23 



with 30 per cent, of clayey material, up to say 80 per cent, lime 

 carbonate with 15 per cent, of silica, alumina and iron. The 

 lower beds of the formation are always higher in lime carbonate 

 than are the beds nearer the top of the formation. The content 

 of magnesium carbonate in these cement rocks is always high, 

 (as Portland cement materials go), ranging from 3 to 6 per cent. 



Xear, and in some cases immediately underlying these cement 

 beds, are beds of purer limestone ranging from 85 to 96 per cent, 

 lime carbonate. The usual practice in the Pennsylvania and 

 Xe\v Jersey plants has been therefore to mix a relatively small 

 amount of thiis purer limestone with the low lime "cement rock" 

 in such proportions as to give a cement mixture of proper com- 

 position. 



The economic and technologic advantages of using such a 

 combination of materials are very evident. Both the pure lime- 

 stone and the cement rock, particularly the latter, can be quarried 

 very easily and cheaply. As quarried they carry but little water 

 so that the expense of dryidg them is slight. The fact that about 

 four-fifths of the cement mixture will be made up of a natural 

 cement rock permits coarser granding of the raw mixture than 

 would be permissible in plants using pure limestone or marl with 

 clay. This point is more fully explained on a later page. 

 It seems probable, also, that when using a natural cement 

 rock as part of the mixture the amount of fuel necessary to clin- 

 ker the mixture is less than when pure limestone is mixed with 

 clay. 



Such mixtures of argillaceous limestone or "cement rock" 

 with a small amount of pure limestone evidently possess import- 

 ant advantages over mixtures of pure hard limestone or marl 

 with clay. They are, on the other hand, less advantageous as 

 cement materials than the chalky limestones discussed on 

 later pages. 



The analyses in Table 2 are fairly representative of the ma- 

 terials employed in the Lehigh district. The first four analyses 

 are of "cement rock"; the last two are of the purer lim>estone 

 used for mixing with it. 



