MATERIALS. 113 



Slag from iron-furnaces can scarcely be regarded as a stone 

 certainly not as a natural onebut it may, perhaps, be as well 

 to notice it here as elsewhere. 



In the year 1857 it was proposed by Mr. J. Dunning to use 

 blocks of slag from iron-furnaces of the Cleveland district, to 

 form a very large breakwater (about 5 miles in length) at the 

 mouth of the river Tees. It was intended to cast blocks weigh- 

 ing about 20 tons each, and to throw them into the sea pierre 

 perdue} 



This proposal, so far as it had reference to the utilization of 

 slag, has since been successfully carried out in the construction 

 of the Tees breakwaters. The blocks used were, however, not so 

 large as those proposed by Mr. Dunning, their weight having 

 been only about 3 tons each. 



It was found necessary to protect the seaward face of these 

 slag mounds by a wall of Portland cement concrete, as the slag 

 blocks, when unprotected, were broken up and moved by the 

 force of the waves. Many of the blocks were thrown into the 

 water while hot, which would, no doubt, cause them to crack. 



The specific gravity of Cleveland slag is 276, that of 

 haematite slag 2*84, representing a weight per cubic foot of 

 172 Ibs. and 177 Ibs. respectively. They, therefore, in respect of 

 specific gravity, rank before most of the granites and syenites. 



The Carron Iron Company state that the specific gravity of 

 their furnace slag is even higher, viz. 3'01S, representing a weight 

 of 188 Ibs. per cubic foot. 



SECTION III. CONCRETE. 



Early use of concrete Value of concrete in sea- works Hydraulic lime Proportion 

 of ingredients in concrete Mixing by hand Mixing by machines Concrete- 

 block making Roman-cement concrete Medina cement Stone-faced blocks 

 Blocks built of rubble in cement mortar. 



Although concrete was used by the Romans and Carthagi- 

 nians in ancient times in the construction of sea-works, it seems 

 to have fallen into disuse for a long period. 



It is only within the last fifty or sixty years that it has 

 again come prominently forward, and its great value in harbour 

 construction been fully recognized. 



Its adoption for sea- works during recent years, together with 

 1 "Minutes Select Committee on Harbours of Refuge, 1857," p. 233. 



I 



