Its strength and toughness render it valuable for naval purposes, to which it is largely applied; its freedom from any tendency to split adapts it for clinker-built boats.
The output will be about 30 tons of "clinker" ready to be ground into cement.
The grinding of the hard rock-like masses of clinker is effected between millstones, or in modern plants in ball-mills, tube-mills and edge-runners.
At this middle portion and in the upper part of the lower shaft the burning proper proceeds; the upper shaft is full of unburnt raw material which is heated by the hot gases coming from the burning zone, and the lower shaft contains clinker already burned and hot enough to heat the incoming air which supplies that necessary for combustion at the clinkering zone.
In each case the clinker which has just been burned and is fully hot serves to heat the air-supply to the compart ment where combustion is actu ally proceeding; in like manner the raw materials about to be burned are well heated by the waste gases from the compartment in full activity before they them selves are burned.
(It may be noted that here and generally in this article "burn" is used in the technical sense; it is technically correct to speak of cement clinker Surninq being "burned," although it is not a fuel; in accurate terms it is the fuel which is burned, and it is the heat it generates which raises the clinker to a high temperature, i.e.
_ rying space or urry Kiln Lower shaft containing hot clinker Grate.,› Upper shaft containing raw material FIG.
The burner where the temperature is highest, and is there heated so highly that the union of the lime, silica and alumina is complete, and fully burnt clinker falls out of the kiln.
On its way down the cylinders the clinker meets a current of cold air and is cooled, the air being correspondingly warmed and passing on to aid in the combustion of the fuel used in heating the kiln.
Rotatory kilns of various other makes are now in use, but the same principles are embodied, namely, the employment of a rotating inclined cylinder for burning the raw materials, a burner fed with powdered coal and a blast of air, and some device such as a cooling cylinder or cooling tower by which the clinker may be cooled and the air correspondingly heated on its way to the burner.
The high temperature necessary to fuse cement clinker makes this process difficult to accomplish commercially, but it has many inherent merits and may be the process of the future, displacing the rotatory method.
Portland cement clinker, however produced, is a hard, rock-like substance of semi-vitrified appearance and very dark colour.
Well-burnt, well-picked clinker when ground yields good Portland cement.
For the same purpose a small quantity of water (up to 2%) may be added either by moistening the clinker or by blowing steam into the mills in which the clinker is ground.
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.