CEMENT in building, is used to denote any kind of mortar of a stronger kind than ordinary. The cement commonly used is of two kinds; hot, and cold. The hot cement is made of rosin, bees wax, brick-dust, and chalk, boiled together. The bricks to be cemented are heated, and rubbed one upon another, with cement between them. The cold cement is that above described for cementing china, &c. which is sometimes, though rarely, employed in building.
The ruins of the ancient Roman buildings are found to adhere so strongly, that most people have imagined the ancients were acquainted with some kind of mortar,
Cement. mortar, which, in comparison of ours, might justly be called cement; and that to our want of knowledge of the materials they used, is owing the great inferiority of modern buildings in their durability. In 1770, one M. Lorio, a Frenchman, pretended to have discovered the secret of the ancient cement, which, according to him, was no more than a mixture of powdered quick-lime with lime which had been long slaked and kept under water. The slacked lime was first to be made up with sand, earth, brick-dust, &c. into mortar after the common method, and then about a third part of quick-lime in powder was added to the mixture. This produced an almost instantaneous petrification, something like what is called the setting of alabaster, but in a much stronger degree; and was possessed of many wonderful qualities needless here to relate, seeing it has never been known to succeed with any other person who tried it. Mr Anderson, in his essays on agriculture, has discussed this subject at considerable length, and seemingly with great judgment. He is the only person we know, who has given any rational theory of the uses of lime in building, and why it comes to be the proper basis of all cements. His account is in substance as follows:
Lime which has been slaked and mixed with sand, becomes hard and consistent when dry, by a process similar to that which produces the natural stalactites in caverns. These are always formed by water dropping from the roof. By some unknown and inexplicable process of nature, this water has dissolved in it a small portion of calcareous matter in a caustic state. As long as the water continues covered from the air, it keeps the earth dissolved in it; it being the natural property of calcareous earths, when deprived of their fixed air, to dissolve in water. But when the small drop of water comes to be exposed to the air, the calcareous matter contained in it begins to attract the fixable part of the atmosphere. In proportion as it does so, it also begins to separate from the water, and to reassume its native form of limestone or marble. This process, Mr Anderson calls a crystallization; and when the calcareous matter is perfectly crystallized in this manner, he affirms that it is to all intents and purposes limestone or marble of the same consistence as before: and "in this manner, (says he,) within the memory of man, have huge rocks of marble been formed near Matlock in Derbyshire." If lime in a caustic state is mixed with water, part of the lime will be dissolved, and will also begin to crystallize. The water which parted with the crystallized lime, will then begin to act upon the remainder, which it could not dissolve before; and thus the process will continue, either till the lime be all reduced to an effete, or, (as he calls it) crystalline state, or something hinders the action of the water upon it. It is this crystallization which is observed by the workmen when a heap of lime is mixed with water, and left for some time to macerate. A hard crust is formed upon the surface, which is ignorantly called frosting, though it takes place in summer as well as in winter. If therefore the hardness of the lime, or its becoming a cement, depends entirely on the formation of its crystals, it is evident, that the perfection of the cement must depend on the perfection of the crystals,
and the hardness of the matters which are entangled among them. The additional substances used in making of mortar, such as sand, brick-dust, or the like, according to Mr Anderson, serve only for a purpose similar to what is answered by sticks put into a vessel full of any saline solution, namely to afford the crystals an opportunity of fastening themselves upon it. If therefore the matter interposed between the crystals of the lime is of a friable, brittle nature, such as brick-dust or chalk, the mortar will be of a weak and imperfect kind; but when the particles are hard, angular, and very difficult to be broken, such as those of river or pit-sand, the mortar turns out exceedingly good and strong. Sea-sand is found to be an improper material for mortar, which Mr Anderson ascribes to its being less angular than the other kinds. That the crystallization may be the more perfect, he also recommends a large quantity of water, that the ingredients be perfectly mixed together, and that the drying be as slow as possible. An attention to these circumstances, he thinks, would make the buildings of the moderns equally durable with those of the ancients; and from what remains of the ancient Roman works, he thinks a very strong proof of his hypothesis might be adduced. The great thickness of their walls necessarily required a vast length of time to dry. The middle of them was composed of pebbles thrown in at random, and which have evidently had mortar so thin as to be poured in among them. By this means, a great quantity of the lime would be dissolved, and the crystallization performed in the most perfect manner; and the indefatigable pains and perseverance for which the Romans were so remarkable in all their undertakings, leave no room to doubt that they would take care to have the ingredients mixed together as well as possible. The consequence of all this is, that the buildings formed in this manner are all as firm as if cut out of a solid rock; the mortar being equally hard, if not more so, than the stones themselves. For the proper proportion of lime and sand requisite in building, see the article MORTAR.