CEMENT.

Lyttelton Times, Volume LVI, Issue 6496, 22 December 1881, Page 3

CEMENT.

In connection with the various projects which are at the present time being put forward, those having reference to the local manufacture of cement are of special importance to this Colony. Of the requisite materials wo possess 'a practically unlimited supply: and even the production of so much cement as is used in New Zealand would be an undertaking of considerable magnitude. Some interesting details of the natural and artificial cements in use have recently been compiled in a condensed form, as follow*: Certain limestones, which contain upwards of 10 per cent, silica, possets the property, when burned, of forming a cement or mortar which hardens nnder water. Such limestone is coiled hydraulic lime, and the mortar is called hydraulic mortar. This stone, before burning, consists of a mixture of c -rbonato of lime and silica, or a silicate, ohitfiy silicate of alumina. The latter is insoluble in hydrochloric acid, hence remains undissolved when the stone is treated with this acid, but in burning this silicate le Hazed by the alkaline carbonates and becomes soluble in acid, the oarbonio acid being expelled. When common lime is slaked it swells enormously and devevelops a great deal of heat; this is not the ease in slacking hydr*u!iclime, which absords water without any considerable increase of temperature or volume. If ordinary lime bo mixed with a suitable quantity of silica or sand, an ortifical hydraulic mortar is obtained, to which we apply the name of cement. These cements may be either natural or artificial. The former are found in volcanic redone, having beenprodueed by the terrestrial heat. Poasnolana, found at PoizuoU, near Naples, is a natural cement of the following composition: Silica, 445; alumina, 150; lime, 8*8; magnesia, 4*7; oxide of iron, 12 0 (with oxide of titanium).; potash and sods, 5*5; water, 9*3; total, 100*8. The quantity of lime is, however, so small that it requires to be mixed with ordinary lime to form hydraulic mortar. It was employed In combination with an equal quantity of lime In building the Eddy stone Lighthou* e. Artificial cement, also called “ Roman cement" because it is not made in Borne, has been manufaotnredin England, on the Thames, and in the isles of Wight and Sheppey since 1796. It is made by burning the calcareous nodules which overlie the chalk in that country. A sample analysed by MichaSUs contained: lime, 68*38; magnesia, S; silica. 2883; alumina, 6.40 j oxide iron, 480. When mixed with water it hardens in fifteen or twenty minutes, and possesses great firmness md strength. Portland cement was patented in England by Joseph Aspdin in 1824. He took the limestone of Leeds, pulverised and burned it, then mixed it with water and an equal weight of olay to a plastic mass. When dry this was broken up and burned again until the carbonic acid was all expelled. It was then pulverised and was ready for use. Fasiey made it from ebalk or limestone with Medway Biver clay, which contains salt. IVttenkofer suggest* that cement ie improved by soaking the clay in salt water. Portland cement is now made, say • Wagner, by making bricks of an Intimate mixture of limestone and clay, drying them in the air, and burning them in a tall shaft furnace from 45 to ICO feet, 12 feet in diameter, with a strong grate 4 foot from the bottom. It is charged with alternate layers of ooal and cement stone. The properties of the cement are largely dependent on the temperature employed in burning ; a white heat le best, but if the temperature is too high it will no longer unite with water, and may even be melted to a glass. If the temperature does not exceed a red heat it unites readily with water and gets hot, like ordinary lime, but possesses very little strength. The colour changes with the burning, and forms a criterion for judging the quality. In its normal condition it forint a gray, sharp, powder, with a shade of green, but not glassy. The manufacture of Portland cement is now carried on in every part of the world where limestone and ofay are to bo found. In order to obtain a good cement, not only must the proper heat bo employed in burning, but the proper proportion of clay, usually 85 per cent, must bo used, and the olay must have certain properties, such as a large proportion of silica, must be very finely divided, and must be very intimately mixed with the limestone. Analyses of Portland cement from various sources show the percentage of lime to vary from 65 to 63j silica, 23 to 35 1 alumina, 6to 9; oxide of iron, Bto 6 1 toda and potash, usually less than 1 per cent.