CEMENT: ITS DISCOVERY AND MANUFATURE.

South Canterbury Times, Issue 2533, 4 May 1881, Page 2

CEMENT: ITS DISCOVERY AND MANUFATURE.

—* (By J.. Brough Stansell.) Sustratus, the architect who built the tower of Pharus, near Alexandria, on top of which a fire was kept burning to guide such ships as sailed at night on those dangerous coasts, resorted to a stratagem to hand down to posterity the fact of its being his handiwork. The potentate (Ptolemy Philadelphus), under whose authority and command the tower was erected, had the usual inscription placed thereon, but it was evanescent in character, being cut in a coating of lime, under which was cut in marble the following dedication :—“ Sustratus, the Caidian, son of Dexiphanes, to the protecting deities, for the use of seafaring people.” We may briefly give the chronological order of the experiments so as to show how much ingenuity had been expended before the realisation of the Portland cement desideratum was reached.

Ist. Smsaton, in 1756, endeavored to obtain a special cement for his oven works; 2nd. Higgins, in 1780, furnished illustrations of his views and theories concerning a cement suited for stuccoing or house fronting purposes ; 3rd. Parker, in 1796, made the most fortunate and useful discovery of a mode of utilising the nodules (septaria) found in the London clays. The Thames Tunnel could not have been made but for the advantages this preparation secured, and many of the early railway tunnels were built with it as a cementing agent; 4th. Dobbs, in 1810, shows by his specifica. tion that he had a good mechanical knowledge of the subject, but his chemistry was insufficient to guide him ; sth. Foreign experimenters, such as Vicat, John Treussart, and St. Leger, all of whom aimed at the obtainment of an artificial hydraulic lime during and about 1818; 6th. Aspdin is entitled to the next position, from the date of his patent in 1824 ; 7th. Pasley, for many years after 1826, when he first entered seriously on the subject in the extensive experiments which he so ably conducted ; Bth. Frost, in 1826, from his being the first to erect a manufactory, near London, for the purpose of making Portland cement for constructive purposes. THEORY OF THE CHEMISTRY OF CEMENT. Professor Petenhofer asserts that the objects to be effected in the calcination of a cement compound are twofold, Ist the conversion of the carbonate of lime into an oxide of calcium at a dull red heat, and 2nd the production at a temperature approaching fusion, of a chemical combination of silicic acid with alumina, iron and the alkalies, the silica being by this means protected from at once combining with the lime, but made available for future chemical action under the changed conditions brought about in the water. The due formation of these silicates must greatly depend upon the physical condition of the mixture of lime and clay, and upon the presence in sufficient quantities, of bases for which, silicic acid has, at a red heat, a more powerful affinity than it has for lime. Among such bases are prominently the alkalies—potash and soda, and in a less degree, alumina, and the oxides of iron. It is not, however, alone imperative that such silicates should be produced in the calcination. Due in order tnat the hydraulic properties of the cement should be exerted to the utmost. The compound, when treated with water, must contain these partly! soluble and partly insoluble silicates, in a form capable of permitting the substitution of the hydrate of lime for certain of the bases previously in combination with the silica. On treating the powdered cement with water a chemical change immediately takes place. The silicic acid previously combined with the alkalies in the form of soluble compounds goes to the recently formed hydrate of lime,leaving the alkalies with which it was first free to be washed out of the compound. The newly formed silicate of lime partially decomposes the other silicates of alumina and iron in the act of hydration, forming double hydrated silicates which are practically insolable. Analysis of a German cement by Pettenkofer and Hopfgarten

Lime ... ... ... 54.11 Magnesia 0.75 Potash 1.10 Soda 1-66 Alumina 7.75 Iron ... ... 5.30 Silica ... 22.23 Carbonic acid ... ... 2.15 Phosphoric acid ... 0.75 Sulphuric acid 1.00 Water 1.00 Insoluble residue ... 2.20 100.00 Knapp, in his work on “ Technology,” puts the case somewhat different. He states of hydraulic limes, that lime rendered caustic by calcination “ reacts upon the siliceous clay, converting it into a compound that is easily decomposed by acids. The excess of caustic lime, as well as the compound into which the siliceous clay has been converted, then react upon each other when mortar is prepared from the ground burnt lime, in such a manner that a solid stone like silicate is produced in the humid way. The water here has obviously a double action. Dry substances like lime and the silicate of alumina act very little upon each other, unless the solvent power of water is employed to bring them into intimate contact. The setting of hydraulic mortars is in nowise due to the formation of simple hydrates. The process of solidification is not so much the conversion of a ready formed silicate into a hydrate as the formation of a hydrated silicate in one and the same operation.” It will be manifest from these observations that the nature of the clay employed to mix with the lime is a matter of considerable importance. The following analysis of the Medway clay will shew the kind of material suitable for cement purposes ' — Silica ... ... ... 70.56 Alumina 14.52 Lime ... ... ... 4.43 Oxide of iron 3.06 S&h} 3 - 95 Carbonic acid 3.48 100.00 (To be continued.)