:ckel.] PUZZOLAN CEMENTS. 359 
In composition trass varies between the following extremes: Silica, 
L5 to 65 per cent; alumina, 10 to 23 per cent; iron oxide, 3 to 12 per 
ient; lime, 1 to 8 per cent; magnesia, to 3 per cent; alkalies, 1 to 7 
Der cent; water, carbon dioxide, etc., 3 to 12 per cent. 
Santorin, another puzzolanic material, is obtained from the island 
)f Santorin, or Thera, one of the most southeasterly of the islands of 
he Grecian Archipelago. 
ARTIFICIAL PUZZOLANIC MATERIALS. 
By far the most important of the puzzolanic materials is blast- 
urnace slag, especially in the United States, where natural puzzolanic 
naterials of domestic origin have never come into use, though trass 
md puzzolanic cements made from it are imported to a small extent. 
■>lag, on the other hand, is the basis of an important industry — the 
nanufacture of slag (puzzolan) cement. The materials and processes 
ised in making this product will be described in some detail on the 
Allowing pages. 
THE MANUFACTURE OF SLAG CEMENT. 
Slag (puzzolan) cement is made by intimately mixing granulated 
blastfurnace slag of proper composition with slaked lime, and reduc- 
ng this mixture to a line powder. This product, though usually 
called a Portland cement by the manufacturers, is different from a 
;me Portland in both its rational and ultimate compositions and in 
ts processes of manufacture. Further than this and more important 
(from the purchasers 1 standpoint, a cement of this class has certain 
jualities which prevent its being used as an exact substitute for Port- 
and cement, though it is a good enough material for certain uses. 
COMPOSITION OF THE SLAG. 
The slag used in cement manufacture must be basic blast-furnace 
slag. Tetmajer, the first investigator of slag cements, announced as 
the results of his experiments (a) that the hydraulic properties of the 
slag increased with the proportion of lime contained in it, and that 
slags in which the ratio ^ was so low as to approach unity were val- 
ueless for cement manufacture; (h) that, so far as the alumina content 
of the slag was concerned, the best results were obtained when the ratio 
iAI O 
Q .' A 3 gave a value of 0.45 to 0.50; and (c) that with any large increase 
oivJ 3 
of alumina above the amount indicated by this value of the alumina- 
silica ratio the tendency of the cement to crack (when used in air) was 
increased. 
