RAW MATERIAL: NATURAL-CEMENT ROCK. 215 



to make. Wherever a good natural-cement rock is obtainable, therefore, 

 these " artificial-natural " cements are driven out of the market. 



Natural-cement materials of Belgium. " Natural Portland'! 

 cement, as well as Roman cement, is extensively manufactured in cer- 

 tain parts of Belgium. 



The following analysis is representative of the composition of the 

 rock from which these " natural Portlands " are made: 



Silica (SiO 2 ) 15.75 



Alumina (A1 2 O 3 ) 3.95 



Iron oxide (Fe 2 O 3 ) 1 .00 



Lime (CaO) 43. 10 



Magnesia (MgO) 0.49 



Sulphur trioxide (SO 3 ) . 50 



Carbon dioxide (CO 2 ) 1 



Water j 35 ' 21 



Cementation Index 1 . 12 



It will be seen that if rock of the composition represented by the 

 above analysis could be steadily obtained it would certainly be an 

 excellent natural mixture for a Portland cement. In composition it 

 is admirable, while its index is about that of the average commercial 

 Portland. In practice, however, the variations in composition of the 

 rocks from various parts of the quarry are sufficient to prevent the 

 product from being sufficiently uniform to be considered a Portland 

 cement in our modern use of that term. This will be seen on referring 

 to the analyses of these Belgian products given on page 261. 



A report * on the Belgian cement industry states : 



" The most important center for the manufacture of natural and 

 artificial Portland cement in Belgium is the calcareous district of Tournai, 

 in the consular district of Brussels. Some of the quarries in this district 

 date back several centuries, when they were principally worked for 

 building stone and for the manufacture of hydraulic lime. The cal- 

 careous stone of these quarries, which originated the now extensive 

 and important industry of cement -manufacture, extends for many 

 miles in length in apparently inexhaustible quantity. Ordinary lime, 

 best hydraulic lime, slow-setting cement (Portland), and quick-setting 

 cement (Roman) are especial products of these immense quarries. 



" Natural Portland cement is obtained from calcareous stone, which 

 is carefully analyzed and dosed, calcined in coke-heated kilns, and, after 

 burning, finely pulverized. Before burning, the stone presents a fine, 



* Roosevelt, R. W. Manufacture of Portland cement in Belgium. 



