130 



HARBOUR CONSTRUCTION. 



Only the first three constituents are essential to good cement, 

 but the presence of the others is generally unavoidable. 



A slow-setting cement, unless the slowness be due to age and 

 aeration, indicates an excess of lime, and is therefore unsafe 

 for use. 



Roman Cement is manufactured from septaria nodules of the 

 London clay formation, found in the Isle of Sheppey and else- 

 where. The stone is broken into small pieces, and calcined in 

 kilns in much the same way as Portland cement, but at a some- 

 what lower temperature. It is then ground into a fine powder. 



The colour of the calcined stone is generally a rich brown, 

 but this depends much upon the percentage of oxide of iron 

 which it contains, and is no indication of quality. 



The composition of the stones from various districts varies 

 considerably. The Sheppey stone usually contains about 55 parts 

 of lime, 38 of clay, and 7 of iron ; while Yorkshire stone contains 

 as much as 62 per cent, of lime, and Harwich stone as little as 

 49 per cent. The natural stones have a fine close grain, pasty 

 appearance, and greasy surface when broken. 



Unlike Portland cement, the best Roman cement is that 

 which is the lightest. It should be ground very fine, and its 

 weight per cubic foot when freshly ground should not exceed 

 60 Ibs. 1 It should set very quickly after being gauged say in 

 about fifteen minutes. 



It is chiefly used in sea- works for temporarily pointing or 

 protecting new work, but it is sometimes employed for making 

 concrete and for setting masonry in situations where the work 

 is much exposed. It is more costly than Portland cement, and 

 never attains the same strength. Moreover, what strength it 

 possesses is quickly reduced if sand be mixed with it, as will be 

 seen from the following table : 



TENSILE STRENGTH PER SQUARE INCH OF SECTION. 



1 The specific gravity of Roman cement varies more than that of Portland 

 cement, but may be taken at from 2'50 to 3'00. 



