HYDRAULIC CEMENT 831 



bushels, which is what bought lias usually weighs. A bushel of lime, ground when 

 fresh burnt, contains, therefore, one-seventh more lime than a bushel of stale lime ; 

 and a cubic yard of concrete, of specified proportions, is so much the better when 

 made with fresh lime. 



Coke was used in the kiln for a short time, but it was found to be 8 per cent, 

 dearer than "Welsh coal ; and, moreover, the heat given out by it was too quick and 

 strong, casing many of the stones over with a vitrified silicate, which hindered the 

 free escape of carbonic acid. When coke was burnt, therefore, the percentage of 

 unburnt stone was raised much above the usual average of 1 or 2 per cent. 



The equally burnt and softest lumps, usually of a buff colour, were picked out for 

 grinding; and the remainder, more of a liver colour, slaked for mortar. The lumps 

 were first broken tolerably small by hand, and then crushed still smaller between iron 

 rollers revolving in the hopper of the grindstones. These rollers were at first made 

 fluted, but it was found that strong projecting cogs did the work more effectually. 

 The hopper was fed with lumps of lime by an endless chain of small wrought-iron 

 buckets worked by the engine. It was intended that these should be self-acting, and 

 dredge the lime up from a well ; but it was found necessary to have a man constantly 

 feeding them with a spade, or the supply was not regular. 



The lime was ground to a fine powder between two pairs of horizontal French burr 

 millstones; the upper one revolving at a speed of 90 revolutions per minute. Each 

 pair of stones was able to grind 3 tons of quick-lime per hour, at a total cost for 

 grinding of Id. per bushel when the consumption was 360 bushels per diem; less, if 

 more lime was used. This is made up as follows : feeding and attending to the 

 hopper and lift, \d. ; engine power, %d. ; measuring the lime into bags for the con- 

 tractor, and recutting the stones as the furrows became worn, the remaining \d. A. 

 .bushel of lime ground fresh from the kiln weighed 84 Ibs. ; and at this weight the 

 total cost was llfd. In buying ground lime from a merchant, if the purchaser buys 

 by weight, he pays for the water absorbed from the atmosphere; if he buys by 

 measure, he pays for the expansion caused by that moisture : the fairest way for both 

 parties would be to specify the bushel to be of a certain average weight, say for lias 

 .from Lyme Kegis, 80 Ibs. This would allow for the lime not being quite fresh, but 

 would prevent it from being too stale. 



The grindstones were composed of burrs from the fresh-water beds of the Paris 

 basin, set in two radiated rings in cement, and backed up with plaster-of-Paris and 

 mortar. The ' skirts ' or outside burrs were five inches thick ; the central or ' high 

 burrs ' somewhat thicker, to allow for the ' swallow/ which is a slight depression 

 in the centre of the upper stone, about two feet in diameter, and at most 3" deep. 

 This acts as a kind of distributing reservoir for the lime as it falls from the hopper 

 between the stones. 



The face of the stones was divided into ten quarterings ' by ' master furrows,' 

 each of them being tangential to an imaginary circle concentric with the stone, and 

 called its ' draft.' The size of this regulated the quantity of lime passing through 

 the stones in a given time. A radius of five inches was found to grind ninety 

 bushels per hour of a sufficiently fine quality. The particles of lime, whirling round 

 near the centre of the stone, by their centrifugal velocity pass towards the outside 

 along the master furrows, being ground finer as they recede from the central depres- 

 sion. Each master furrow had two other distributing furrows leading out of it, 

 parallel to the former master furrow. The furrows are shallow grooves, or rather 

 nicks about l^ inch wide, with the cutting edge sharp, and the other bevelled. 



The hydraulic lime being thus prepared, it was employed in the formation of 

 the required bed of concrete. This was formed by cementing a mixture of 

 sand and gravel such as is found in the London basin, called ' ballast,' in the 

 following manner : 



When the ballast was moderately dry, 12 cubic yards of gravel and 2 cubic yards 

 of lime made 11 cubic yards of concrete, mixed and deposited. The shrinkage 

 from the dry materials was then 22 per cent. ; but if the ballast happened to be 

 very dry, the shrinkage was more, and the same quantities made only 10 cubic 

 yards. 



A cubic yard of concrete requires about thirty-eight gallons of water to bring the 

 dry materials to the requisite state of fluidity. Of this quantity nearly eight gallons 

 enter into chemical combination with the oxide of calcium in the lias, and thirty 

 gallons are either absorbed mechanically by the pores of the lime, retained by 

 capillary attraction between the grains of sand, or lost by evaporation. After the 

 concrete has been mixed and deposited, a gradual expansion takes place from the 

 chemical action of the lime slaking ; the less of this swelling, however, the better, as 

 it disturbs the setting of the mortar round the pebbles, and causes friability in the 

 concrete. Whenever concrete is made with quick-lime (as it usually is), there must 



