Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/507

Rh CEMENTS.] BUILDING 459 tunnels, is composed of one part of moderately hydraulic lime, one part of coal &amp;lt;isb.es, one part of burned clay, and ig and two parts of sharp sand. The vitrified refuse of furnaces, .ders. called slag, and the scorito from the iron-works, have also been crushed and used instead of sand ; and with lime, slightly hydraulic, produce good mortar. The former is preferred to the latter, as having sharper and harder par ticles, and containing much less iron. Coal cinders have been used, and seem to have some hydraulic properties ; they should, however, bo employed with caution, for it is considered they make the -lime &quot;short.&quot; Wood cinders are too alkaline to be used with safety. Icanic The vitrified and calcined products of volcanoes make ulucts. most excellent materials for mortars, particularly where required to be eminently hydraulic. The principal of them .zuolana. is the pozzuolana, which abounds in Italy. It is called so from being found in great abundance at Pozzuoli, near Naples, and is, in fact, the basis of all the best Roman mortars, ancient as well as modern. It is usually sent to England from Civita Vecchia. It varies in colour from reddish brown to violet red, and is sometimes greyish; it has a roughly granulated appearance, and sometimes resembles a cinder in texture, and has frequently a spongy appearance. Acids have little effect on it, and it is not soluble in water. A similar earth is found in the centre of France. But one long known in this country comes from the village of Brohl, near Andernach, on the Rhine ; this is called tarrass or ss. trass. These materials have a wonderful effect in rendering even the rich limes eminently hydraulic, and in less propor tions improving the hydraulic limes. Vicat says, these mortars begin to set under water the first day, grow hard in the third, and in twelve months are as hard as the bricks themselves. The mixture of common lime with these materials, according to the French writers, should be 1 of pounded lime to 2 of pozzuolana, or to 2 of trass; or 1 of lime to 1 of sand and 1 of pozzuolana. In addition to the hydraulic limes, which have been thus described, there is a peculiar class of stones, which, when burned and pulverized, may be used as mortar, without admixture of sand or any similar substance ; and which will not only set rapidly under water, but will acquire an unusual degree of hardness and tenacity. These are ,-cnts. been the inventor ; at any rate, that gentleman took out a patent about sixty years ago for what he called Roman cement. His materials were the argillo-calcarcous nodules, or septaria, found in the Isle of Sheppey, and commonly called bald-pates. They contain about 70 per cent, of carbonate of lime, about 4 per cent, of oxide of iron, 1 8 per cent, of silica, and 6 or 7 per cent, of alumina. The process is simply to break the stones into small pieces, and burn them in running kilns with coal or coke ; they are then ground to a powder, and headed up into casks for use. The success of Parker s cement led to experiments in other places, and the same process was carried on with other nrgillo-calcareous nodules, as the septaria at Hawick ; those in Yorkshire, which produce the cement called Atkinson s ; and those in the Isle of Wight, which produce the Medina cement. Similar substances were also discovered, and the same processes carried on in France and in Russia. All these cement-stones effervesce with acids, and lose about one-third of their weight in burning. Parker considered that the more the stones were burned short of absolute vitrifica tion the better ; but this is not the practice in the present day, though, no doubt, sound in theory. When taken from the kiln these stones will not slake without being pulverized ; and if kept dry, and not exposed to the air, the cement will be good almost any length of time ; but it rapidly absorbs both water and carbonic acid if not carefully closed up, and falls back into a state of subcarbonate, from which it is said it may be recovered by fresh burning, but it is doubted whether it is ever so good as on the first calcination. The great utility of these cements, and the expense of Artificial obtaining the stone, induced manufacturers to endeavour cements, to discover some method of making an article by artificial means which should resemble the natural cements. Mr Frost seems to have been the first who attempted it on a large scale ; but though he was assisted by the science of General Pasley, the results did not come up to the expected standard. Of course, the object was to produce an argillo-calcareous substance containing the same chemi cal qualities as the natural nodules, which might bo burned in kilns as they are. The attempt to combine argil in the form of burned clay, to be mixed with lime instead of pozzuolana, had partially failed, as has been stated above. The experiments conducted by General Pasley, and by Vicat at Meudon in France, were all based on the principle of mixing together, in a mill, with a quantity of water, masses of chalk and clay, just as the brickmakcrs do for the production of malm bricks, but in the proportion of about four of the former to one of the latter. The fluid mixture is run out into shallow receivers, and when dry is cut into small blocks or lumps, and burned exactly as the natural nodules are. The difficulty was to give the materials the full degree of calcination short of vitrification. A successful result seems to have been at last attained by the inventors of the Portland cement, so called Portland from its near resemblance to Portland stone in its colour. cement - It not only possesses the property of setting more quickly, and has greater powers of cohesion than the natural cements, but it may be used with a superabundance of water in the form of grout, which they cannot be ; above all, it seems to resist the action of sea-water beyond all others ; and it is proof against water when used as a mortar in setting brickwork, and in the composition of concrete for founda tions. It also forms a very superior cement for plasterer s. work. A prepared patent carbonate (Westmacott s patent) Westma- is used in combination with chalk, grey, and all other cott&amp;gt;s limes. All the carbonic acid being removed from the lime T atent - in its burning, 75 per cent, of this acid is restored by its mixture with the prepared patent carbonate, and its indura tion immediately commences, instead of the lime gaining the carbonic acid by atmospheric influence through a lengthened period. It is used as a quick stucco for rapid plastering ; and as a carbonated lime for external use it is in the course of a few days converted into a stone mortar. Selenitic mortar is the name given to a composition Scott s lately invented by Major-General Scott. He was the first selenitic to observe, about eighteen years ago, that a limestone mortar - capable of conversion by burning into a hydraulic lime might furnish a good cement by simply allowing a small portion of sulphuric acid gas to pass into the kiln during the burning of the lime. Having found difficulty in carry ing out this process, he now mixes with the water used in the preparation of the mortar a small quantity of sulphate of lime (i.e., plaster of Paris, or gypsum) or green vitriol. It has the advantage, when used for plastering, of allowing the setting coat to be applied in forty-eight hours after the first coat has been put on. This mortar is said to save half the lime, is four times as strong, and sets in one quarter of the time required by common mortar. The lime will take six parts of sand, and is said to be an excellent substitute for Portland cement for concrete at less cost. Asphalt, or mineral pitch (see ASPHALT), has lately Bituminous come into extensive use for paving, for covering the backs cements, of arches, or rendering the walls of basements where wet is likely to soak through, also as a damp-course over the footings of walls to prevent the rise of damp, and for lining cisterns and tanks to prevent the escape of the fluid. The best qualities arc the Val de Travcrs and the Seyssel,
 * ural called natural cements. Mr Parker is supposed to have