Chap. V.

MOKTAU AND CEMENTS.

721

the strength which it acquires from the flare or flame heat: pure lime and clay thusheated in a kiln will produce good hydraulic lime.

When water is thrown upon a subcarbonate, however, it will form a hydrocarbonate,which will not set under water, and with an excess of base is more difficult to reduce intolime than a neutral carbonate by a second calcination; this will in some degree accountfor the difficulty of converting limestone into lime, when it has been chilled beforecalcination is complete.

Artificial hydraulic lime, which is so necessary for all engineering works, is now generallyprepared by mixing with a rich lime a certain proportion of alumina or clay, and thensubjecting it to the process of calcination: with lime so prepared the beton, a masscomposed of hydraulic lime and rubble, is made, the lime being slacked previous to itsmixture; this sets under water, and is universally employed in France , where the piers ofbridges are founded on a beton composed of sand, flint, and artificial hydraulic lime; it isoften applied at great depths in a caissoon without a bottom, and after it has been deposited8 months, has been found hard enough to bear 2500 tons on a surface of less than 100square yards. Chalk is sometimes used, which being quickly reduced to a powder is formedinto a paste by the addition of water, but does not produce so good an artificial lime, inconsequence perhaps of the less perfect state of the combination of the materials. Cal-careous substances cannot, without slaking or subjecting them to heat, be reduced to thesame state of fineness. The proportion of lime and clay for the manufacture of hydrauliclime must vary according to their quality; but 20 parts of dry clay added to 80 of richunslacked lime, or 1*10 of carbonate of lime, is found to be a good proportion; the finestand softest clays are always preferred.

Parker's cement was first patented in 1796 : it contains 45 per cent, of clay, and 55 of car-bonate of lime, according to Sir Humphry Davy , and the mineral substance used for itsmanufacture is a reniform limestone, found in nodules in beds of clay; they are mostabundant in the argillaceous strata, which alternate with those of oolite, and the claystratum, which reposes on the chalk and often on the London clay. In Kent they arefound on the coast of the Isle of Sheppev, and are called Septaria; the siliceous clay ofwhich they are composed contains veins of ealespar; after these Septaria or cement-stonesare collected, they are subjected to calcination in kilns, and the cement is packed in casks ;their analysis shows usually 55 parts of lime, 38 of alumina, and 7 of oxide of iron.

In Yorkshire there is a cement made for hydraulic purposes, which contains 34 partsof clay, and 62 of the carbonate of lime, and that obtained at Harwich, which sets veryquickly, has 47 parts of clay, and 49 of carbonate of lime.

When this material is properly burnt it is of a light brown colour; the cement, whentaken from the kiln, requires grinding before it is fit for use, and when mixed with waterit regains all the carbonic acid that was driven oft' by burning. The stone or septariaof which this invaluable cement is made is fine grained, and susceptible of polish, its spe-cific gravity being about 2-59.

Its components after a careful analysis are found to be,

Carbonate of lime

-

-

_

- 657

--

of magnesia -of iron

-

-

-

5

60

—

of manganese

-

-

-

19

Clay -

J silica

.

_

-

- 180

\ alumina

.

_

- 66

Water

'

13

1000

Two measures of sand and one of good cement powder form an excellent compositionfor ordinary building purposes, though many prefer equal measures of each ; cement,however, unites more readily and powerfully with brick or stones when it is perfectly pureand unmixed with sand: for the lining of reservoirs or cisterns it is used pure, and ifmixed with sand is seldom found to be water-tight; hence where a thickness is required,it is better to dub out with tiles as much as is necessary, using only pure cement.

Kilns for burning Parker's Cement -That in Her Majesty’s dockyard at Shccrness

is circular, 17 feet in diameter from out to out, and about 21 feet 6 inches in height: aninverted cone occupies the middle, which has a clear diameter at top of 8 feet, and atbottom of 5 feet 6 inches, where there is a conical mass of brickwork, which spreadsthe cement as it falls through the ash-holes or eyes; there are four of these placed at re-gular distances, each 30 inches in width, and 18 inches in height, to the crown of the flatarch that covers them ; within are fire-holes a foot square, which have iron bars to sup-port the brickwork above them. Around the entire cylindrical kiln are four wrought-ironhoops | uf an inch in thickness, and 3 inches in width, for the purpose of holding the work

3 A