Voelcker yielded 34.92% tricalcium phosphate, a specimen of the shale 52.15% (Report of Brit.
As the outcome of these inquiries it has been established that tricalcium silicate 3CaO S10 2 is the essential constituent of Portland cement.
The constituent of next importance is an aluminate, but whether this is dicalcium aluminate, 2CaO Al 2 0 3, or tricalcium aluminate, 3CaO-Al 2 0 3, is still in doubt.
In the following description it is assumed to be the tricalcium aluminate.
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.
Both the components of alite react, and for simplicity their reactions may be stated in separate equations, thus: 2(3Ca0 S10 2)+9H 2 O =2(Ca0 S102) 5H20+4Ca(OH)2 Tricalcium silicate.
3CaO Al 2 0 3 +12H 2 O =3CaO Al203.12H20 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.
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.
It will be observed that in the hydration of tricalcium silicate, the main constituent of Portland cement, a large portion of the lime appears as calcium hydroxide, i.e.
In France this slaking is conducted systematically by the makers, the freshly burned lime being sprinkled with water and stored in large bins where slaking proceeds slowly and regularly until the whole of the surplus uncom bined lime is slaked and rendered harmless, while the cementitious compounds, notably tricalcium silicate, remain untouched.