Mortar made of pure lime, sand, and water, may be employed in the linings of reservoirs and aqueducts, pro vided it have sufficient time to dry ; but it' the water he put in while it is wet, it will tall to pieces in a short time ; and, coysequendy, if the circumstances of the building he such as render it impracticable to keep out the water, it should not be used : there are, however, certain ingredients put into common mortar, by which it is made to set imme diately under water, or it the quick-lime contain in itself a certain portion of burnt clay, it will possess this property.
From the triable and crninbling nature of our mortar, a notion has been entertained by many persons, that the ancients possessed a process in its fitbrication, which has been lost at the present slay ; but the experiments of .Mr. Smeaton, Dr. Ilie-gins, and others, have shown this to be un founded, and that nothing more is wanting than that the chalk, lime-stone, or marble, be well burned, and thoroughly slacked immediately, and to mix it up with a certain propor tion of clean large-grain sharp sand, and as small a quantity of water as will be sufficient for working it ; to keep it a considerable time from the external air, and to beat it over again before it is used : the cement thus made will be suffi ciently hard.
The practice of our modern builders, is to spare their labour. and to increase the quantity of materials they pro duce, without any regard to its goodness : the badness of our IllOderll Mortar Is to be attributed both to the faulty nature of the materials. and to the slovenly and hasty methods of using it. This is remarkably instanced in London, where the lime employed is chalk lime, indiff.:rently burnt, con. vet ed flom Essex or Kent, a distance of ten or twenty miles, then kept many days without any precaution to pre vent the access of external air. Now, in the course of this time it has absorbed so much carbonic acid as nearly to lose its cementing properties, and though chalk lime is dually good with the hardest lime-stone, when thoroughly burned. ) et. by this treatment, when it is slacked, it falls into a thin powder, and the core or unburned lumps are ground down, and mixed lip in the mortar, and not rejected, as it ought to be.
The sand is equally defective, consisting of 'small globular grains, containing a large proportion of clay, which prevents it from drying. and attaining the necessary degree of hard ness. These materials being compounded in the most hasty manner, and beat tip with water in this imperfect state, can not fail of producing a crumbling and bad mortar ; and to complete the miserable composition, screened rubbish, and the scraping of roads, are thrown in, as substitutes tOr pure sand.
How very different was the practice of the Romans ! The lime which they employed was perfectly burnt, the sand sharp, clean, and large-grained ; when these ingredients were mixed in due proportion, with a small quantity of water. the mass was put into a wooden mortar, and beaten with a heavy wooden or iron pestle, till the composition adhered to the mortar : being thus fitr prepared, they kept it till it was at least three years old. The beating of mortar is of the utmost consequence to its durability, and it would appear that the effect produced by it, is owing to something more than a mere mechanical mixture. See MORTAR.
Water cements are such as are impervious to water : they are generally made of common mortar, or of pure lime and water, with the addition of some other ingredient to give it the property of hardening under water.
For this purpose there are several kinds of ingredients, as puzzolana, cellular basalt, or wakke, compact basalt, coal ashes, coal-cinders, wood-ashes, putnice-stonc, brick-dust, powder of quick-lime, forge-seales, roasted iron-ore, &c.
The cement employed by Mr. Smeaton, in the construction of the Eddystone lighthouse, was composed of equal parts, by measure, of slacked Aberthaw lime and puzzolana ; this proportion was thought advisable, as the building was exposed to the utmost violence of the sea : but for other aquatic works, as locks, basins, canals, &c.. a composition made of lime, puzzolana, sand, and water, in the following proportion, viz., two bushels of slacked Aberthaw lime, one bushel of puzzolana, and three of clean sand, has been found very effectual. It is well known, that sand and lime, mixed toge ther with care. will incorporate and form a mortar imper vions to water, and sufficient even for the linings of cisterns and reservoirs ; but then time mot-tar must be hardened before it is exposed to the water, or it will crumble to pieces ; and theref ire, it' the situation be such as to require the mortar to be dried in a certain time, the use of this cement must be Among the ancient nations, the Romans appear to have been the only people who practised building in water to any great extent, particularly in the sea. The discovery ofpuz•olana is attributed to the following circumstance. ainotg this great people. The Bay of Bake, like on• fashionable watering places. was the summer resort of all the %%ealthy in Rothe the inhabitants of this place did not content themselves with erecting their houses as near the shore as possible. bat they even constructed moles and small islands, on which they erected their summer-houses in the more she'tered parts of the bay. Ily the tbrtunate discovery of an earthy substance at the neighbouring town of Puteoli, they were enabled to build both expeditiously and securely in water. From this circumstance, the earth thus diseovered was called pulvis Puteolanus, "powder of Puteoli," " Puteolean powder," or, as it is now denominated. puzzolana, which is a mineral of a light, porous, friable nature. and I a red colour, supposed to be formed by of the volcanic ashes of Vesuvius, near to which mountain the town of Plite4 di is situated. The original material seems to he a ferruginous clay, which, baked and calcined by the force of volcanic tire, and mixed with common mortar, not only enablus it to acquire a remark ab'e hardness in the air, but to become as firm as stone under water. The only preparation which puzzolana undergoes, is that of pounding and sifting, by which it is redueed to a coarse powder; in this state it is beaten up with lime, either with or without sand, which limns a mass of remark able tenacity, that sets under water with great celerity, and at last acquires a strength and hardness equal to those of tree-stone.