20 



which it occurs is of great importance in connection with cernert 

 manufacture. 



(1) In perhaps its commonest form, silica is present in 

 nodules, masses or beds of flint or chert. Silica occurring in 

 this form will not readily enter into combination with the lime of 

 a cement mixture, and a chert y or flinty limestone is therefore 

 almost useless in cement manufacture. 



(2) In a few cases, as in the hydraulic limestone of Teil, 

 France, a large amount of silica is present and very little 

 alumina; notwithstanding which the silica readilv combines with 

 the lime on burning. It is probable that in such cases the silica 

 is present in the limestone in a verv finely divided condition, or 

 possibly as hydrated silica, possibly as the result of chemical 

 precipitation or of organic action. In the majority of cases, 

 however, a highly siliceous limestone will not make a cement on 

 burning unless it contains alumina in addition to the silica. 



(3) In the crystalline limestone (marbles) and less commonly 

 in uncrystalline limestones, whatever silica is present may occur 

 as a complex silicate in the form cxf shreds of mica, hornblende, 

 or other silicate mineral. In this form silicate is somewhat in- 

 tractable in the kiln, and mica and other silicate minerals are 

 therefore to be; regarded as inert and useless impurities in- a ce- 

 ment rock. These silicates will fitix at a lower temperature than 

 pure silica and are thus not so troublesome 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 limestone, and are of 

 special interest 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 Portland cement mixture. 



Iron. Iron when present in a limestone occurs commonly as 

 the oxide (Fe2C>3), or sulphide (FeS2) ; more rarely as iron 

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

 or silicate form, is a useful flux, aiding in the combination of the 



