MORTAR, HYDRAULIC, called also Roman Cement, is the kind of mortar used for building piers, or walls under or exposed to water, such as those of har bors, docks, &c. The poorer sorts of limestone are best adapted for this pur pose, such as contain from 8 to 25 per cent. of foreign matter, in silica, alumina, magnesia, &c. These, though calcined, do not slake when moistened ; but if pulverized they absorb water without swelling up or heating, like fat lime, and afford a paste which hardens in a few days under water, but in the air they never acquire much solidity. Smeaton first discovered these remarkable facts, and described them in 1759.
The following analyses of different hy draulic limestones, by Berthier, merit confidence : All good hydraulic mortars must con tain alumina and silica ; the oxides of iron and manganese, at one time consid ered essential, are rather prejudicial in greaients. By adding silica and alumi na, or merely the former, in certain cir cumstances, to fat lime, a water-cement -lay be artificially formed ; as als.' by adding to lime any of the following na tive productions, which contain silicates ; puzzolana, tress or tarrass, pumice-stone, basalt-tutf, slate-clay. Puzzolana is a vol canic product, which forms hills of con siderable extent to the south-west of the Apennines, in the district of Rome, the Pontine marshes, Viterbo, Bolsena, and in the Neapolitan region of Puzzuolo, whence the name. A. similar volcanic tufa is found in many other parts of the world. According to Berthier, the Ita lian liouzzolana consists of 44.5 silica ; 15.0 alumina ; 8.8 lime ; 4.7 magnesia ; P4 potash ; 4.1 soda ; 12 oxydes of iron and titanium ; 9.2 water ; in 100 parts. The tufa stone, which, when ground, forms tress, is composed. of 57.0 silica, 16.0 clay, 2.6 lime, PO magnesia, 7.0 pot ash, PO soda, 5 oxides of iron and tita nium, 9.6 water. This tuff is found abundantly filling up valleys in beds of 10 or 20 feet deep, in the north of Ireland, among the schistose formations upon the banks of the Rhine, and at Manheim in Bavaria.
The fatter the lime, the less of it must be added to the ground puzzolana or tress, to form a hydraulic mortar ; the mixture should be made extemporane ously, and must at any rate be kept till about to be applied. Sometimes a pro portion of common sand mortar instead of lime is mixed with the tress. When the hydraulic cement hardens too soon, as in 12 hours, it is apt to crack ; it is better when it takes 8 days to concrete. Through
the agency of the water, silicates of lime, alumina (magnesia), and oxyde of iron are formed, which assume a stony hard ness.
In England the stones are calcined in shaft-kilns, or sometimes in mound-kilns, then ground, sifted, and packed in casks. The color of the powder is dark-brown red. When made into a thick paste with water, it absorbs little of it, evolves hard any heat, and soon indurates. It is mixed with sharp sand in various pro portions, immediately before it ; and is employed in all marine and river embankments, for securing the seams of stone or brick floors or arches from the percolation of moisture, and also for facing walls to protect them from damp.
The cement of Pouilly is prepared from a Jurassic (secondary) limestone, which contains 89 per cent. of silica, with alu mina, magnesia, and iron oxide. Vicat forms a factitious Roman cement by mak ing bricks with a pasty mixture of 4 parts of chalk, and 1 part of dry clay,, drying, burning, and grinding them. River sand must be added to this powder ; and even with this addition, its efficacy is some what doubtful; though it has, for want of a better substitute, been much em ployed at Paris.
The cement of Dihl consists of porce lain or salt-glaze potsherds ground fine, and mixed with boiled linseed oil.
All sorts of lime are made hydraulic, in the humid way, by mixing slaked lime with solutions of common alum or sul phate of alumina ; but the best method consists in employing a solution of the silicate of potash, called liquor of flints, or soluble glass, to mix in with the lime, or lime and clay. A hydraulic cement may also be made which will serve for the manufacture of architectural orna ments, by making a paste of pulverized chalk, with a solution of the silicate of potash. The said liquor of flints will like wise give chalk and plaster a stony hard ness, by merely soaking them in it after they are cut or moulded toa proper shape. On exposure to the air,' they get pro,gres sively indurated. Superficial hardness may be readily procured by washing over the surface of chalk, Sze., with liquor of flints, by means of a brush. This method affords an easy and elegant method of giving a stony crust to plastered walls and ceilings of apartments ; as also to statues and busts, cast in gypsum, mixed with chalk.