Thus the Warwickshire (Rugby and Stockton), Somersetshire (Bridgwater), Dorset (Poole and Wareham), Portland cements are made from Lias limestone and clay, and in Cheshire (Doveholes) the limestones of the Carboniferous formation are used for the same purpose.
When dense limestones are used for the manufacture of Portland cement, they must be crushed by machinery. The shale or clay is roughly burnt to ballast (see p. 193), the two are then mixed in the proper proportion (according to their composition) to give the percentage of clay and lime required - and are ground to a fine powder.
This powder is passed into the pug-mill of a brick-making machine - thoroughly mixed - slightly moistened, and then moulded semi-dry into bricks. These bricks are then dried upon hot plates to drive off any remaining moisture - burnt in kilns as hereinafter described - and then ground to powder.
The process of manufacture just described is adapted for hard limestones and shaly clays, which cannot be reduced to liquid and thus mixed together. The dry process is stated by Mr. Reid to be very efficient and economical. He says, moreover, "The carbonate of lime is in so finely comminuted a state, and so accurately blended with the silica and alumina, that no injurious development from this source can possibly arise, at all events in the direction of the cracking or blowing danger."
The plant required for a Portland cement manufactory is so extensive that it can hardly ever be worth while for an engineer or builder to manufacture for himself. This branch of the subject will, therefore, not be pursued further; but any one who is interested in it will find full details of the processes of manufacture, with much other useful information, in Mr. Reid's works: A Practical Treatise on the Manufacture of Portland Cement; The Practical Manufacture of Portland Cement, translated from Lipowitz; and The Science and Art of the Manufacture of Portland, Cement; also in a paper by General Scott and Mr. Redgrave, in the Minutes of the Proceedings of Civil Engineers, vol. lxii. p. 67.
Ordinary blast furnace slag (see p. 259) contains nearly the same constituents as Portland cement, but not in the same proportions - the proportion of lime being too small. Mr. Ransome runs the molten slag into water, so that it forms a sort of sand, grinds this with the required extra proportion of lime in the form of chalk, and then by burning the mixture in a revolving kiln produces good Portland cement. The writer has no experience of this material, but it is said to attain the same strength as ordinary Portland cement, and in a shorter time.1 The process is, of course, useful only in blast-furnace districts, for it would not pay to transport slag to other places for the purpose.
" Great caution is necessary in adopting a cement of this nature, more especially when it is recollected that blast-furnace slags differ materially in their composition. ... It would appear, however, that when care is taken to see that the constituents of the cement exist in suitable proportions, a very serviceable article is capable of being produced." 1
1 Engineer, 4th March 1887.
Portland cement differs very considerably in its characteristics and action.
It can be manufactured more cheaply when under-burnt, because then a greater bulk of cement is produced with a given quantity of material, and it requires less fuel and less grinding; it also sets more quickly, but never arrives at the same ultimate strength as a properly burnt cement. Under-burnt cement contains, moreover, an excess of free quicklime, which is apt to slake in the work and cause great mischief. This may be remedied by exposing the cement, and allowing these particles to become air-slaked.