MATERIALS. 125 



mechanically mixing or blending together the chalk, clay, and 

 water in the washing-mills, that the right proportions can be 

 ensured. 



Mr. Bamber's description of the process of Portland cement 

 manufacture is so clear and concise, that I cannot do better than 

 give it in his own words, as follows : 



"In the older works the mixture of clay, chalk, and water 

 obtained in the washing-mills is run into " backs," J and allowed to 

 settle. The water is then drawn off, and the solidified mixture 

 carried to drying-plates, to remove the greater part of the remaining 

 moisture. These drying-plates are heated by coal or coke, and in 

 some cases are so arranged that the waste heat of coking-ovens is 

 used for the purpose. 



" In more recently erected works, the slurry is only mixed with 

 sufficient water to enable it to run along the troughs that conduct it 

 to the horizontal flues in direct communication with the interior of 

 the kilns. All the heat that passes from the kilns during the burning 

 of the cement is thus carried over the slurry, which is dried rapidly 

 and cheaply. These flues also being part of the kilns themselves, less 

 labour is required in packing the kilns with the dried slurry and 

 coal or coke. In either case, when the slurry has been sufficiently 

 dried, it is carefully packed into the kilns with the proper mixture 

 of coal or coke. The quality of the resulting cement depends to a 

 great extent on the careful performance of this part of the process ; 

 for if the clay and chulk mixture be added in too large lumps, the 

 outside only will be properly clinkered, while the interior will remain 

 an imperfectly burned powder ; whereas thorough clinkering is abso- 

 lutely necessary in order to obtain good cement. 



"The first effect of the heat on the mixture in the kilns is to 

 drive off water and carbonic acid, thereby converting the chalk into 

 quicklime, which decomposes the clay, combining chemically with 

 both the silica and alumina, forming calcium silicate and calcium 

 aluminate, of which properly burned cement chiefly consists. 



" Clinkering is a state of semi-fusion, and it is requisite that this 

 should take place in the kilns, so that when they are emptied there 

 should be only good clinkers without dust. It is on this process that 

 the production of strong cement depends, for it is at the time of this 

 semi-fusion that the chief chemical action takes place. In the clay, 

 which is mainly a silicate of alumina, the silica and alumina are in 

 intimate chemical combination, the silica acting the part of an acid, 

 and the alumina that of a base. Alumina, however, has also tho 



1 Settling-pouds or reservoirs. 





