kvkk.I CEMENT INVESTIGATIONS IN ARIZONA. 377 
imum of voids, the compressive strength of the concrete increases as 
the proportion of stone increases and as the volume of voids between 
the stone decreases, and decreases as the proportion of sand in the 
mortar increases. The rule, therefore, holds: to secure the greatest 
strength mix the maximum quantity of stone with a minimum of 
sand mortar sufficient to bond the stone together, the sand mortar 
being rich in cement. An extensive bed of exceptionally good sand 
for mortar was found near the Buttes, the grains graduating in size 
from very small to large sizes. It shows only 35 per cent of voids, 
while the standard cement-testing sand used in laboratories has 45 
per cent of voids. 
A large tube mill will grind 10 barrels of sand cement per hour to 
the requisite degree of fineness, at an estimated cost of 20 cents per 
barrel. The cost of power for grinding is calculated at 3 cents per 
horsepower per hour at the dam site. 
( 'ost of sand cement with Portland cement at ss per barrel. 
One-half barrel. Portland $4. 00 
One-half barrel crushed and coarse-ground qnartzite . .18 
Grinding same in a tube mill ... .20 
Royalty on account of sand-cement patent .05 
Total cost of sand cement per barrel (375 pounds) : 4. 43 
The cost of sand cement per pound, qnartzite being used as the 
source of the sand cement, would therefore be 1.2 cents. 
The cost of the mill per barrel of cement, 20 cents, is not included 
in the above. 
It is estimated that the cost per barrel of Colton Portia cement 
delivered at the dam site will be $8. This, for 375 pounds weight 
cement, would make the cement cost 2.12 cents per pound. 
USE OF ROCKS AT THE DAM SITES. 
For the purpose of ascertaining whether the rocks at the dam sites 
could be used in the manufacture of Portland cement the localities 
were visited and samples were obtained of those rocks which occurred 
in sufficient quantities to furnish the supply necessary for manufac- 
turing the large amounts of cement needed. In looking for the raw 
materials it must be borne in mind that, chemically considered, Port- 
land cement consists of a compound of tricalcium silicate and dical- 
cium aluminate, accompanied by small percentages of ferrate and 
sulphate of lime and traces of alkalies. It is made by grinding and 
burning together either natural or artificial mixtures of carbonate of 
lime and silicate of alumina. Limestones, chalks, or mails usually 
furnish the carbonate of lime, and clays are the ordinary source of 
the alumina and silica. The mixtures are burned at a high tempera- 
ture to a blackish clinker of a semivitrihed character. After cooling, 
this clinker is reduced by grinding loan impalpable powder, in which 
form it is known by the generic name of Portland cement. 
