CEMENT. The word cement may be defined as any glutinous or other substance, capable of uniting bodies in close cohesion, or making them adhere firmly together, so as to form of the whole one solid mass—as mortar, glue, solder, asphaltum, &c. Ceriteuts are of various kinds, lout, for conve nience, may be divided into NATURAL and ARTIFICIAL.
Natural cements are found in Russia, France, and other countries, and indeed the substance so extensively used in England, and very improperly termed Roman cement, is nothing than a natural cement, resulting fmnr a slight calcination of a calcareous mineral, containing about 31 per cent of ochreous clay, and a few hundredths of carbonate of magnesia and manganese. It may be observed, that when the proportion of clay in calcareous minerals exceeds 27 to 30 per cent, it is seldom converted into lime by calcination, but they then furnish a kind of natural cement, which can be used by pulverizing it, and kneading it with water.
There are some natural cements which do not set in water for many days, but these are now rarely used ; those which solidify quickly, being generally preferred. The adhesive power of some cements in the open air, is very remarkable ; and we have ourselves seen 33 bricks stuck to one another by Roman cement, and projecting at right angles from the side of a wall.
The argillaceous limestones, and the artificial mixtures of pure lime and clay, in the proportions requisite to con stitute hydraulic lime by the ordinary calcination, become natural or artificial cements, when they have been subjected merely to a simple incandescence, kept op for some minutes.
Calcareous cements may be classed according to the three following divisions, namely, simple calcareous cements, water cements, maltlta, and mastics.
lst, Simple calcareous cements include those kinds of mortar which are employed in land-building, and consist of lime, sand, and fresh water.
Calcareous earths are converted into quick-lime by burn ing, which being wetted with water falls into an impalpable [so vder, with great extrication of heat : and if in this state it is beat with sand and water, the mass will concrete and become a stony substance, which will be more or less perfect according to its treatment, or to the quality and quantities of ingredients.
When carbonated lime has been thoroughly burned, it is deprived of its water, and all, or nearly all, of its carbonic acid ; much of the water, during the process of calcination, being carried elf in the form of steam.
Lime-stone loses about g of its weight by burning, and when fully burned, falls freely, and will produce something more than double the quantity of powder, or slacked lime, in measure. that the burnt lime-stone consisted of Quick-linse, by being exposed to the air, absorbs carbonic acid with greater or less rapidity, as its texture is less or more hard ; and this, by continued exposure, becomes unfit for the composition of mortar ; hence it is that quick-lime made of chalk, cannot be kept for the same length of time between the burning and slacking, as that made from stone.
Marble. chalk, and limestone, with respect to their use in cements, may be divided into two kinds—simple lime-stone, or pure carbonate of lime, and argillo-ferruginous lime, which contains from to of clay, and oxide of iron, previous to calcination : there are no external marks by which these can be distinguished from each other, but whatever may have been the colour its the crude state, the former, when calcined, becomes white, and the latter more or less of an ochrey tinge. The white kinds are more abundant, and when made into mortar will admit of a greater portion of sand than the brown, consequently are more generally employed in the composition of mortar ; but the brown lime is by fir the beg for all kinds of cement. If white, brown, and shell lime, recently slacked, he separately heat up with a little water into a stiff paste, it will he found that the white lime, whether made from chalk, Hine-stone, or marble, will not acquire any degree of hardness; the brow» lime will become considerably indurated ; and the shell lime will be concreted into a firm cement, which, thongh it will fill to pieces in water, is well qualified for interior fmishings, where it can be kept dry.