<ki:i. I PUZZOLAN CEMENTS. 371 
lepend on the use to which the cement is to be applied. As before 
Mentioned, the rapidity of set increases naturally with the amount of 
dumina in the slag. Set can be artificially hastened by the addition 
)f puzzolanic material to the cement. Burned clay, certain active 
forms of silica, slags high in alumina, etc., are additions which are 
>oth effective and cheap. The treatment of the cement during manu- 
acture with alkalies to accelerate the set has already been discussed. 
Strength. — While slag cements fall below high-grade Portlands in 
ensile strength, good American slag cements develop sufficient 
strength to pass the usual specifications for Portlands. Tested neat, 
hey do not approximate so closely to the Portlands as they do if 
ested in 2:1 or 3:1 mortars. Part of this property may be due to the 
act that they are, in general, ground finer than Portlands, especially 
han foreign Portlands. A few years ago Prof. W. K. Hatt made a 
series of tests on American slag cements, and reported that there was 
10 noticeable deficiency in strength of briquettes kept in air as corn- 
Dared with those kept in water. Other investigators have arrived at 
)pposite conclusions, and it is probable that these conflicting results 
irise from differences in the chemical composition of the various 
grands tested. 
Resistance to mechanical icear. — Slag cements are notably deficient 
n this property, and are therefore not available for use for the sur- 
faces of pavement, floors, etc., where this quality- must be highly 
leveloped; they seem to be well fitted, however, for pavement foun- 
lations, or indeed for any work which will not be exposed to dry air 
md in which a high strength is not necessary. 
PLANTS, PRICES, AND USES. 
Of the seven slag-cement factories now in operation in the United 
States, two each are located in Alabama and Ohio, and one each in 
[llinois, New Jersey, and Pennsylvania. Most of the plants now in 
)peration are connected closely in ownership with the furnaces from 
tvhich the slag is obtained. This condition is almost a necessity, since 
common ownership or control furnishes the only possible guaranty 
;hat a sufficient supply of slag, of proper quality, will be always 
Available. 
The selling price of slag cements is highly variable, as can be under- 
stood from statements made on previous pages. Whenever possible, 
:hey are sold as Portlands, and in that case approximate in price to 
ements of that class. When necessary, however, they are marketed 
it prices below those of natural cements. Taking the entire annual 
product into consideration, its price per barrel will probabty average 
ibout 20 to 25 per cent higher than the best brands of natural cements 
