Cementite

“ But, generally speaking, in steel concrete the cost of the cement is but a small item of the whole expense, and it is worth while to be generous with it. If It is used in piles or structures where it is likely to be bruised the proportion of cement should be increased. The mixing and laying should all be done very thoroughly; the concrete should be rammed in position, and any old surface of concrete which has to be covered should be cleaned and coated with fresh cement. ”

“ The burning drives off the small amount of combined water and all the carbon dioxide from the stone, leaving the lime and magnesia as oxides, while the grinding to a powder puts it into the best possible condition for mixing with sand and gravel, and moistening to form a mortar. Artificial cement consists of about sixty-two per cent of lime mixed with silica and silicate of alumina in nearly the same proportions as those found in the cement stone, and it is free from magnesia. ”

“ Three of the most common cements are iron oxide, calcium carbonate, and silicon dioxide, although a number of other materials also serve as cements.
Dissolved rock materials come out of solution not only to serve as cementing agents but to form the chief mineral of some sedimentary rocks as well. Sedimentary rocks of this kind form mostly in lakes and seas into which much dissolved material is carried by rivers. When the dissolved material comes out of solution, it is said to be precipitated and the mineral sediments it forms are the chemical sediments. ”

“ Where can better concrete be found than that which has set under water? A certain definite amount of water is necessary and sufficient to hydrate the cement; less than that amount will be detrimental, while an excess can do no harm, provided, as before mentioned, that it does not wash the mortar from the stone. Again, dry concrete is apt to be very porous, which in certain positions is a very grave objection to it—this, not only from the fact of its porosity, but from the liability to disintegration from water freezing in the crevices. ”

“ In the manufacture of natural cement the raw material generally used is some stone in which the carbonate of lime, silica, and alumina are present in more or less correct proportions, while in the manufacture of the artificial cement the raw material used consists of the essential ingredients, each in a comparatively pure state, thoroughly mixed in theoretically the correct proportions. It is due to this fact that artificial or Portland cement is not only much superior to natural cements, but that it is much more uniform in its quality. ”

“ Indeed, it is more easily done than sand, as the water flows through the larger voids in the gravel more readily than through the voids in the sand. Dust may be left in the crushed stone without fear of its interfering with the strength of the cement, but care should be taken to see that such dust is distributed evenly through the whole mass, and when dust is found in stone, slightly less sand should be used than ordinarily. ”

“ Cement for uniting metals. A strong cement, which hardens rapidly, is made by stirring the finest whiting in a solution of soda-glass of 33° B., made so as to form a plastic mass. This can be readily colored to any desired shade. The addition of sifted sulphide of antimony gives a black cement, which by polishing acquires a metallic lustre; iron filings render it grayish-black; zinc dust turns it green, but after polishing, it appears like metallic zinc, and may be employed for the permanent repair of zinc ornaments, etc. Carbonate of copper imparts a light green shade. ”

“ With the growth of engineering in the early part of the 19th century arose a great demand for hydraulic cement. The supply of materials containing naturally suitable proportions of calcium carbonate and clay being limited, attempts were made to produce artificial mixtures which would serve a similar end. Among those who experimented in this direction was Joseph Aspdin, of Leeds, who added clay to finely ground limestone, calcined the mixture, and ground the product, which he called Portland cement. ”

“ But where the concrete or mortar is not well made and is porous, the continual passage of water through it will gradually break up and dissolve away the calcareous constituents of the cement until its strength is utterly destroyed. This destructive action is increased if the water contains sulphates or magnesium salts, both of which act chemically on the calcareous constituents of the cement. As sea-water contains both sulphates and magnesium salts, it is especially necessary in concrete for harbour work to take every care to produce an impervious structure. ”

“ The common cement used in building, which is called mortar, is made of sand and quicklime mixed with water. Roman and Portland cements consist of certain kinds of clay burnt and ground. Plaster of Paris forms a useful cement, it is to be mixed with water to the consistence of cream, and it hardens in a few minutes. Glue is an invaluable cement for wood and many other purposes, it also joins any kind of broken glass or china that will not have to be subjected to much wetting. Shellac dissolved in spirit is a useful cement ”

“ Improvements in the manufacture of hydraulic cements have given an impetus to the use of concrete, but its use is by no means of recent date. It is no uncommon thing in the taking down of heavy walls several centuries old to find that the method of building was to carry up face and back with rubble and stiff mortar, and to fill the interior with bowlders and gravel, the interstices of which were filled by grouting—the whole mass becoming virtually a monolith. Modern quick-setting cement accomplishes this object within a time consistent with the requirements of modern engineering works ”

“ Soft, spongy foundations of natural cement or Portland cement of inferior quality are not only an inadequate support for the yielding surface, but they are easily penetrated by water from below and act as conveyors of water, while a sound and firm foundation of the best Portland cement concrete not only presents an unyielding support to the surface, but also, when properly set and dried before the surface is laid, keeps the surface as free as is possible under the circumstances from ground water, and thus contributes to the longevity of the surface. ”

“ The ideal mixture is one in which all the spaces or voids between the grains of sand are filled with the cement and all the voids in the gravel are filled with the cement-sand mortar. This perfection is seldom attained, because the voids in each lot of gravel and sand vary slightly, and in order to be absolutely safe a little more sand and cement than will just fill the voids are used.
The strongest concrete is not required in every structure, and, in many instances, the cost of it would be unwarranted. ”

“ The amount required to combine chemically with the cement is about 16% by weight, but in practice much more than this is used, because of loss by evaporation, and the difficulty of ensuring that the water shall be uniformly distributed. If the situation is cool, the stone hard, and the concrete carefully rammed directly it is laid down and kept moist with damp cloths, only just sufficient to moisten the whole mass is required. On the other hand, water should be given generously in hot weather, also when absorbent stone is used or when the concrete is not rammed. ”

“ There are various minor external causes for the failure and ultimate destruction of cement mortar and concrete, but their discussion is a matter for the specialist. Failure from inherent vice in the cement has been already touched on; it can always be traced to want of skill and care in manufacture.
Calcium Sulphate Cements.—Under this term are comprehended all cements whose setting properties primarily depend on the hydration of calcium sulphate. They include plaster of Paris, Keene’s cement and many variants of these two types. The raw material is gypsum (q.v.) ”

“ Inasmuch as the variation in sizes of the individual particles of rounded gravel is such that a dense concrete results naturally, it is quite reasonable both to increase the amount of stone and decrease the amount of cement if that variation in size seems to be one which will produce a dense mixture. Thus one cubic yard of stone, one-third cubic yard of sand, and four bags of cement may be used and will, under favorable conditions, result in a good concrete. ”

“ Wilhelm Michaelis has found that Portland cement does not resist the chemical action of such water so well as do Roman cement and the hydraulic cements. The soluble sulphates in the sea water appear to enter into a substitution combination with the lime which exists in the cement in a free state or is liberated in the hardening, and it is converted into a sulphate, while disintegration ensues. In Roman cement the lime exists in combination, and there is no inclination toward the formation of a sulphate, and hydraulic limes resemble Roman cement in physical qualities. ”

“ The foundations of all important structures, in situations where they can not rest directly upon solid rock, owe their strength to cement. They are usually made of concrete, cement mixed with sand or gravel, and they are often strengthened by iron beams so as to bind the whole into one continuous mass. Tunnels under rivers, sewers, cable trenches, and all the numerous subways of our large cities, are either concrete or masonry laid in cement mortar. Their strength, again, is the strength of the cement used. ”

“ These impurities that are inherent in the nature of the materials from which the natural cement is made and also in the process of manufacture that is of necessity followed, result in a cement that can be made and sold at a less price than Portland cement and that is inferior to it for many purposes, while, on the other hand, for a great many purposes natural cements have been found to answer every requirement and are made and used in enormous quantities. ”

“ The moral of that is to keep your cement in a dry place.
The second substance needed in concrete is broken stone or gravel. Of course a hard rock must be selected, such as granite or trap rock. Limestone calcines in a heat exceeding 1000° F., and therefore it cannot be used in fireproof construction. Soft rock, like slate or shale or soft sandstone, will not answer because it is not strong enough. Gravel is always hard. If you look at a cut in a gravel bank, you will usually see strata of sand and then strata of rounded pebbles of different sizes. The sand was once an ancient sea beach ”

“ This clinker is ground until it is reduced to a powder, and this is the Portland cement. Portland cement is generally mixed with an aggregate to produce strength and this aggregate is usually stone, gravel or sand. The third material needed to complete the combination is water.
SUCCESS IN THE USE OF CEMENT depends largely upon cement that is fresh. Cement is very sensitive to moisture and if kept where fogs, dew or moisture of any nature is absorbed into it, the first set or “hydration” takes place and destroys its use for fine handicraft. ”

“ The most characteristic constituent is the oxide of iron, which gives the cement a reddish colour, and the presence of manganese also differentiates Roman from Portland cement, which rarely contains appreciable quantities of that element. The high percentage of alumina causes the cement to be quick-setting, and it becomes hard in about five minutes. It resists the action of water, salt or fresh, very well, and is therefore useful in situations where the work is likely to be submerged immediately after it has been put in place. ”

“ Mahogany cement, for filling up cracks in wood, consists of 4 parts of beeswax, 1 of Indian red and yellow-ochre to give colour. Cutler’s cement, used for fixing knife-blades in their hafts, is made of equal parts of brick-dust and melted rosin, or of 4 parts of rosin with 1 each of beeswax and brick-dust. For covering bottle-corks a mixture of pitch, brick-dust and rosin is employed. A cheap cement, sometimes employed to fix iron rails in stone-work, is melted brimstone, or brimstone and brick-dust. ”

“ This quality and the rapid setting, tenacity, and strength of brickwork in cement make it a most valuable material to use in those buildings or parts of a building where great steadiness and strength are wanted, and in sewage and dock work, where there is water to contend with. A good many cements made from natural stones used to be employed, such as Medina, Harwich, Atkinson's, or Roman cement. ”

“ This cement will stand heating to the boiling-point of mercury.
Linseed oil, zinc and manganese cement. Knead 10 parts of pyrolusite, 20 of zinc white, and 40 of clay with sufficient boiled linseed oil (not exceeding 7 parts) to a plastic mass. This cement will stand a somewhat higher temperature than the preceding one.
Cements resisting very high temperatures. I. Clay 100 parts, powdered glass 2.
The glass melts on exposure to great heat and slags the clay to a hard mass. The same effect is produced by adding small quantities of soda and borax to the clay. ”

“ Cement is more expensive than brick. The surface to be covered should, first, be levelled, then saturated with water; after which is laid a three-inch bed of cement concrete, made of gravel, sand, and cement in proper proportions. Upon this is placed a three-fourth-inch layer of cement mortar. Ordinary American, hydraulic cement may be used for concrete, but for the three-fourth-inch layer nothing but best Portland cement should be considered. Sometimes the cement work in the cellar is done by the plasterer. Outside cement work for walks requires special skill. ”