The essential constituents of every good hydraulic mortar, are caustic lime and silica ; and the hardening of this com pound under water consists mainly in a chemical combination of these two con stituents through the agency of the water, producing a hydrated silicate of lime. But such mortars may contain other bases be sides lime, as for example clay and mag nesia, whence double silicates of great solidity are formed ; on which account dolomite is a good ingredient of these mortars. But the silica must be in a pe culiar state for these purposes ; namely, capable of affording a gelatinous paste with acids ; and if not so already, it must be brought into this condition, by calcin ing it along with an alkali or an alkaline earth, at a bright red heat, when it will dissolve, and gelatinize in acids. Quart zose sand, however fine its powder may be, will form no mortar with lime • but if the powder .be ignited with the, lime, it then becomes ht for hydraulic work. Ground felspar or clay forms with slaked lime no water cement; but when they are previously calcined along with the lime, the mixture becomes capable of hardening under water.
The mastic called Hamelin' s, and so much employed in London, is composed of ground Portland stone (roe stone), sand, and litharge, in the proportion of 62 of the first, 35 of the second, and 3 of the third, in 100 parts ; but other propor tions will also answer the purpose. Chalk will not make a good mastic, being too compact to permit the air to insinuate be tween the pores, and to produce the con cretion of the linseed oil, with which the above mixture is worked up and applied. This mastic soon acquires great hard ness, and is totally impervious to water. The surface to which it is to be applied must be dry, and smeared over with lin seed oil. Considerable dexterity is re quired to make good work with it. The
fine dust of sandstone alone, mixed with 10 or 12 per cent. of litharge, and 7 per of linseed oil, forms an excellent mastic.
Limestone, which contains so much as 10 per cent. of clay, comports itself after calcination, if all the carbonic acid be ex pelled, just as pure limestone would do. When it is less strongly burned, it affords, however, a mass which hardens pretty speedily in water. If the argillaceous proportion of a marl amounts to 18 or 20 per cent., it still will slake with water, but it will absorb less of it, and forms a tolerably good hydraulic mortar, especial ly if a little good Roman cement be added to it. When the proportion of clay is 25 or 30 per cent. after burning, it heats but little with water, nor does it slake well, and must therefore be ground by stom pers or an edge millstone, when it is to be used as a mortar. This kind of marl yields commonly the best water cement without other addition. Should the quantity of clay be increased further, as up to 40 per cent., the compound will not bear a high or long-continued heat with out being spoiled for making hydraulic mortar after grinding to powder. When more strongly calcined, it forms a vitri form substance, and should, after being pulverized, be mixed up with good lime, to make a water mortar.
The Manlius or water limestones of the New-York system of rocks, furnishes in some of its courses, when quarried, a very good water cement, but the compo sition of the stone varies in different places. Some of this rock, taken from South Fayette, Seneca Co., N. Y., afforded to the Editor, on treatment by muriatic acid, the following composition: There was apparently in this stone too much silica, and too small a proportion of lime, to make a good hydraulic stone.