulty. The
foundations of our knowledge on this subject were laid by H. le
Chatelier, whose work has since been supplemented by that of Spenser
B. Newberry, W.B. Newberry and Clifford Richardson. As the outcome of
these inquiries it has been established that tricalcium silicate
3CaO.SiO2 is the essential constituent of Portland cement. The
constituent of next importance is an aluminate, but whether this is
dicalcium aluminate, 2CaO.Al2O3, or tricalcium aluminate, 3CaO.Al2O3,
is still in doubt. In the following description it is assumed to be
the tricalcium aluminate. The remaining silicates and aluminates
present, and ferric oxide and magnesia, if existing in the moderate
quantities which are usual in Portland cement of good quality, are of
minor importance and may be regarded as little more than impurities.
The silicates and aluminates of which Portland cement is composed are
believed to exist not as individual units but as solid solutions of
each other, these solid solutions taking the form of minerals
recognizable as individuals. The two principal minerals are termed
alite and celite; according to the best opinion, alite consists of a
solid solution of tricalcium aluminate in tricalcium silicate, and
celite of a solid solution of dicalcium aluminate in dicalcium
silicate. Celite is little affected by water, and has but small
influence on the setting; alite is decomposed and hydrated, this
action constituting the main part of the setting of Portland cement.
Both the component