STATE GEOLOGIST. DAL 
of the character of magnesia, its slow hydration, must lead to the general 
conclusion that dolomitic Portland cements are found to show two dis- 
tinct rates of hydration, one due to the hydration of the lime compounds, 
the other due to the magnesium ‘reactions. 
Magnesia is active only at lower temperatures, and its rate of hydra- 
tion is the slower the higher the compound has been burnt. But since 
two hydrations, a rapid and a slow one, cannot take place in a cement 
without injuring the structure, we must conclude that the cement should 
not contain enough magnesia to exert any: appreciable influence. How- 
ever, a content of magnesia up to 5 per cent., according to the experi- 
ments made so far, has proven harmless. 
CALCULATION OF PORTLAND CEMENT MIXTURES. 
For the calculation of the necessary amounts of clay and lime 
carrying material we have several available methods which we shall 
discuss briefly. 
The Chatelier Formula.—American cement chemists make exten- 
sive use of the limiting Chatelier formula modified by Newberry: 
4 (3CaO+Si0,) +y(2CaO+Al,O,). 
Let us assume we have a clay of the formula: 
IN {O).5 SiO),O.22 18 CO.alal(O 
and, say, a pure limestone. We would have then the formula: 
34 CaO 7Si0,--2yCaO-+-yAl,O,, or 
(34-L2y)CaO+45i0,+ yAI,O,. 
But the clay contains 5 molecules of silica and 1 of alumina and the 
required molecules of calcium oxide will be: 
(1s-+2)=17CaO. 
We find, hence, that the values x and y are the co-efficients of the 
molecules of silica and alumina in the clay. The formula of the cement 
is then: 
IWjCa@sSi® yA @,o22he,O0;. 
The sum of the molecular weights of the clay formula is: 
5910,+Al,O,+0.22Fe,0,+2H,0=—473.2. 
_ Therefore the limestone and the clay must be mixed in the ratio of 
I7X100 (molec. weight of calcium carbonate) =1700 :473.2, or for each 
part by weight of the clay 1700+473.2—3.59 parts of limestone must be 
taken. 
