124 



HARBOUR CONSTRUCTION. 



SECTION IV. CEMENTS. 



Portland cement Analyses of clay and chalk Process of making Portland cement 

 Air-slaking Porosity in concrete Indications of good Portland cement 

 Bate at which Portland cement briquettes gain strength with age Apparent 

 decrease of strength at age of seven or eight months Analysis of good Portland 

 cement Koman cement Medina cement. 



The very existence of a sea-work may depend upon the 

 quality and manipulation of the cement used in its construction ; 

 so it is necessary that those engaged upon such works should 

 have a thorough knowledge of the subject. 



Portland Cement, which is much more largely employed in 

 harbour construction than any other, is generally made by 

 intimately mixing and calcining together clay and chalk ; but, 

 seeing that clays and chalks vary greatly in their composition, 

 the proper proportions can only be determined by careful 

 analysis. 



Mr. H. K. Bamber, whose large experience in such matters 

 is well known, gives the average composition of a good clay and 

 of a good average soft chalk, suitable for the manufacture of 

 Portland cement, as follows : l 



CLAY. 



Silica 



Alumina and oxide of iron 



Lime ... 



Magnesia 



Carbonic acid ... 



Sulphuric acid ... 



Organic matter and moisture 



Silica 



Alumina and oxide of iron 

 Lime ... 

 Magnesia 

 Carbonic acid ... 

 Organic matter, etc. 



CHALK. 



54-84: 



25-08 



0-90 



0-80 



0-83 



1-20 



16-35 



100-00 



1-15 



0-78 

 54-00 



0-25 

 42-50 



1-32 



100-00 



Although clays and chalks similar to the above are doubtless 

 very suitable for the manufacture of Portland cement, others of 

 very different composition are frequently used, and it is only by 

 making constant chemical tests, especially during the process of 



Proc. Inst. C.E., vol. cvii. 



