Common mortar crumbles away, if laid under water before it has had time to harden.
Portland cement is an artificial cement, of a dark grey colour. It is made by grinding chalk, mixing it with blue clay or river-mud in certain proportions, and then burning it in a kiln and afterwards grinding it to a fine powder. It is used, mixed with sand, for external plastering (“compoing”) of walls, for making concrete, or instead of lime for making mortar if extra strength is required.
Compo consists of Portland cement and sand, and is used for covering walls when an impervious smooth surface is required, and for keeping out rain. It is laid on in two coats. The first or rough coat ¾ inch thick, is composed of one part cement to 5 parts compo sand, i.e. coarse sand mixed with fine beach. The outer or fine coat is composed of two parts fine or washed sand to one part cement. To “render” or “compo” a wall is to cover it with this material. The internal plastering of a chimney flue is called “pargetting.”
Concrete is of two kinds, lime or cement concrete. It is composed of three parts broken ballast or large beach, two parts of sand, and one part of lime or cement. Lime concrete has no resisting strength, and is only used for surrounding drain-pipes, or where no great strength is required.
Stone varies very greatly in character. It is uncommon for the whole thickness of the walls of a house to be built of stone; usually there is merely a facing of stone and a backing of brickwork. If good stone is not available, the less it is used the better.
The stone chosen should be durable, and able to resist the action of the sulphuric, sulphurous, and carbonic acids absorbed from the atmosphere, and brought in contact with it by means of rain. The stone of which a considerable part of the Houses of Parliament consists is dolomite, a double carbonate of lime and magnesia. The acid fumes in the air produce on its surface sulphate of magnesium, which is washed away in successive layers.
If the stone presents any stratification, it should be laid in the wall in the same position as that in which it was originally deposited in the quarry. Thus, any planes of stratification will be horizontal, and the scaling off by the action of frost and rain is minimised. Comparatively homogeneous stones, such as granite and millstone-grit, can be laid in any position. In testing the character of any stone, the least porous, densest, and most resistent to crushing, will as a rule be the most durable.
The chief difficulty in the use of stone for the walls of houses, is that of keeping out the wet. To obviate this, stone-houses are often built of great thickness, and are consequently cooler in summer and warmer in winter.
In and near large towns brick is chiefly used for walls of houses, and stone employed only for window-sills, columns, steps, etc. It is even more important in these cases to carefully select the stone, as the parts where it is placed are those most exposed to the weather. If a soft, friable freestone is used, after a sharp frost large scales are seen falling off in flakes, owing to the freezing and subsequent thawing of the moisture in the stone.
Portland stone is the best-wearing stone to be had in the neighbourhood of London. Bath stone is also considerably used, but it varies greatly in quality, and should be very carefully selected. For landing-steps and paving, Yorkshire stone is extensively used, but artificial cement pavings are replacing it to some extent. Most kinds of stone can only be economically used near the quarries from which they are derived.