s of alite react, and for simplicity their reactions
may be stated in separate equations, thus:--

(1) 2(3CaO.SiO2) + 9H2O = 2(CaO.SiO2).5H2O + 4Ca(OH)2
Tricalcium silicate. Hydrated mono- Calcium
calcium silicate. hydroxide.

(2) 3CaO.Al2O3 + 12H2O = 3CaO.Al2O3.12H2O
Tricalcium aluminate. Hydrated tricalcium
aluminate.

Since alite is a solid solution and, although an individual mineral,
is not a chemical unit, the proportion of tricalcium silicate to
tricalcium aluminate in a given specimen of alite will vary; but,
whatever the proportions, each of these substances will react in its
characteristic manner according to the equations given above.

The precise mechanism of the process of setting of Portland cement is
not known with certainty, but it is probably analogous to that of the
setting of plaster of Paris, consisting in the dissolution of the
compounds produced by hydration while they are in a more soluble form,
their transition to a less soluble form, the consequent
supersaturation of the solution, and the deposition of the surplus of
the dissolved substance in crystals which interlock and form a
coherent mass. This theory being accepted, it is evident that a small
quantity of water, by successive dissolution and deposition of a
substance capable of existing in a more soluble and in a less soluble
form, is able to bring about the crystallization of an indefinitely
large quantity of material. It is not necessary that there should be
present sufficient water to dissolve the whole of the reacting
substance at any one time; it is sufficient if there is enough for
hydration and a small surplus for the crystallization by successive
stages as above described. It is generally admitted that the aluminate
is the chief agent in the first setting of the cement, and that its
ultimate hardening and attainment of strength are due to the
tricalcium silicate.

As mentioned above, the constituents other than the tricalcium
silicate and tricalcium aluminate of which alite is composed, are of
minor importance. The function of the ferric oxide present in ordinary
cement is little more than that of a flux to aid the union of silica,
alumina and lime in the clinker; its role in the setting of the cement
is altogether secondary. In fact, excellent Portland cement can be