Generally speaking, cement is made from cement-stones or pebbles, containing from 30 to 35 per cent, of clay, which proportion is the best for the complete combination of the different constituents, as previously pointed out.
This natural cement is made chiefly at Rugby and Stockton, in Warwickshire, and in Kent; while artificial varieties are made in different parts of the country (principally in the south) from a mixture of white chalk and unburnt clay, in the proportion of about 2 to 1, which is calcined at a very high temperature to produce a heavy, slow-setting cement.
Good cement should be of a very fine powder, leaving no more than 10 per cent, residue when sifted through a sieve of 2,500 meshes per square inch. It should be clean and cool to the arm when thrust into it, and it may be either quick or slow-setting - the slower it is the stronger it ultimately becomes, weighing up to 120 lbs. per striked bushel; while the quick-setting quality weighs from 95 lbs. to 112 lbs. per bushel. It is capable of bearing a tensile strain of 350 lbs. per inch superficial, and is of a greenish-grey colour; a brown tint indicating excess of clay, and blue excess of lime. It should be stored on wooden floors in dry sheds.
Roman cement contains too much clay (from 30 to 45 per cent.), producing a brown quick-setting material, weighing no more than 75 lbs. per striked bushel, and of little ultimate strength compared with Portland. It is useful, however, in work which has to be done quickly, such as sea-walls, etc., requiring that the work should set in the interval between tides.
Selenitic is a patent artificial cement, made from lias lime, by the admixture of plaster-of-Paris or a sulphate of lime, which stops all slaking, and makes the cement set quicker, with a greater proportion of sand, without apparent loss of strength.
Pure and poor limes are sold in lumps by the ton; lias limes in lumps or in bags, when ground for concrete purposes; while cements are sold either in bags or casks.
Burning Lime, of whatever quality of the stone or lias kinds, is burnt in clamps or kilns of both Tunnel and Flare varieties, on both the intermittent and continuous systems; while cements are burnt in conically shaped, brick-lined kilns, as they require much more heat.
Clamps are very little used, as they are a poor and expensive way of working; the lime and fuel being placed in the clamps in alternate layers, and covered over with some material to keep in the heat.
Tunnel kilns are of different sections, on the continuous principle; the lime and coal being added from time to time through an orifice at the top, heated gradually, and abstracted, as it gets down in parts, from a hole at the bottom and away from the fire; while flare kilns are on the intermittent system, the usual cylindrical brick-lined spaces being filled with lime and fuel, and burnt all together, emptied, and then refilled.
Mortar is a mixture of sand with lime or cement in various proportions, according to requirements, to bind together the solid materials used in a building.
Limes should be first thoroughly slaked, and then mixed in a mortar mill with certain proportions of sand, ashes, brick, or stone dust, etc., being thoroughly ground and amalgamated together. When mixed by hand care should be taken that the lime and sand are screened, in order to separate all large lumps, etc., before the water is added to make it into the creamy mass required, no more being mixed under any circumstances than is required for the day's consumption.
Cement is generally mixed, by hand, with very clean, sharp sandmortar, which must be washed if necessary, though, on large buildings, cement is mixed by machinery; and although lime-mortar made by hand is inferior to that made by machinery, cement-mortar is not affected either one way or the other, on account of its fineness of grit and freedom from lumps.
The usual proportion for ordinary work, under no special circumstances, is: -
Poor and Pure lime...
1 part to 3 parts of sand.
1 „ 2 to 3 parts of sand.
1 " 3 to 5 „ „
Roman „ . • •
1 „ 1 part of sand.
Selenitic „ ...
1 „ 4 to 6 parts of sand.