214 ANNUAL REPORT 
The calcium aluminate may have one of the following formulas: 
CAOQALO- 
AC AQUA (Os 
aCalOVAlLOr, 
But unless there is an excess of lime or unless certain conditions of 
burning prevail, it seems that. only CaOAI,O, and 2CaOAlI,O, should 
be considered. 
The quantity of water chemically fixed by hydration cannot be 
determined beforehand. The cements hydrate in different degrees ac- 
cording to whether they harden in pure or lime water. To this are 
due many peculiarities in setting. 
In setting, the calcium ortho-silicate is hydrated to form 2(2CaO 
SiO,)-+H,O, which is followed by the hydration of the calcium alumi- 
nate. These two reactions take place in different phases for different 
cements. 
With cements of crystalline structure, it must be assumed that the 
calcium. ortho-silicate is found combined with CaO and the calcium 
aluminate in a definite crystalline condition. Portland cement in the 
hydrated condition contains but small quantities of calcium aluminate. 
This explains that in cements of a crystalline character the setting may be 
caused by the hydration of the calcium ortho-silicate according to the 
following recation:: 
2(3CaOSiO,) +3H,O—2(2CaOSiO,) H,0+2Ca(OH),. 
This is associated with the hydration of the aluminate, which in 
Portland cement consists mainly of mono-calcium aluminate and hydrates 
with seven molecules of water. 
CaOAl,O,-+-7H,O=CaOAl,O,7H,0. 
Rebuffat found that all the water of crystallization does not become 
free at low heat, but part is expelled at a bright red heat. 
Other Investigators. —Dr. Tomei treated hardened cement with an 
aqueous solution of ammonium chloride and found that the amount of 
lime thus dissolved decreased in amount after some time. He con- 
cluded from his experiments that the stability of the lime compounds 
increases with time, and that part of the silica from the sand of the mortar 
enters into combination with the lime. 
In 1895, A. Hauenschild® treated a number of hydraulic cements with 
various solutions of ammonium salts and found that the salts did not 
permit of an exact determination of the lime compounds. 
1Tonindustrie Ztg 1895, p. 177 ! 
2 Tbid, 1895, p. 239, 
