HYDRAULIC CEMENT 829 



layers are crusted, the upper half may be removed and employed as ordinary lime, 

 and the lower half ground down as cement. If the process should be judged to have 

 been quite ineffectual, the removed portion of the charge can be replaced with fresh 

 lime, and the operations of heating the lime to a faint glow, and of burning sulphur 

 under it, be repeated ; with ordinary care a failure can hardly occur, and should the 

 sample specimens set firmly ' in pats ' six inches square and half an inch thick, but yet 

 grow too hot in setting to be safely trusted in work, a few days' exposure on the floor 

 of the shed will render the cement fit for use. It is better, however, that ' in pats ' of 

 the above size, made up with water to a stiff consistency and without admixture of 

 sand, the warmth in the setting even when the cement is quite fresh from the oven 

 should be barely perceptible. 



The grinding of the cement is the most serious part of the manufacture. Should 

 the extent of the works justify a considerable outlay in grinding apparatus, the flour- 

 mill construction is to be preferred, horizontal crushing rollers being used to reduce 

 the lumps of cement to a size which the eye of the runner mill-stone can readily 

 take. It is better also to sift it through a sieve, the coarse particles being again 

 passed through the stones. For grinding, on the small scale, a pair of vertical wheels 

 of iron or stone, turning on a pivot between them by horse-power, must be resorted to, 

 the sifting apparatus being attached by suitable machinery to the gearing of the 

 wheel, or worked by manual labour. The wire- gauze used for the sieves should have 

 thirty meshes to the inch. The construction of both descriptions of mills is well 

 understood by mill-makers. 



When the cement is prepared from the pure and feebly-hydraulic limes, and it 

 becomes necessary to impart to them properties which will enable them to resist the 

 action of water, artificial or natural puzzolanas must be restored to. If such sub- 

 stances are ground down with the cement to a fine powder, and intimately mixed with 

 the cement, one part of puzzolana will give to two parts of pure lime-cement, by 

 weight, the necessary degree of resistance; but if mixture by hand-labour be alone 

 available, and the puzzolana be not in a fine state of division, its quantity in proportion 

 to the cement should be increased, the amount of sand used for mortar being corre- 

 spondingly diminished. 



The clays best adapted for the manufacture of artificial puzzolana are such as are 

 greasy to the touch. The temperature at which they ought to be calcined depends 

 upon their chemical composition, but the action of the constituents is complicated, 

 and actual trial must be resorted to for the determination of the most efficient degree 

 of calcination. To make the trial, a portion of the clay under examination may be 

 moulded into a cylinder five or six inches long, and one and a half inch diameter ; 

 this cylinder when dried is to be exposed, the one end in a violent fire, the other on 

 or near the outside of it. When the more highly-heated end begins to vitrify, the 

 cylinder is withdrawn from the fire and divided into three or four portions, according 

 to the general appearance and colour of the different parts. 



By pulverising each part, and mixing this puzzolana with the cement in the propor- 

 tion of one part of burnt clay to two or three parts of cement, by measure, it is easy to 

 determine by actual trials in water what degree of calcination gives the best results. 

 To prevent the cylinder from breaking in the fire, it is moulded round a wire. As a 

 general rule, it will be found that clays containing lime and other metallic oxides, 

 especially alkalis, become energetic at low temperatures, though they may also succeed 

 when burned at a violent heat. The poorer clays will only give good results when 

 calcined at a very high temperature, and they have the further disadvantage of being 

 difficult to reduce to powder. 



^ Clay should be made ready for burning by moulding it into rounded balls of the 

 size of an apple, and leaving it in the air to dry. A thick layer of coals is placed on 

 the bars at the bottom, then layers of the clay balls and coal alternately ; when the 

 kiln is about two-thirds full, faggots should be placed under the bars and ignited, and 

 as the combustible is consumed, more coals and clay should be added ; the appearance 

 of the clay will at once show whether a proper quantity of fuel has been used. 



Concrete. The following is an account, by a thoroughly practical man, of the use of 

 hydraulic limes in the formation of concrete. It is from a paper read by George 

 Kobertson, C.E. before the Scottish Society of Arts, On the Concrete used in the late 

 Extension of the London Docks.' 



The great mass of the concrete was made with naturally hydraulic lime, blue lias 

 from Lyme Kegis in Dorsetshire, which requires no artificial mixture with puzzolana 

 or minion to render it capable of setting permanently under water. The word con- 

 crete ' in this paper implies, therefore, that made with blue-lias lime, unless otherwise 

 specified. The Dorsetshire lias was the only lime burned on the works ; all lias 

 from Warwickshire or Leicestershire was bought ready burned from the merchants. 

 Lias requires much greater care in burning than richer limes, because any sudden 



