Portland cement is sold in casks or in sacks for home consumption, and in casks for export.
The inside dimensions of the casks are sometimes as follows, but they vary. Length, 271/2 inches. Diameter at middle, 19 inches. Diameter at ends, 16 inches.
Each cask usually contains 400 lbs. (net).
Those for export should be well hooped and nailed, and lined with stout brown paper.
The sacks measure 22 inches x 38 inches, and each usually contains 2 trade bushels or 200 lbs. of cement, but sometimes is filled so as to contain 2 cwt.
Good Portland cement is slow-setting as compared with the cements made from most natural cement stones, but surpasses them in ultimate strength; and is more extensively used than any other, for all kinds of work for which cement is suitable. It is particularly well adapted for making concrete.
Scott's Processes - General Scott's two processes depend upon an intimate admixture with the lime of a small quantity of a sulphate, usually sulphate of lime,1 before, or at the same time that, the water is added.
All limes are improved by them, and converted into a kind of cement, the slaking action being suppressed, and the lime setting without expansion, thus forming a denser and harder mortar.
The quickness of setting is greatly increased by these processes for all limes, and their ultimate strength is also improved.
Scott's Cement is prepared by passing the fumes of burning sulphur through lumps of quicklime placed on gratings, and raised almost to a red heat, by which about 5 per cent of it is turned into a sulphate.
The calcined stone, if properly burnt, will be found to have lost all power of slaking; upon being ground it becomes a fine homogeneous powder, of a tint similar to that of the unslaked lime from which it is prepared.
Good Scott's cement should be finely ground, and contain not less than 10 per cent of soluble silica; it should weigh fully 60 lbs. per striked bushel, and when mixed with two parts of sand should be strong enough to come out of the mould in twenty-four hours. After being left for seven days in a dry place, the weight required to break it should be not less than 66 lbs. per square inch.
This material was coming into considerable use some years ago for making mortar, but especially for plastering. It is not now in the market, having been superseded by selenitic cement, in which the same qualities and characteristics are obtained by a much simpler process of manufacture. It has, however, been described here, as there is sometimes a confusion between the two.
Selenitic Cement,2 sometimes known as selenitic lime, is also an invention of General Scott's.
This cement, like the other, contains a small proportion of sulphate of lime, which is added in the form of plaster of Paris, mechanically mixed and ground with lime. Lime may, however, be selenitised by adding a small proportion of any sulphate, or by mixing it with sulphuric acid.
1 Calcium sulphate.
2 So called from Selenitic, the scientific name for gypsum which is sulphate of lime, and forms, when burnt and gronnd, plaster of Paris.
The sulphate begins to take effect directly water is added. Its presence arrests the slaking action, causes the cement to set much more quickly, and enables it to be used with a much larger proportion of sand than ordinary lime without loss of strength.
This cement may be made from any lime possessing hydraulic properties. The limes from the magnesian limestones are much used for the purpose, also those from the grey chalk. But the best limes for selenitising are those from the Lias formation and grey chalk.
The cement should be finely ground so as to pass through a sieve of 900 meshes to the inch.
The quantity of sulphate the cement should contain depends upon the quality and description of the lime used for its manufacture, and varies from 4 to 7 per cent, the usual proportion being about 5 per cent.
When more than 71/2 per cent of sulphate is required to stop the slaking action, the lime may be considered not suitable for making selenitic cement. In this case, however, the lime may be rendered suitable by mixing it with one containing more clay.
Where used. - This cement has been used at the New Law Courts; Grosvenor Mansions; Chesterfield Mansions, etc.; and for plastering at the Alexandra Palace, Manchester New Town Hall, and several of the principal new buildings in London, and other large towns.
The Table on the opposite page is taken from a circular issued by the Selenitic Cement Company.
It shows the comparative strength of selenitic and Portland cement, with different proportions of sand, and also the increase of strength which accrues to the lime when it is prepared by the selenitic process.
Ordinary lime may be selenitised during the process of mixing it into mortar. The method of doing this is described at p. 207.
In addition to the manufacture of hydraulic limes and cements by the intimate mixture and calcination of the necessary ingredients, hydraulic properties are sometimes conferred upon mortars made from fat lime by adding to them such substances as are known to produce hydraulicity.