STATE GEOLOGIST. 43 
Trass also is a kind of volcanic tufa, it being conyolidated volcanic 
dust which has been subjected to the action of pressure and water. It 
is found in large quantities in the Rhine province, Germany, the top layers 
being loose in structure, the lower rock-like. Trass, like pozzuolane, is 
partly soluble in hydrochloric acid and contains water of hydration. 
Santorin-earth is found on the Island of Santorin and is likewise a 
kind of volcanic material, like pumice-stone, but is softer than the two pre- 
ceding materials. Itdiffers in its high content of free amorphous silica ;it is 
not so easily decomposed by acids and does not gelatinize. About one-fifth 
of the Santorin-earth is supposed to be pumice stone. 
ARTIFICIAL POZZUOLANES. 
Similar to the natural pozzuolane rock products, we can produce such 
materials artificially, and the two most important pozzuolane substances of 
this kind are the slags and the dehydrated clays. 
Slags.—The slags used for this purpose are the waste products of 
the blast furnaces smelting iron from its ores. Slags from the refining of 
pig iron, or the manufacture of steel, or from the manufacture of other 
metals than iron are not used. 
The slag as it leaves the furnace is converted into a true pozzuolane 
by being quenched in cold water while still in the hot liquid condition. 
But not every slag is available for hydraulic purposes. Several factors, 
of which the most important is the composition, govern the properties of 
the material. Tetmayer* in his extensive and thorough examination of 
the subject says the following in regard to the composition of suitable 
slags: | 
“The hydraulic activity of a slag depends in the first place on its 
basic character expressed sufficiently by the ratio of the lime to the silica. 
All slags corresponding to a ratio of Oe 
CaO: SiO,< 1.00 
are not suitable for cement making. The most favorable ratio of lime to 
to silica and to alumina is approximately 
% CaO :% SiO, :% Al,O,=46 :30 :16=1.00 :0.65 :0.35 or CaO :SiO,+ 
ENIE@) p= ie COMI OO! 
Magnesia is not a desirable constituent of slags and should be as low 
as possible. A high content of sulphur is very objectionable and for 
best results the content of sulphur should not be more than 1.25 per cent. 
With the same composition the greenish gray, more glassy, slag from gray 
iron is to be preferred to the yellowish foamy material. In the granu- 
lating process this slag should be as thin liquid as possible and the water 
used in quenching quite cold. 
*Resultate Spezieller Untersuchungen auf dem Gebiete der Hydraulischen Bindemittel 
page 152. 
