126 HARBOUR CONSTRUCTION. 



power, tinder certain circumstances, of taking the place of an acid, 

 and this occurs during the semi-fusion in the kiln ; the lime decom- 

 poses the clay, and combines both with the silica and alumina, 

 forming calcium silicate and calcium aluminate, which, when finely 

 ground, constitute good cement. When mixed with water, the 

 cement combines chemically with a certain quantity, and crystallizes 

 into a solid block, impervious to water. In the case of concrete 

 where sharp sand is used, some of the lime gradually combines 

 chemically with it, forming an additional quantity of silicate of lime, 

 and this still further improves the strength of concrete. During the 

 process of absorption of water and crystallization, the whole should 

 be kept as much at rest as possible, for excess of water, especially in 

 motion, would separate the fine particles, and weaken the concrete, 

 and form an incrustation of white chalk, etc. 



"In order to know whether cement obtained from any manu- 

 facturer is good, a complete chemical analysis is requisite, in the first 

 place, to show if it has the right composition. This will, however, 

 only give the proportions in which the ingredients exist, which may 

 be exactly the same in a well-clinkered cement as in one that has 

 only been burned enough to drive off the water and carbonic acid 

 without causing any further chemical action between the several 

 ingredients. The usual test of proper burning is the weight of a 

 striked bushel of the cement, which is required to be not less than 

 112 pounds to 115 pounds. This is a very uncertain test, as many 

 little points during the filling of the mixture may make several 

 pounds difference in the weight. A much more reliable test is the 

 specific gravity, which should accompany the analysis. A properly 

 clinkered and ground cement, when new, will have a specific gravity 

 of 3'1 to 3*15. If it is as low as 2 - 9, or less, it shows that the 

 cement has not been properly burned. Thus the chemical analysis 

 gives the actual composition of the cement, and the specific gravity 

 gives the density, which cannot be brought up to 3'1 unless the 

 ingredients have been thoroughly and properly clinkered." 



In what is termed the "dry process" of making Portland 

 cement, the ingredients, instead of being mixed together with 

 water and run into settling-ponds or "backs," are thoroughly 

 dried and reduced to powder. This powder, in which the pro- 

 portions of clay and lime are carefully adjusted, is then moistened 

 and formed into bricks, which are dried, calcined, and ground 

 into cement in precisely the same way as the slurry is treated in 

 the wet process. 



The next thing to be considered is the fineness to which 



