As long as the burning is incomplete, and any carbonic acid is left in the stone, it will remain of a dull red colour. When the carbonic acid is all expelled, the stone in the kiln becomes peculiarly bright, which is a sign that the calcination is complete, and that the lime may be withdrawn.
The Temperature at which a lime or cement should be burnt depends upon its composition.
A pure or fat lime requires only heat enough to drive off the carbonic acid and moisture.
Limes containing clay require a somewhat greater heat, in order that the silicates and aluminates may be formed which give the hydraulic properties required.
A great deal depends, however, upon the composition of the clay.
A large proportion of iron and alumina (especially of iron) as compared with the silicic acid, greatly facilitates the action which takes place in calcination, and the prepared mortar also sets more quickly.
Great care must be taken, however, that the heat is not sufficient to fuse the particles of the lime or cement.
Thus Roman cements, in which the quantity of iron and alumina together nearly equals the silicic acid, are burnt with little fuel at a low temperature.
Portland cement, on the other hand, in which the iron and alumina are less than half the silicic acid, is burnt at very high temperatures. There is very little danger of fusing the particles, and the heat may with advantage be raised to a point just short of vitrification.1
The denser the stone and the higher the temperature at which it is to be. burnt, the smaller must be the pieces into which it is broken.
Pure or fat limestones are broken into pieces containing from one to two cubic feet.
Hydraulic limestones into pieces containing about a quarter of a cubic foot.
Roman cement stones and others of the same quick-setting class are broken into pieces containing one or two cubic inches.
The Quantity of Fuel is of course influenced partly by the form of kiln, but chiefly by the nature of the stone and by the temperature at which it is to be burnt.
Thus, for the calcination of pure dense limestones about 1/3 to 1/4 their weight of coal is required.
For hydraulic limestones about 1/5 to 1/7 their weight.
For Roman and other quick-setting cements about 1/9 to 1/12 of the weight of stone.
For Portland cement about 1/3 the weight of the dried slurry.