Cements. These may be roughly divided into three classes: (1) The stone cements, including Roman and Portland cements, and ordinary mortar, which are used in thickish layers for uniting stone and brick work, and for protective coverings to buildings; (2) substances which form binding joints of much less but still appreciable thickness, such as white lead, red lead, and putty; and (3) cements which require to be used in extremely thin coatings, such as glue, isinglass, and dissolved caoutchouc.
Ordinary Mortar is a mixture of slaked lime (calcium hydroxide) and sand, made into a paste with water. Generally one part of lime to three or four parts of sand are used, but the proportions vary according to the purity of the lime employed. Very pure or fat lime, such as that made by burning white chalk or white marble does not make so good a mortar as lime obtained from less pure limestones, which are by far the most abundant. The more thoroughly the ingredients are intermixed, the more complete will be the subsequent hardening of the mortar. As commonly laid in the joints of brick or stone work, mortar sets sufficiently fast to allow building operations to proceed from day to day with occasional longer intervals, but it takes years—perhaps in many cases centuries—to reach its maximum hardness. The setting and subsequent slow hardening of mortar are usually considered to be due, in the first instance, simply to the loss of water, and afterwards to the absorption by the lime of carbonic acid from the atmosphere, the carbonate of lime thus formed binding together the sand and stone. It is doubtful, however, if this is an altogether satisfactory explanation. The mortar used in many medieval buildings is largely mixed with small pebbles. In a number of cases this has proved to be of a more durable nature than the stone used along with it.
Puzzolana or Pozzuolana, a loosely coherent volcanic sand found at Pozzuoli, near Naples, has been long celebrated for its property of forming a hydraulic cement when mixed with ordinary lime. It is composed of silica, with a little magnesia and potash or soda, alumina, lime, and oxide of iron.
Roman Cement.—Certain natural mixtures of lime and clay are called cement-stones. The clays of some of the newer geological formations in the south of England, for example, contain courses of septarian nodules (see SEPTARIA), which have been in great request for making the best kinds of Roman cement. They are concretions of impure calcareous matter, many of them having this analysis: Carbonate of lime, 66; silica, 18; alumina, 7; and protoxide of iron, 6; or consist of these substances in nearly that proportion. Cement-stones are carefully calcined in kilns, and afterwards ground and sifted. Good Roman cement should set in about 15 minutes, and this quick-setting property makes it valuable for work which requires to be executed between tides and for other purposes where the cement used must harden quickly. It is at best of but medium strength. Some natural cements are slow-setting, and these do not contain more than 22 per cent. of clay. They set under water when half their weight consists of clay. The proportion of sand used with Roman cement should not much exceed that of the cement. When employed for external coatings of buildings it is apt to effloresce and become unsightly.
Portland Cement.—This is considered by far the most important of the stone cements. It is an artificial product, named from its resemblance to Portland Stone, but is much more largely used than Roman cement. In the manufacture of Portland cement on the banks of the Thames and the Medway by the wet process, three parts of white chalk are mixed with one part of clay or mud from the lower reaches of these rivers. The two substances, along with water, are placed in a 'wash mill' in which strong revolving knives or cutters reduce the whole to a creamy 'slurry' or slip. The slurry then passes by gravitation to backs or reservoirs. There it is allowed to settle for some weeks, when the superfluous water is removed by decantation. The mixture is next dried on heated iron plates or on the floor of a heated chamber, and then burned in kilns. Finally it is ground to a fine powder. Modifications of the wet process by which the large reservoirs are dispensed with have been introduced in recent years. In other parts of the country Portland cement is manufactured by the dry process from the hard limestones of other formations than the chalk, along with clay or shale. These limestones are crushed small, mixed in the proper proportion with clay or shale, then roughly burned, and ground to powder. This powder slightly moistened is passed through a pug-mill, and then made into bricks, which are afterwards burned in kilns and reduced to powder.
Since Portland cement is hardly ever employed in the pure or neat state, its strength is perhaps best tested when it is mixed with an equal weight of sand. The best cement so mixed and moulded in the state of a stiff mortar, into any convenient shape, when tested after the lapse of seven days, during six of which it is customary to keep it immersed in water, exceeds in tensile strength 200 lb. per square inch, and in crushing strength, tested by -inch cubes, 1000 lb. for the same area. Its strength in the unmixed state is much greater. Much of the Portland cement made is, however, little more than half as strong as the best kind. Roman cement of good quality mixed to the same extent with sand as the above, and tested under the same conditions, has on an average a tensile strength of 30 lb., and a crushing strength of 200 lb. in each case per square inch. Portland cement is slow in setting compared with most varieties of Roman cement. Both Portland and Roman cement form hydraulic mortars—that is, they set under water. No mortar will do this which contains less than 10 per cent. of silica.
Till close on 1840 Portland cement was hardly known, but the use of it has extended rapidly, especially in recent years. Its most important application is in the construction of docks and harbours, many of which are partly or wholly built of it, mixed with sand and broken stones, in the form of a concrete. In this state, or simply mixed with sand, it is also much employed for other purposes where strength and durability are required. Owing to the nature of some of the extensive engineering works in which Portland cement is largely used, it is plainly of great consequence that its properties should be thoroughly understood. Numerous failures with it have taken place. The chemical investigation into the case of the Aberdeen docks in 1887 distinctly showed the deleterious action of sea-water upon this sub- stance. But it is also known that objects made of unmixed Portland cement from the works of some of the best makers will sometimes keep good for nearly twenty years, and then crumble to pieces even when not exposed out of doors at all. Of course explanations of these failures are forthcoming. They are generally attributed to carelessness in the manufacture of the cement, or in the selection of the materials for it. But if they occur, as they have done, with cements that have stood very well the ordinary mechanical tests, how can any cement of this kind be entirely depended upon for durability? Twenty, thirty, or even fifty years is far too short a time to test the lasting property of a building material of this nature. The use of Portland cement in pavements and for architectural ornaments is not attended with much risk, and for such purposes it is very suitable. The capital employed in the manufacture in Great Britain is probably near two millions sterling. For American cements, see ROSENDALE.
Scott's Selenic Cement consists of burnt limestone mixed with about 5 per cent. of sulphate of lime in the form of plaster of Paris, and ground to powder. The presence of the sulphate arrests the slaking action of the lime, causes the cement to set more quickly, and admits of more sand being used with it than ordinary lime does. This cement has been a good deal used for plastering, and to some extent also for mortar.
Plaster of Paris (see ALABASTER and GYPSUM).—This material is used for cementing marble and alabaster in much the same way as mortar is in brick-work. It is also employed for uniting the separately moulded pieces of any large object cast in the same material. Sometimes it is selected for fixing metal mounts to glass.
Keene's Cement is made by saturating plaster of Paris in small lumps with alum and recalcining it. It then forms a hard plaster for the projecting portions of halls and rooms, such as pilasters, columns, and skirtings. It is capable of taking a high polish.
Parian or Keating's Cement somewhat resembles Keene's. In its manufacture borax as well as alum is added to the plaster of Paris.
Martin's Cement is another kind, with plaster of Paris for its basis, but instead of borax, carbonate of potash is added, and sometimes hydrochloric acid as well. With the exception of Scott's, these plaster of Paris cements are only used in plastering or other internal work—not for mortars.
Mastic Cement, consisting of a mixture of burnt clay or limestone in a powdered state, with boiled oil and litharge, was more in use formerly than now; but though expensive, it is an excellent material for preventing the admission of rain-water at certain joints about buildings, such as where wood and stone work come together at windows. It was also used for covering external mouldings.
Rust or Iron Cement.—Joints in iron-work, such as those for hot-water pipes, are filled up with a cement of iron borings or turnings, mixed with at least 2 per cent. of sal-ammoniac. Sometimes sulphur in powder is added. The iron oxidises and forms a firm joint.
Sulphur Cement.—For jointing earthenware pipes, and occasionally for fixing bars of iron into stone, a cement is made of sulphur, resin, and brick-dust. It is a cheap but not a strong cement where metal is concerned.
Water-glass Cements.—For furnaces one kind consists of burnt and unburnt fireclay made plastic with silicate of soda or water-glass. Another cement, capable of standing a high heat, is formed of asbestos powder made into a paste with silicate of soda. The same silicate mixed with ground glass makes an acid-proof cement.
White and Red Lead Cements.—Either white lead or red lead by itself, or a mixture of both, is much in request as a cement for the joints of slate or glass cisterns, such as aquariums. These are also employed for the joints of gas-pipes, for cementing metal mounts to glass tubes, and other chemical and electrical purposes. White and red lead cements are made up with boiled linseed-oil, and sometimes gold size is added. Mixed white and red lead make a very hard and firm cement. A cement of these two substances and ground plum-bago in equal parts, mixed with oil, is said to stand a great heat in steam-joints.
Shell-lac Cements.—An excellent cement is made by digesting 4 oz. of the finest shell-lac in 3 oz. of methylated spirit in a warm place. It should be made into a consistency like thick syrup. This makes a firm cement for mending pieces of glass, china, ornamental stones, and ivory. It is not soluble in water. A cheaper, but still very serviceable cement can be formed by dissolving shell-lac in wood naphtha. For some purposes shell-lac itself is used as cement by simply melting it.
Marine Glue is a mixture of shell-lac in a solution of india-rubber. It is made into thin sheets, and melted when required for use in shipbuilding, &c.
Gelatin and Isinglass Cements.—Fish-glue, gelatin, or Isinglass (q.v.), made up with dilute acetic acid and other bodies into a jelly or thick liquid, produces a cement slightly varying in its nature, for mending china, glass, ivory, bone, and other substances. Foulke's cement and liquid fish-glue are cements of this class. These can be obtained in a convenient form for use in hardware or druggists' shops. They are more or less soluble in water, so that articles mended with them must be quickly washed. Cement of mixed glue and glycerine, sometimes with tannin added, is occasionally used for leather and cloth.
Armenian or Diamond Cement.—The following is the reputed formula for preparing the cement used by the Armenian jewellers for attaching diamonds, &c., without any metallic setting: 'Dissolve five or six bits of gum-mastic, each the size of a large pea, in as much rectified spirit of wine as will suffice to render it liquid; and in another vessel dissolve as much isinglass, previously a little softened in water—though none of the water must be used—in French brandy, or good rum, as will make a 2-ounce phial of very strong glue, adding two very small bits of galbanum or ammoniacum, which must be rubbed or ground till they are dissolved. Then mix the whole with a sufficient heat. Keep the glue in a phial closely stopped, and when it is to be used, set the phial in boiling water.'
Elastic Cements.—One part of caoutchouc dissolved in 3 parts of chloroform; also, 5 parts of caoutchouc in 3 parts of chloroform, with 1 part of powdered gum-mastic added. Benzole is sometimes used instead of chloroform as the solvent. Another elastic cement can be made by a mixture of gutta-percha and caoutchouc dissolved in bisulphide of carbon. The solvents of these cements must not be exposed to any but a gentle heat.
Resin Cements.—There are a great number of cements partly formed of ordinary resin. One kind consists of resin 4, beeswax 1, and whiting 1 part. The proportions of these ingredients in the same order for another are 15, 1, and 4. Another is made from resin 4, and plaster of Paris 1 part. These cements are used to fix pieces of stone, glass, &c. to handles when grinding them. Resin, pitch, beeswax, and plaster of Paris or brick-dust are made up in various proportions into cements.
Cutters' Cement, used for fixing knives and forks in handles, is made of equal weights of resin and brick-dust melted together; or, for a superior quality, 4 parts of resin, 1 of beeswax, and 1 of brick-dust.
Copal varnish, mastic varnish, Canada balsam, and gold size are each useful occasionally for cementing substances like two pieces of glass together.
Cement-stone, a somewhat argillaceous and ferruginous limestone, generally compact, which is occasionally employed for making hydraulic mortar or cement. The Cement-stone Series is the name of a group of strata occurring in the Carboniferous System of Scotland. See CARBONIFEROUS SYSTEM.
Cemetery (from the Greek kōimētērion, literally 'a sleeping-place') may mean any graveyard, or other place of deposit for the dead; but it has lately acquired a special meaning, applicable to those extensive ornamental burial-grounds which have recently come into use as the practice of burying within and around churches was gradually abandoned (see BURIAL). The fine burial-grounds of the Turks, extending over large tracts adorned by cedars and other trees, may have suggested the plan to western Europeans. Those round Constantinople are famous, and are dense forests of cypresses. A Moslem grave is never reopened, and a cypress is usually planted after every interment. Of western cities, Paris took the lead in this respect; and in Britain there are now no considerable towns near which there is not at least one cemetery, and the legislation mentioned under the head of BURIAL has rendered their establishment, to a certain extent, a legal necessity. There was at first a natural feeling of regret at the prospect of deserting places of deposit for the dead so hallowed by ancient use and recent associations as the church and the churchyard. On the other hand, the new places of interment began to become attractive in virtue of their trees and flowers, natural scenery, and works of monumental art. The new cemeteries are in many instances cheerful open places, and in them the place of rest for the dead has rather tended to improve than to undermine the health of the living. One of the first and most celebrated of modern European cemeteries is that of Père la Chaise (q.v.), near Paris, the arrangements of which have been generally followed in the cemeteries of London and other English cities. It was laid out in 1804, and is now within the enceinte of the city. The Campo Santo of Pisa (1228-83), the pantheon of the Pisans, has been the model of many Italian cemeteries. It is an oblong court, surrounded by lofty arcades of marble, and adorned with famous frescoes and works of art. In the centre is a mass of earth brought from the Holy Land. The Genoese Campo Santo contains an enormous wealth of sculpture. One Neapolitan cemetery (the Campo Santo Vecchio) differs widely from most others. It contains 366 deep pits, one of which is opened each day, and in it all the interments of the day take place. At night a funeral service is performed, and the pit is filled with earth and lime, not to be reopened till the year after. The Sicilian catacombs are also a kind of cemetery. Kensal Green Cemetery dates from 1832; other well-known London cemeteries are those of Highgate and Woking (1855), near Guildford, 7000 acres in area, with a crematory. The Dean Cemetery at Edinburgh, and the Necropolis of Glasgow, are notable; that of Glasnevin, outside of Dublin, is the most celebrated in Ireland. English cemeteries are usually divided into two portions—one consecrated for the burials of members of the Established Church, over whose remains the funeral service is read, and one unconsecrated, for the burials of dissenters. In the United States great pains are bestowed on the adornment of cemeteries. The most famous are Mount Auburn, near Boston, Greenwood in Brooklyn, and Laurel Hill, near Philadelphia. See BURIAL.