Class XXVII.]
PASSIVE AND INCRUSTED CEMENTS.
times as great as that of any natural kind. Portlandcement makes an admirable and most powerful concrete,the proportion of cement required being only a tenth ortwelfth part *
It is not difficult to procure artificially mixtures oflimestone and clay, which are less costly than the naturalkinds, though not equal in value.
On the Continent similar material is obtained in variouslocalities, of which we may mention Boulogne, Vassy,Pouilly in Burgundy, and some parts of Russia . In somecases these cements appear to owe their hydraulic qua-lities to the presence of silicate of lime and* silica ratherthan clay.
Certain substances also exist in nature, and can beimitated by art which, on being mixed with commonquick-lime, form hydraulic mortar. These are calledpuzzuolana, and consist generally of volcanic ash, ofwhich a vast quantity is found in the modern volcanicdistricts of South Italy , Greece , &c., and in the extinctvolcanic districts of the Rhine , Auvergne , &c., but similarsubstances are found in our own country. Mortar madewith puzzuolana has extraordinary hardness and dura-bility, this substance having a great affinity for lime andhydrate of lime, and forming a perfectly insoluble com-pound.
The cements hitherto described are composed essen-tially of carbonate of lime, from which both the carbonicacid and water are driven off by burning in a kiln. An-other group, however, exists which we may call plasters,of which the base is sulphate of lime, or gypsum, usuallyfound in combination with nearly 24 per ceut. of water,and often containing carbonate of lime and clay in smallproportions. On being burnt at a low temperature thismineral simply parts with its water, which it absorbsagain readily, and with great rapidity, on subsequent ex-posure; but if a greater heat is applied it melts, and whencooled assumes a new and permanent form, not altered by-wetting.
The property possessed by gypsum, of parting readilywith its water of solidification, and re-absorbing it asreadily, is the basis of a vast variety of uses to whichthis mineral is applied. When mixed with water mecha-nically, after being calcined, a part of the water entersinto combination with the powder, and forms a truehydrous sulphate of lime, which collects into minutecrystals, fitting into each other, and the mass becomesrapidly solid, although not very hard. If the solidifica-tion takes place in a mould, the most minute cavities arefound to have been filled, and the result is a perfect cast,owing to the expansion that takes place while solidifica-tion goes on. Thus the calcined gypsum, mixed withwater, becomes an admirable material for casting, and isgreatly used for this purpose. Vast quantities of gypsum,of various degrees of purity, are found in the immediatevicinity of Paris , and thus the powder obtained afterburning is generally designated Plaster of Paris . It isemployed very extensively in constructions for variouskinds of internal work. Stucco is a combination of thesame substance, with a solution of gelatine or strongglue. This mixture dries more slowly than that madewith Mater, but is more durable.
Sulphate of lime is the basis of all the cements knownas Keene's, Martin's, Parian, and some others, but inthese the plaster in the state of fine powder is throwninto a vessel containing a saturated solution of alum, sul-phate of potash, or borax. After soaking for some hoursit is removed and air-dried, and subsequently rebaked ata browuish red heat. When taken out of the oven it isonce more reduced to a fine powder, and carefully sifted,alter which it is fit for use, but when slacked a solutionof alum is employed instead of pure -water. When boraxis used, the plaster is called Parian, but in the other caseit forms Keene’s cement. The kind called Martin’s cementis made with pearlash as well as alum, and is baked at amuch higher heat than the rest.
The raw material for these various cements, of whichthe consumption is now extremely great, is obtained from
* See'an account of experiments on the strength of Port-land cement, p. 587.
Derbyshire , Nottinghamshire , and Cumberland, besidesthe neighbourhood of Paris . The quality differs much,but for the finer kinds of cement the most crystalline isconsidered the best. Vast quantities of gypsum exist inTuscany , much of it in the crystalline form called ala-baster. The English gypsum is sometimes in beds ofvery variable thickness, sometimes in lenticular masses,and occasionally in veins. It occurs extensively in thenew red sandstone, but also in the clay deposits of theoolitic period. The harder kinds make the soundestplaster, containing generally a little lime, hut the purerkinds are much whiter when burnt. In France thegypsum is burnt in open kilns, and is thus discoloured,but in England this is avoided, the fuel not being allowedto come in contact with the plaster.
A highly ornamental material, consisting of a coatingof plaster mixed with alum and colour into a paste, andafterwards beaten on a prepared surface with fragmentsof marble, See., has long been known under the name ofscagliola , and is greatly used as an excellent and econo-mical means of imitating the finer kinds of marble, thematerial being as hard as marble, very durable, cold tothe touch, and taking a perfect polish. The name seag-liola is derived from the Italian, where the process is saidto have been invented more than two centuries ago, butit is now very extensively used for decorative purposes inEngland. The cement is prepared from the finest gypsum,broken up before calcining, and afterwards reduced to afine powder and passed through a sieve. It is then mixedwith aluminous matter, and isinglass, and also withcolouring matter, and is afterwards made up with alum;and as it is generally made use of only where the morebeautiful and veined marbles are to be imitated, as manydifferent colours and shades of colour must be mixed upseparately as there are in the kind of marble to be repre-sented. Thus prepared, it is ready to be laid on to "thesurface intended to receive it, which has a rough coatingof lime and hair already prepared.
The different colours having to be laid on and mixedby the hand, the -work somewhat resembles that of thefresco painter, everything depending on the skill of theoperator in imitating the style, beauty, and reining of theoriginal. When the cement is laid on and has hardened,the surface is prepared for polishing by rubbing it withpumice-stone, and cleansing with a wet sponge. It isthen polished by nibbing, first with tripoli and charcoal,then with felt dipped in tripoli and oil, and lastly with oilalone. A durable lustre is thus obtained, equal to that ofmarble.
With this general outline of the nature and use of thevarious cements and plasters in -which carbonate and sul-phate of lime are the essential ingredients, we pass on nowto the different exhibitors in these materials. There areseveral in each, chiefly from our own country and Italy ,so far as larger and more ornamental specimens are con-cerned. They show some novelty and much excellentwork, and form a very interesting and important groupof objects. We commence with those who chiefly exhibitthe cements properly so called, and the larger groups ofobjects, as applied rather to construction than decoration,and shall afterwards proceed to the plasters and scag-liolas.
a. Hydraulic Cements.
Messrs. Robins, Aspdix, and Co. (Outside, West, 5,and ClassXXVII., 103, p. 772) are exhibitors of a gigauticslab of Portland cement, measuring 20 feet by 12, and 10inches thick, weighing 15 tons; numerous blocks ofcement and concrete, proved to various pressures, up to154 tons, showing the strength to be greater than that ofPortland stone; of bricks cemented together and placedso as to give a pressure of 3 tons on the first brick ; andof several other similar illustrations.
It has already been said that Portland cement is ahydraulic mortar, made of a mixture of chalk anda peculiar river silt. In working it is sometimes mixedwith sand and even with broken brick, forming a kindof concrete of extraordinary strength. It receives itsname from its peculiar colour, which approaches that ofPortland stone, and not from being made from that stone,