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CEMENT into contact with, lime in the presence of water, and causes the formation of a true hydraulic silicate of lime. The following analysis shows the composition of a typical slag:— Insoluble residue Silica (SiOo) , Alumina (A1203) Manganous oxide (MnO) Lime (CaO). Magnesia (MgO) Soda (Na20). Sulphuric anhydride (S0:i) Sulphur (S).

131-50 18-56 0-44 42-22 3-18 0-70 0-45 2100-30 1-10

Deduct oxygen equivalent to sulphur

99-20 Granulated slag of this character is ground with slaked lime until both materials are in a state of fine division and intimately mixed. The usual proportions are three of slag to one of slaked lime by weight. The product termed slag cement sets slowly, but ultimately attains a strength scarcely inferior to that of Portland cement. Puzzuolanic cements are little used in England. Generally speaking they are only of local importance, their cheapness depending largely on the nearness and abundance of some suitable volcanic deposit of the trass or tufa class. They are not usually manufactured by the careful grinding together of the puzzuolana and the lime, but are mixed roughly, a great excess of puzzuolana being employed. This excess does no harm, for that part which fails to unite with the lime serves as a diluent, much as does sand in mortar. In fact, ordinary puzzuolanic cement made on the spot where it is to be used may be regarded as a better kind of common mortar having hydraulic qualities. Good hydraulic mortars may be made from lime mixed with furnace ashes or burnt clay as the puzzuolanic constituent. Cements of the Portland type differ in kind from those of the puzzuolanic class ; they are not mechanical mixtures of lime and active silica ready to unite under cement*1 suifable conditions, but consist of definite chemical compounds of lime and silica and lime and alumina, which, when mixed with water, combine therewith, forming crystalline substances of great mechanical strength, and capable of adhering firmly to clean inert material, such as stone and sand. They are made by heating to a high temperature an intimate mixture of a

calcareous substance and an argillaceous substance. The commonest of such substances in England are chalk and clay, but where local conditions demand it, limestone, marl, shale or any similar material may be used, provided the correct proportions of calcium carbonate, silica, and 0 alumina are maintained. The earliest forms of cements of the Portland class were the hydraulic limes. These are still largely used, and are prepared by burning limestones containing clayey matter. Some of these naturally possess a composition differing but little from that of the mixture of raw materials artificially prepared for the manufacture of Portland cement itself. Although hydraulic limes2 have been in use from the most ancient times, their true nature and the reason of their resistance to water have been known for less than 100 years. Next in antiquity to hydraulic lime is Roman cement, prepared by heating an indurated marl occurring naturally in nodules. Its name must not be taken to imply that it was used by the ancients ; in point of fact the manufacture of this substance dates back only about 100 years. With the growth of engineering in the early part of the 19th century arose a great demand for hydraulic cement. The supply of materials containing naturally suitable proportions of calcium carbonate and clay being limited, attempts were made to produce artificial mixtures which would serve a similar end. Among those who experimented in this direction was Joseph Aspdin, of Leeds, who added clay to finely-ground limestone, calcined the mixture, and ground the product, which he called Portland cement. The only connexion between Portland cement and the place Portland is that the cement wrhen set somewhat resembles Portland stone in colour. True, it is possible to manufacture Portland cement from Portland stone (after adding a suitable quantity of clay), but this is merely because Portland stone is substantially carbonate of lime ; any other limestone would serve equally well. Although Portland cement is later in date than either Roman cement or hydraulic lime, yet on account of its greater industrial importance, and of the fact that, being an artificial product, it is of approximately uniform composition and properties, it may conveniently be treated of first. The greater part of the Portland cement made in England is manufactured on the Thames and Medway. The materials are chalk and Medway mud; in a few wrorks the latter is replaced by gault. The composition of typical samples of chalk and clay is shown in the following analyses :—

Chalk. Per cent. Silica (Si02) .... Alumina + ferric oxide ALOo + Fe203) • • • ' • Lime (CaO) Magnesia (MgO) Carbonic anhydride (C02)

0-92 0-24 55-00 0-36 43-40 99-92

Insoluble silicious matter Silica (Si02) Alumina (A1203) Ferric oxide (Fe203) Lime (CaO) , Magnesia (MgO) Soda (Na20) Potash (K20) Sodium chloride (NaCl) Combined water, organic matter, and loss.

Clay. Per cent. 26-67 Consisting of 31-24 Quartz (Si02) 16-60 Silica (Si02) 8-66 Alumina (A1203) 0-25 Magnesia (MgO) 1-91 Soda (Na20) 1-00 0-45 1-86

19-33 5-19 I Felspar 1-47 per 0-03 IJ-7'34 0-65 cent. 26-67

11-36 100-00

These materials are mixed in the proportion of about 3 :1 by weight so that the dried mixture contains approximately 7 5 per cent, of calcium carbonate, the balance being Mixing. clay- T^1C mixing may be effected in several ways. The method once exclusively used consists in mixing the raw materials with a large quantity of water in a wash mill, a machine having radial horizontal arms

driven from a central vertical spindle and carrying harrows which stir up and intermix any soft material placed in the pit in which the apparatus revolves. The raw materials in the correct proportion are fed into this mill together with a large quantity of water. The thin watery “ slip ” or slurry flows into large settling tanks (“ backs ”) where the solids in suspension are deposited ; the water is drawn off.