21 



lime and silica in the kiln. When present as a sulphide, in the 

 form of the mineral pyrite it is to be avoided in quantities over 2 

 or 3 per cent. 



Physical characters of limestones. in texture, hardness, and 

 compactness, the limestones vary from the loosely consolidated 

 marls through the chalks to the hard compact limestones and 

 marbles. Parallel with these variations are variations in absorp- 

 tive properties and density. The chalky limestones may run as 

 low in specific gravity as 1.85, corresponding to a weight of say 

 110 pounds per cubif oot, while the compact limestones com- 

 monly used for building purposes range in specific gravity be- 

 tween 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 x counterbalance each other; the softer the limestone 

 the more absorbent is it likely to be. These purely economic fea- 

 tures \vill be discussed in more detail in later chapters. 



Effect of heating on limestone. On heating a non-magnesian 

 limestone to or above 300 C., its carbon 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 th,e burning mass. In practice this is ac- 

 complished 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 sim- 

 ple a character. At temperature of 800 C. and upwards these 

 clayey impurities will combine with the lime oxide, giving sili- 

 cates, aluminates and related salts of lime. In this manner a 

 natural cement will be produced. An artificial mixture of cer- 

 tain and uniform composition, burned at a higher temperature, 

 will give a Portland cement, the details of whose manufacture 

 are discussed on later pages. 



