30 CEMENT MATERIALS AND INDUSTRY. [bull. 243. 
1.85, corresponding to a weight of 110 pounds per cubic foot, while 
the compact limestones, commonly used for building purposes, range 
in specific gravity between 2.3 and 2.9, corresponding approximately 
to a range in weight of from 140 to 185 pounds per cubic foot. 
From the point of view of the Portland-cement manufacturer these 
variations in physical properties are of economic interest chiefly in 
their bearing upon two points — the percentage of water carried by 
the limestone as quarried and the ease with which the rock may be 
crushed and pulverized. To some extent the two properties counter- 
balance each other; the softer the limestone the more absorbent is it 
likely to be. These purely economic features will be discussed in 
more detail on later pages. 
EFFECT OF HEAT ON LIMESTONE. 
On heating a nonmagnesian limestone to or above 300° C. its car- 
bon dioxide will be driven off, leaving quicklime (calcium oxide, CaO). 
If a magnesian limestone be similarly treated, the product would be a 
mixture of calcium oxide and magnesium oxide (MgO). The rapidity 
and perfection of this decomposition can be increased by passing 
steam or air through the burning mass. In practice this is accom- 
plished either by the direct injection of air or steam or more simply 
by thoroughly wetting the limestone before putting it into the kiln. 
If, however, the limestone contains an appreciable amount of silica, 
alumina, and iron, the effects of heat will not be of so simple a charac- 
ter. At temperatures of 800° C. and upward these cla}^ey impurities 
will combine with the lime oxide, giving silicates, aluminates, and related 
salts of lime. In this manner a natural cement will be produced (see 
pp. 333-334). An artificial mixture of a certain uniform composition, 
burned at a higher temperature, will give a Portland cement. 
ARGILLACEOUS LIMESTONE (CEMENT ROCK). 
An argillaceous limestone containing approximately 75 per cent of 
lime carbonate and 20 per cent of clayey materials (silica, alumina, 
and iron oxide) would, of course, be the ideal material for use in the 
manufacture of Portland cement, as such a rock would contain within 
itself in the proper proportions all the necessary ingredients. It 
would require the addition of no other material, but when burnt alone 
would give a good cement. This ideal cement material is, of course, 
never found, but certain argillaceous limestones approach it very 
closely in composition. 
The most important deposit of these argillaceous limestones or 
"cement rocks" is that which is so extensively utilized in Portland- 
cement manufacture in the Lehigh district of Pennsylvania and 
New Jersey. As this area still furnishes about two-thirds of all the 
Portland cement manufactured in the United States, its raw materials 
are described in some detail on pages 31-32. 
