eckel.] COST OF MATERIALS. 43 
cent. A Portland-cement mixture composed of a pure chalky lime- 
stone and a clay might, therefore, average 10 to 20 per cent of water; 
consequently about 700 pounds of such a mixture would be required 
bo make one barrel of finished cement. 
With marls the loss on drying- and burning is much greater. Rus- 
ell states a that according to determinations made by E. D. Campbell 
L cubic foot of marl, as it usually occurs in the natural deposits, 
ontains about 47i pounds of lime carbonate and 48 pounds of water, 
[n making cement from a mixture of marl and clay, therefore, it would 
je necessary to figure on excavating and transporting over 1,000 
sounds of raw material for every barrel of finished cement. 
Thus the cost of raw materials at the mill, per barrel of cement, 
vill vary not only with the cost of excavation but with the kind of 
naterials in use. In dealing with hard dry materials extracted from 
)pen quarries near the mills the cost of raw materials may range from 
> to 15 cents per barrel of cement. The lower figure is probably 
.bout the lowest attainable under good management and favorable 
latural conditions; the higher figure is probablv a maximum for 
airly careful management of a difficult quarry under Eastern labor 
onditions. When it is necessar}^ to mine the materials the cost will 
>e somewhat increased. Cement rock has been mined at a cost equiva- 
Bnt to 10 cents per barrel of cement, but only under particularly 
avorable conditions. The cost of mining and transportation may 
each 20 cents per barrel. 
| With regard to wet marls and clays, it is difficult to give even an 
pproximate estimate. It seems probable, however, when the dead 
e|/eight handled is allowed for, that these soft materials will cost about 
alf as much delivered at the mill per barrel of finished cement as the 
ard dry limestones and shales. 
METHODS OF MANUFACTURE OF PORTLAND CEMENT. 
If, as in this bulletin, the so-called "natural Portlands" (see p. 
2) are excluded, Portland cement ma}^ be regarded as an artificial 
roduct obtained by burning to semifusion an intimate mixture of 
lulverized materials containing lime, silica, and alumina in varying 
-oportions within certain narrow limits, and by crushing finely the 
inker resulting from this burning. If this restricted definition of 
ortland cement be accepted, four points may be regarded as being of 
irdinal importance: (1) The cement mixture must I >e of the proper 
hemical composition; (2) the materials must be carefully ground and 
itimately mixed before burning; (3) the mixture must be burned at 
le proper temperature; (1) after burning, the resulting clinker must 
e finely ground. 
igl a Twenty-second Ann. Rept. V . s. Geol. Survey, pt. 3, p. 657. 
