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 harbours, docks, . The poorer sorts of limestone are best adapted for this purpose, such as contain from 8 to 25 per cent. of foreign matter, in silica, alumina, magnesia, . 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 hydraulic limestones, by Berthier, merit confidence :— | No. 1. | No. 2. | No. 3. | No. 4. | No. 5. | | A. Analyses of limestones. | | | | | | | Carbonate of lime | 97·0 | 98·5 | 74·5 | 76·5 | 80·0 | | Carbonate of magnesia | 2·0 | — | 23·0 | 3·0 | 1·5 | | Carbonate of protoxide of iron | — | — | — | 3·0 | — | | Carbonate of manganese | — | — | — | 1·5 | — | | Silica and alumina | | | | | | - | 15·2 | | | - | 18·0 | | Oxide of iron | 1·0 | 1·5 | 1·2 | | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | | B. Analyses of the burnt lime. | | | | | | | Lime | 96·4 | 97·2 | 78·0 | 68·3 | 70·0 | | Magnesia | 1·8 | — | 20·0 | 2·0 | 1·0 | | Alumina | 1·8 | 2·8 | 2·0 | 24·0 | 29·0 | | Oxide of iron | — | — | — | 5·7 | — | | 100·0 | 100·0 | 100·0 | 100·0 | 100·0 | No. 1. is from the fresh-water lime formation of Château-Landon, near Nemours; No. 2. the large-grained limestone of Paris; both of these afford a fat lime when burnt. Dolomite affords 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 poor lime, po e ing no hydraulic property; No. 4. is the secondary limestone of Metz; No. 5. is the lime marl of Senonches, near Dreux; both the latter have 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 considered e ential, 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 by adding to lime any of the following native productions, which contain silicates; puzzolana, tra or tarras, pumice-stone, basalt-tuff, slate-clay. Puzzolana is a volcanic product, which forms hills of considerable extent to the south-west of the Appenines, in the district of Rome, the Pontine marshes, Viterbo, Bolsena, and in the Neapolitan region of Puzzuoli, whence the name. A similar volcanic tufa is found in many other parts of the world. According to Berthier, the Italian puzzolana consists of 44·5 silica; 15·0 alumina; 8·8 lime; 4·7 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 forms tra , is composed of 57·0 silica, 16·0 clay, 2·6 lime, 1·0 magnesia, 7·0 potash, 1·0 soda, 5 oxides of iron and titanium, 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 Monheim in Bavaria. The fatter the lime, the le of it must be added to the ground puzzolana or tra , to form a hydraulic mortar; the mixture should be made extemporaneously, and must at any rate be kept dry till about to be applied. Sometimes a proportion of common sand mortar instead of lime is mixed with the tra . 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 oxide of iron are formed, which a ume a stony hardne . Besides the above two volcanic products, other native earthy compounds are used in 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 clay and lime results, a ociated with lime in a sub carbonated state. A lime-marl containing le clay will bear a stronger calcining heat without prejudice to its qualities as a hydraulic cement; but much also depends upon the proportion of silica present, and the physical structure of all the constituents. The mineral substance most used in England for making such mortar, is vulgarly called cement-stone . It is a reniform limestone, which occurs distributed in single nodules or rather lenticular cakes, in beds of clay. They are mostly found in those argillaceous strata which alternate with the limestone beds of the oolite formation, as also in the clay strata above the chalk, and sometimes in the London clay. On the coasts of Kent, in the isles of Sheppey and Thanet, on the coasts of Yorkshire, Somersets hire, and the Isle of Wight, ., these nodular concretions are found in considerable quantities, having been laid bare by the action of the sea and weather. They were called by the older mineralogists Septaria and Ludus Helmontii (Van Helmont’s coits). When sawn acro , they show veins of calc-spar traversing the siliceous clay, and are then sometimes placed in the cabinets of virtuosi . They are found also in several places on the Continent, as at Neustadt-Eberswalde, near Antwerp, near Altdorf in Bavaria; as also at Boulogne-sur-mer, where they are called Boulogne-pebbles ( galets ). These nodules vary in size from that of a fist to a man’s head, they are of a yellow-gray or brown colour, interspersed with veins of calc-spar, and sometimes contain cavities bestudded with crystals. Their specific gravity is 2·59. Analyses of several cement-stones, and of the cement made with them :— | No. 1. | No. 2. | No. 3. | No. 4. | No. 5. | | A. Constituents of the cement-stones. | | | | | | | Carbonate of lime | 65·7 | 61·6 | | 82·9 | 63·8 | | Carb —— e of magnesia | 0·5 | | | | 1·5 | | Carb —— e of protoxide of iron | 6·0 | 6·0 | | | | - | 4·3 | 11·6 | | Carb —— e of manganese | 1·6 | | | | | Silica | 18·0 | 15·0 | | 13·0 | 14·0 | | Alumina or clay | 6·6 | 4·8 | | trace | 5·7 | | Oxide of iron | | 3·0 | | | | | Water | 1·2 | 6·6 | | | 3·4 | | B. Constituents of the cement. | | | | | | | Lime | 55·4 | 54·0 | 55·0 | | 56·6 | | Magnesia | | | | | 1·1 | | Alumina or clay | 36·0 | 31·0 | 38·0 | | 21·0 | | Oxide of iron | 8·6 | 15·0 | 13·0 | | 13·7 | No. 1. English cement-stone, analyzed by Berthier; No. 2. Boulogne stone, by Drapiez; No. 3. English ditto, by Davy; No. 4. reniform limestone nodules from Arkona, by Hühnefeld; No. 5. cement-stone of Avallon, by Dumas. In England the stones are calcined in shaft-kilns, or sometimes in mound-kilns, then ground, sifted, and packed in casks. The colour of the powder is dark-brown-red. When made into a thick paste with water, it absorbs little of it, evolves hardly any heat, and soon indurates. It is mixed with sharp sand in various proportions, immediately before using 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 Jura ic (secondary) limestone, which contains 39 per cent. of silica, with alumina, magnesia, and iron oxide. Vicat forms a factitious Roman cement by making 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 somewhat doubtful; though it has, for want of a better substitute, been much employed at Paris. The cement of Dihl consists of porcelain or salt-glaze potsherds ground fine, and mixed with boiled linseed oil. Hamelin’s mastic or lithic paint to cover the façades of brick buildings, ., is composed of 50 measures of siliceous sand, 50 of lime-marl, and 9 of litharge or red-lead ground up with linseed oil.