Polytechnic Association. 899 



Induration in Fresh Water. 



If Portland cement is mixed with water to the consistency of a 

 stiff paste, it turns darker in consequence of the displacement of the 

 air by the water, and sets more or less rapidly, according to its com- 

 position and the temperature to which it has been exposed. From 

 the moment of setting, it hardens gradually in air, as well as in water, 

 but more rapidly in the former medium. If finely divided cement 

 is exposed to the action of a large quantity of water, it will be com- 

 pletely decomposed. Lime and a small quantity of silica are dis- 

 solved, while the hydrates of silica, alumina and oxide of iron remain 

 behind. 



If the action of the water went in to such an extent, the cement 

 could not be used in water ; it is, however, protected from decompo- 

 sition by its great density on the one hand, and by the action of car- 

 bonic acid on the other. Whenever this gas comes in contact with 

 hydrate of lime or with silicate of lime, and at the same time comes 

 in contact with a silicate of an alkali, a corresponding carbonate is 

 formed, while silicic acid is separated. This latter, as Fuchs has 

 shown, possesses strong binding qualities. 



But, however, since the atmospheric air affords a greater amount 

 of carbonic acid than water does, and since water acts to some extent 

 as a solvent, it is easy to see why the hardening of cements takes place 

 sooner and in a more perfect manner in the air than in the water. As 

 to the interior, it hardens equally as well in the air as in the water. 

 One may readily convince himself of this fact by making the tests 

 with samples, one set in the air, the other in the water. No difference 

 will be found, no matter after what length of time they have been 

 tested. The lesser the degree of density of a mortar, the more it will 

 be decomposed by water and carbonic acid. But, as there is no 

 hydraulic mortar having a greater density than Portland cement, there 

 is, therefore, none that is capable of resisting that decomposing action 

 better than this. 



Induration in Salt Water. 



Experience shows that the setting of hydraulic mortars takes place 

 more slowly in the sea than in fresh water. The principal substances 

 in sea-water are, as is well known, chloride of sodium, chloride of 

 magnesium, and sulphate of magnesia ; of these, the two last act the 

 most as decomposing agents. Aside from these salts, sea- water con- 

 tains other agents which act on the mortar, carbonic acid (either in a 

 free state or as a bicarbonate), and sulphuretted hydrogen, which is. 



