The Crystalline Cements.—Eckel. 153 
that it contains cements very different in composition and 
character. In use, cements of this class may be distinguished 
from Portlands by their lower specific gravity, more rapid set, 
anal smaller ultimate strength. They carry 20%-30% silica, 
30% to 60% lime, 2% to 40% magnesia, 6% to 20° 
aluminum and iron. The mere statement of this range in com- 
position shows what widely different products are here in- 
cluded. The silica content is the least variable, as might in- 
deed be expected in view of the fact that to a large extent it 
causes the hydraulicity of the product. 
Portland Cement. Portland cement is the product obtained 
by burning an intimate mixture (in definite proportions) ot 
lime, silica and alumina to the point of incipient vitrefaction ; 
and pulverizing the resulting clinker. The mixture may be 
either natural or artificial, The proportions of its essential 
components, before burning, will vary little from lime 75%, 
silica and alumina 25%. The composition of the cement will 
oscillate within narrow limits about the proportions lime 60% 
silica 22%, alumina 8%. 
Portland cement is distinguished from natural cements by 
the fact that it is calcined at a higher temperature and from 
slag cements by the fact that, after calcination, no material 
is added (except a small amount of gypsum, to regulate the 
set). The specific gravity of Portland cement averages about 
ae 
The lime in the Portland cement mixture is generally de- 
rived from pure limestone or marl; the argillo-siliceous ele- 
ments from shales, clays or argillaceous limestones. 
Pozsuolanic Cements. In this sub-group are included those 
cementing materials made by mixing in proper proportions 
without subsequent calcination, powdered slaked lime with cer- 
tain alumino-siliceous materials. As to origin, the materials 
added may be artificial (blast-furnace slag) or natural (trass, 
pozzuolana, santorin). Slag is the only one of these at pres- 
ent used in the United States. 
The slag used must be basic, running high in lime. im- 
mediately on issuing from the furnaces it must be “granu- 
lated” by a jet of cold water. This material is dried and mixed 
with 15% to 40% of powdered slaked lime; and the mixture 
finely ground. 
