bo 
=] 
STATE GEOLOGIST. 
in the production of a compound corresponding to the type formula 
(2CaO)SiO,. In the Portland cement reaction the burning temperature 
is raised still more and hence the acidity of the silica is increased, enabling 
it to combine with more of the base, and we thus obtain a product corre- 
sponding to the type of (3CaO)SiO,. There is no doubt that if a still 
higher temperature were employed more base could be taken up, chem- 
ically and by solution, and a still more complex hydraulic body obtained. 
THE CONSTITUENTS OF THE HYDRAULIC COMPOUNDS. 
Silicates of lime (or magnesia) constitute the main component of all 
hvdraulic cements of whatever kind. They are, however, invariably 
associated with alumina or ferric oxide, which either may be assumed 
to combine with lime, forming aluminates or ferrates of calcium, as in 
very basic cements, or to enter into the calcium silicate with the formation 
of a complex calcium-alumina (iron) silicate. 
Both the alumina and iron modify the properties of the simple cal- 
cium silicates very decidedly; in fact, they may be said to start the 
hydration reaction, which, without the sesquioxides, would be extremely 
slow, too slow for practical application. But as the aluminates are also 
hydraulic in themselves, their hydraulicity is added to that of the silicates 
and the total strength is thus increased. 
In addition to these functions, the alumina and iron decrease the 
melting point of the compounds, which means, of course, a lower point 
of vitrification, which is of vital importance in the burning of the highest 
type of cements. 
SILICA AND ITS SALTS. 
This highly important compound exists in nature in immense quanti- 
ties, forming a large part of the earth’s crust. It occurs in three principal 
forms: 1. Crystalline, or.glassy, quartz and quartzite. 2. Amorphous, 
dhydrous and anhydrous. 3. Combined in silicate minerals. 
It is derived from all of these forms in greater or less degree for use 
as an ingredient in cements. 
Crystalline Silica.—This, in its various modifications, is known as 
quartz. It is an important constituent of many igneous rocks, and as the 
silicate minerals of the latter decompose, thus breaking down the rock, the 
quartz remains unchanged. Its grains, ground down more or less by me- 
chanical abrasion, are removed by running water or wave action, as sand. 
- When deposited in layers and cemented together by various substances 
like calcium carbonate, ferric hydroxide, etc., they form sandstones. 
These, in turn, hardened and cemented by still further heat and pressure 
become quartzite. 
The purest forms of quartz crystal are perfectly colorless, having a 
specific gravity of 2.69, and are very hard. Quartz may be of almost any 
color, and its structure may be anything from the most perfect crystalline 
to the decidedly amorphous, as in fused quartz. Quartz is insoluble in 
all acids, with the exception of hydrofluoric acid. It is attacked, however, 
