SMART GEOLOGISM: Pleas 
calcium hydrate loses its water at a much higher temperature. The fact 
that half of the fixed water escapes at 160° C. indicates that the molecule 
is a double one. The remaining water leaves at a much higher tem- 
perature. The process of hydration is represented by the following reac- 
tion: 
2(3CaO) SiO,+aq.=2[(2CaO)Si0,H,O]-+2Ca(OH),. 
_DETERMINATION OF FREE LIME. 
The attempts to accomplish the removal of free lime from the anhiy- 
drous as well as the hydrated cement are very numerous indeed, for it 
is fully realized by every investigator that if this could be accomplished 
the problem of the constitution of Portland cement would be cleared up. 
Various reagents for the extraction of the lime have been proposed, and 
we shall consider the. methods proposed briefly in the following para- 
graphs: 
The Work of Rebuffat, —Rebuffat* used for these purposes an 
aqueous sugar solution,which method,however,has been proven byl eret** 
to be erroneous, since the solution acts upon and decomposes the basic 
silicate. Michaelis has raised the same objection and claims that any 
results based upon such a solution must be wrong. The conclusions of 
Rebuffat must, hence, be accepted with considerable caution. In brief 
they are as follows. 
He divides cements into two classes, viz: 
tr. Cements of simple, compact structure, non-crystalline, derived 
from a mixture of calcium oxide, silicate, and aluminate. These com- 
prise the hydraulic limes and quick setting cements. 
2. Cements of crystalline structure derived from a combination of a 
crystalline mass, consisting of 2CaOSiO, and CaO and the calcium 
aluminate in various proportions—Portland cements and cements high 
in silica. 
He assumes that the quantitative constitution of the different hydraulic 
cements after setting is the same. They are formed from a mixture 
of lime hydrate, hydrated calcium silicate, and hydrated calcium alumi- 
nates, with a slight quantity of accessory ingredients. The calcium sili- 
cate is the ortho-silicate—2CaOSiO,. The quantity of water of hydra- 
tion which is absorbed does not exceed 51% per cent., so that the hydrated 
compound has the formula (2CaOSiO,),.+H.O. In cements high in 
silicate the presence of a certain quantity of meta-silicate must be ac- 
cepted. This does not hydrate, but reacts with the aluminates and 
forms double silicates of calcium and alumina. This fact is important, 
as it explains the resistance of cements to sea water. i 
*Gaz. Chim. Ital. 1898 and Tonindustrie Zig. 1901, No. 105. 
**International Cong. for Testing Building Materials, Paris, 1990. 
