MORTAR, I lymtA cue. someti meg also callvd I :wean cement is the composition used ill walls under, or exposed to, the action of water, such as those of harbours, docks, &e. The material best adapted to the tnanufaeture of hydrenlic mortar is the poorer stills of limestone, such as contain from 8 to 2.5 per cent. offoreign matter, in siliettonagnesia, alumina, &e. These, when pulverized, absorb eater without swelling up, or heating, as a richer lime does. and though •a'cined, do not slake when moistened, but makes a paste e hieh hardens in a sew days wider water, though in the air it never acquires much solidity. These filets were discovered by 8meaton.
analvses of different hydraulic limestones, by Berthier. is given by lit'. Lire in his Diet. of Arts and anufactures.
" No. 1, is frem the flesh-water lime f irritation of Chateau Landon. near Nemours ; No. 2, the large-grained limestone of Paris; both of these afford a fat lime when burnt. 1)elormite alfords a pretty fat lime. though it contains 42 per cent. of carbonate of magnesia ; No. 3, is a limestone from the neighbourhood of Paris, which yields a paw lime, pos sessing no hydraulic property ; NI) 4, is the secondary Hine stone of Metz; No. 5, is the lime marl of Senonehes, near 1)renx ; both the latter Intro the property of hardening under water, particularly the last, which is much used at Paris on this account." All good hydraulic mortars must contain alumina and silica ; the oxides of iron and manganese, at one time con sidered essential, are rather prejudicial ingredients. By adding silica and alumina, or merely the former, in certain circumstances, to fat lime, a water-cement may be artificially formed ; as also adding to lime any of the tidlowing native productions, which contain silicates; puzzolana, trass or wrists, pumice-stone, basalt-tuff, slate-clay. Puzzidana is a vide.anic product, which forms hills of considerable extent to the south west of the Appenincs, in the district of the Pontine marshes, Viterbo, Bolsena, and in the region of Puzzuola, whence the name. A similar volcanic teffir is fi.mnd in many other parts of the world. Acei)rding to Berthier, the Italian pezzolana consists of silica ; 15-0 alumina ; lime ; magnesia; 1.4 potash ; 4.1 soda; 12 oxides
of iron and titanium ; 9-2 water ; in 100 parts.
The tufa stone, which when ground fortes trass. is com of 57-0 10 0 clay, 2-; lime, 1.0 magnesia, 7.0 potash, 1.0 stela, 5 oxides of iron and titanium, 9-0 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 }Imitations upon the banks of the Rhine, and at Manhchn itl Bavaria.
The flitter the lime, the less of it must be added to the ground puzzo1ana or tress, to form an hydraulic mortar ; the mixture should be made extemporaneously, and must at any rate be kept dry till abaft to be applied. Sometimes a pro portion of common sand mortar instead of lime is mixed vi lib the t•ass. \V hen the hydraulic cement ha dens to soon. as in 12 hours, it is apt to crack ; it is better when it takes 8 days to concrete. Through the agency of water, silicates of lime, alumina, (magnesia.) and oxide of iron, are formed, which assume a stony hardness.
Beside the above two volcanic products, other native earthy compounds are used hi making water cements. To this head belong all limestones which contain from '20 to 30 per cent. of clay and silica. By gentle calcination. a portion of the carbonic acid is expelled, and a little lime is combined with the clay, while a silicate of (lay and lime results, asso ciated with lime in a subcarbonated state. A Hine-marl con taining less clay will bear a stronger calcining heat without prejudice to its qualities as a hy draulie cement; but much also depends upon the proportions of silica present, and the physical structure of all the constituents." In England, what is commonly called renient-stone, is the substance generally used for Inaking this kind of mortar. It is found in great abundance on the coasts of Kent, in the isles of Sheppey. and Thanet, and various other places. The stones vary in size from that of a fist to a man's head, are of a yellow-gray or brown colour, interspersed with veins of talc spar. Their specific gravity is 2.50.