RAW MATERIAL: NATURAL-CEMENT ROCK. 201 



In determining in advance of actual calcination whether or not 

 a given rock will make a good natural cement the Cementation Index 

 will prove of service. This can be calculated, as explained on page 196, 

 from the analysis of the rock. If the value of the Cementation Index 

 is over 2.00, the rock will make only a very weak sort of cement, not 

 worth putting on the market as a new product in face of competition 

 from older and stronger brands. If, on the other hand, the Cementa- 

 tion Index is less than 1.00, the rock is in most cases unavailable, for 

 after burning it will contain too much free lime and free magnesia to 

 furnish a safe cement. As noted earlier, however, a rock whose index 

 falls between 0.80 and 1.00 can be made into an apparently safe cement 

 if it contains 20 per cent or more of magnesia, by burning at a very high 

 temperature. If the Cementation Index falls between 1.00 and 2.00 it 

 can be assumed that a natural cement of good quality can be made 

 from the rock under proper conditions of burning, etc. Within these 

 limits the properties of the cement will vary with the index. A rock 

 with an index of 1.00 to 1.10, for example, will require burning at high 

 temperature, especially if much lime be present (i.e., over 50 per cent 

 CaO). As the index rises, the temperature necessary for burning de- 

 creases. 



American Natural-cement Rocks. 



In the following pages analyses of the rocks used at almost all of 

 the natural-cement plants of the United States will be given. Notes 

 on the physical character, geology, and other features of these rocks 

 will also be presented. 



Clayey limestones of the composition required for natural-cement 

 manufacture are very widely distributed, both geologically and 

 geographically, in the United States. There is hardly a State, in fact, 

 in which natural cement of more or less value has not been made at 

 one time or another. In order, however, that a natural-cement industry 

 can become well established in any given locality, certain things are 

 requisite in addition to the occurrence of a good natural-cement rock. 



The rock must not only be of the right composition to make a good, 

 sound, and strong cement, but it must be fairly steady in composition, 

 and the beds must be located favorably for cheap extraction of the 

 rock, either by quarrying or by mining. Fuel must also be obtainable 

 at reasonable rates. A good local market and cheap transportation 

 to outside points are necessities. 



Of the many localities in the United States at which deposits of 

 good natural-cement rock occur, so few possess the commercial advan- 



