Bye) ANNUAL REPORT 
ing new silicate compounds from the fact that when one mineral or com- 
pound is broken down by heat or other agency, its ingredients are liberated 
in the nascent state, and will enter at that time into new combinations with 
« vigor which they possess under no other circumstances. It is, therefore, 
advantageous to use natural or artificial silicate minerals as raw materials 
whenever the composition of such minerals lends itself to the matter in 
hand. For instance, we could use kaolinite, or silicate of alumina (pure 
clay), in a cement mixture, because both of its components, silica and 
alumina, are needed in the cement, but we could not use talc, the corre- 
sponding silicate of magnesia, because its magnesia would form an un- 
desirable element in the resultant cement. 
Also, while it is advantageous to use silicates, where those containing 
the proper elements are available, it rarely happens that the proportion in 
which these elements occur are also such as is needed in the compound to 
be made. For instance, kaolinite furnishes both silica and alumina to the 
cement-mix to which it is added, but it can be used in limited quantities 
only, because its proportions of alumina are high and the silica relatively 
low. If enough were used to bring in all the silica desired, the resultant 
cement would be overloaded with alumina. ‘Thus there is this obstacle to 
the free use of the silicates in producing new silicate compounds, and it is, 
therefore, generally necessary to obtain the desired composition by mix- 
ture of different silicates or more commonly by supplying the extra silica 
needed in one of its free or uncombined forms. 
ALUMINA AND FERRIC OXIDE. 
We have now discussed the various sources of silica, the crystalline, 
amorphous and combined, and have shown how the kind of silica influences 
the silicate reaction and how complex silicates are built up in the heat re- 
actions with which we are dealing, based on the conception of the hydrous 
polysilicic acids. But silicates of calcium oxide or magnesium oxide alone 
would not do the work required of mortars. Of the great numbers of 
stich silicates prepared by the writer, none of them showed the strength 
of even weak cements, but remained soft and friable; in water they often 
crumbled, and yet by the very preponderance of the silica in cements the 
calcium silicate must be the main hydraulic agent. But it is evident that 
the alumina must play an important role in causing the hydration reaction 
to take place in the manner which we know in cements. 
In order to show how alumina affects the hydration of cements it is 
perhaps best to indicate how alumina behaves when united with calcium 
oxide alone. Prof. Zulkowski*, thus found that a mixture of calcium 
oxide and alumina corresponding to the formula CaOAl,O,, burnt for 
eight hours when mixed with water, set in two minutes and hardened to 
a very hard mass over night; in three months it became so hard that it 
could hardly be scratched with a knife. A mixture of the formula 
2CaOAl,O, heated to vitrification also hardened with great rapidity in 
*Sonderabdruck aus ‘' Die Chemische Industrie,” 1901. 
