310 CEMENTS, LIMES, AND PLASTERS. 



inert and useless impurities in a cement rock. These silicates will flux 

 at a lower temperature than pure silica and are thus not so trouble- 

 some as flint or chert. They are, however, much less serviceable than 

 if the same amount of silica were present in combination with alumina 

 as a clay. 



Silica with alumina. Silica and alumina, combined in the form 

 of clay, are common impurities in limestones, and are of special inter- 

 est to the cement manufacturer. The best-known example of such 

 an argillaceous limestone is the cement rock of the Lehigh district of 

 Pennsylvania. Silica and alumina, when present in this combined 

 form, combine readily with the lime under the action of heat, and an 

 argillaceous limestone therefore forms an excellent basis for a Port- 

 land-cement mixture. 



Iron. Iron when present in a limestone occurs commonly as the 

 oxide (Fe20s) or sulphide (FeS2); more rarely as iron carbonate or 

 in complex silicate. Iron in the oxide, carbonate, or silicate forms 

 is a useful flux, aiding in the combination of the 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. 



Alkalies. Soda and potash occur usually in small percentages 

 and most commonly in the looser-textured limestones. It is probable 

 that these alkalies are largely driven off in the kiln, so that they do no 

 particular harm to the cement. If the total amount of alkalies is above 

 5 per cent, however, a sufficient amount will be carried over into the 

 cement to cause trouble; and raw materials carrying more than 5 per 

 cent of soda and potash together should therefore be looked upon with 

 suspicion, if not absolutely rejected. 



Sulphur. Sulphur may occur combined with lime as lime sulphate, 

 or combined with iron as the mineral pyrite. In either case it is an 

 injurious impurity, and the presence of over 1 to 1J per cent of total 

 sulphur should cause the rejection of the raw material. 



Physical characters of limestones. In texture, hardness, and com- 

 pactness the limestones vary from the loosely consolidated marls 

 through the chalks to the hard, compact limestones and marbles. Paral- 

 lel with these variations are variations in absorptive properties and 

 density. The chalky limestones may run as low in specific gravity as 

 1.85, corresponding to a weight of, say, 110 Ibs. 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 Ibs. per cubic foot. 



From the point of view of the Portland-cement manufacturer these 



