Polytechnic Association. 897 



application of it for building purposes, I shall feel amply repaid for 

 my labor. 



Portland cements are those artificial hydraulic mortars which have 

 been exposed to a temperature sufficiently high to produce softening, and 

 which, in this condition, contain no free lime, and have a specific gravity 

 above three. The name originated with Joseph Aspdin, in Leeds, in 

 1S24, who first succeeded in producing cement superior to any before 

 made. The solidified cement being in appearance and quality equal 

 to the Portland stone, of which the finest edifices of the English metro- 

 polis are built, he gave to his product the name of Portland cement! 

 When we remember that this stone is classed among the most valuable 

 building materials of England, the above designation may be con- 

 sidered too assuming ; but it will be shown that its qualities were not. 

 overrated ; that, in fact, the name must be considered as very appro- 

 priate. 



Portland Cement, Physically and Chemically considered. 



Portland cement appears as a sharp, crystalline powder, of a color 

 varying from light to dark gray, and having a bluish or greenish tint. 

 Its specific gravity is, for the most part, above 3.1. Chemically con- 

 sidered, it is essentially a combination of lime, silica, alumina and. 

 oxide of iron. 



Michaelis gives the following analysis, showing its average conrpo- 

 sition : 



Lime = 60.05 per cent. 



Magnesia = 1.17 " 



Alumina => 7.50 " 



Oxide of iron = 3.34 " 



Potash = 0.80 " 



Soda = 0.74 " 



Gypsum = „ 1.82 " 



Silica = 24.31 " 



After this cement lias been exposed to a heat of 3,500° Fahr., there 

 is no doubt that a thorough chemical action must have taken place, 

 and that no part of it has remained intact. In fact, the calcined 

 cement consists chiefly of a silicate and aluminate of lime, of which 

 the significance of the latter has only been lately recognized. In 

 properly calcined cement, the oxide of iron is always combined with 

 oxide of calcium ; provided that, in consequence of too high a tem- 

 perature, the acid nature of the silica has not been called into play, 

 in which case it is converted into a protoxide which will then unite 



[Inst.] 57 



