CEMENTS. (FR., Ciment ; GER., Cement.) A cement is a substance, which, on being applied to the surfaces of two bodies, causes them to adhere strongly, when brought into contact. For the purposes of easy reference, a system of classi fication will be adopted in dealing with this subject, the subdivisions being Calcareous cements, Gelatinous cements, Glutinous cements, Resinous cementing compounds, and Nou-resinous cementing compounds. These will be followed by a number of compositions often erroneously termed cements, but more properly designated Lutes.
Calcareous comparatively few in number, these aro by far the most important cements. Their ingredients are all obtained from the mineral kingdom, and their efficacy depends chiefly upon the treatment of the rem materials, and the proportions of their admixture. The principal varieties are the following :— Mortar.—Mortar is composed of two essential ingredients, lime and sand, which are intimately mixed, in a fine state, by the agency of water. The sources of lime, in this country, are very abundant. It is obtained from the crystalline mai bles of the Metamorphic system ; from the coralline and shelly beds of the Silurian ; from the cornstones of the Old Red Sandstone ; from the coralline and shelly marbles of the Devonian ; from the coralline, encrinal, shclly, and fresh-water beds of the Carboniferous ; from the dolomites of the Permian ; from the muschelkalks and gypsums of the Trias ; from the oolites of the Jurassic ; from the shelly bands of the Wealden ; from the chalks of the Cretaceous ; from the gypseous and nummulitic strata of the Tertiary ; from the lacustrine marls of the Post-Tertiary. In composition, these rocks vary considerably, some being essentially carbonates, others sulphates, others again magnesian or dolomitic ; further, these may be argillaceous, bituminous, ferruginous, or siliceous.
The limestones best suited for the manufacture of common mortar are carbonates which are free from silica, alumina, and iron. These, after being quarried, and broken into pieces of a convenient size, are calcined in kilns constructed in a variety of ways. The kiln should be placed as near as possible to the quarries whence the stone is extracted, so as to avoid carriage. The longer the stone has been exposed to the nil., the less fuel will be consumed in driving off its inherent moisture, or " quarry water." Generally, ordinary pit coal is used in the calcination, one ton being necessary for every 4 or 5 tons of limestone ; for some kinds of stone, however, slaty, or shaly coals, are better adapted than superior coal, not only from their being cheaper in price, but also because they burn the stone more slowly and equally, at the same time keeping it " open," and preventing its slagging Etnd sintering. Th&e impure coals may cause a greater amount of kiln-dust ; but the lime will be
more free from cores and slags. When properly burnt, that is to say, when not slagged or coated with a siliceous glaze, from too sudden ignition, the limestone will have lost its carbonic scid, and will have become converted into caustic or " quick '' lime, protoxide of calcium. One hundred parts of raw limestone should yield fifty-six parts of quicklime.
The proper selection of the sand has as great an influence upon the mortar as has the character of the lime. The " sharper " and cleaner the sand, the better ; the finest mortar is made with clean pit or river sand ; the presence of earthy impurities will interfere with the chemical union of the lime and silica ; sea sand is sure to be impregnated with salt, which will subsequently cause deliquescence or efflorescence.
Before mixing the lime and sand together, to form mortar, the former must be " slaked " with water. One volume of water is added to three volumes of lime, when the latter " falls down," with violent evolution of heat, into a powder, whose colour will resemble the tint of the limestone employed, The more rapidly and completely it falls, the better the lime ; a lime that falls slowly and unequally will never be satisfactorily cohesive. When the lime is slaked, more water is applied, to convert it into a pulp or paste ; this paste, thoroughly incorporated with an equivalent of sand, constitutes common mortar. The proportion of sand will vary with the richness or " fatness " of the lime, which latter is dependent upon the purity of the original carbnnate of lime ; for poor limes, 2 or 2i parts of sand will suffice, while some fat limes will take 4 or 5 parts, and yield a superior mortar. The longer a lime remains slaked before being used, the stronger will be the mortar made with it. The admirable solidity of ancient buildings is entirely due to the fact that the slaked lime was covered with turf, and kept for a year, often even three years, before use. It is scarcely necessary to remark that mortar is distinguished from hydraulic cements, by its incapacity to set, or harden, under water.