9 



The natural cements differ from Portland cements in the fol- 

 lowing important particulars : 



(1) Natural cements are not made by burning carefully pre- 

 pared and finely ground artificial mixtures, but by burning 

 masses of natural rock. 



(2) Natural cements, after burning and grinding, are usu- 

 allv yellow to brown in color and light in weight, their specific 

 gravity being about 2.7 to 2.9 ; while Portland cement is com- 

 monly blue to gray in color and heavier, its specific gravity 

 ranging from 3.0 to 3.2. . 



(3) Natural cements are always burned at a lower tempera- 

 ture than Portland, and commonly at a much lower temperature, 

 the mass of rock in the kiln never being heated high enough to 

 even approach the fusing or clinkering point. 



(4) In use, natural cements set more rapidly than Portland 

 cement, but do not attain such a high ultimate strength. 



(5) In composition, while Portland cement is a definite pro- 

 duct whose percentages of lime, silica, alumina and iron oxide 

 vary only between narrow limits, various brands of natural ce- 

 ments will show very great differences in composition. 



The material utilized for natural cement manufacture is in- 

 variably a clayey limestone, carrying from 13 to 35 per cent, of 

 clayey material, of which 10 to 22 per cent, or so is silica, while 

 alumina, and iron oxide together may vary from 4 to 16 per 

 cent. It is the presence of these clayey materials which give the 

 resulting cement its hydraulic properties. Stress is often care- 

 lessly or ignorantly laid on the fact that many of our best known 

 natural cements carry large percentages of magnesia, but it 

 should, at this date, be realized that magnesia (in natural ce- 

 ments at least] may be regarded as being almost exactly inter- 

 changeable with lime, so far as the hydraulic properties of the 

 product are concerned. The presence of magnesium carbonate 

 in a natural cement rock is then merely incidental, while the 

 silica, alumina and iron oxide are essential. The 30 per cent, or 

 so of magnesium carbonate which occurs in the cement rock of 

 the Rosendale District, N. Y., could be replaced by an equal 

 amount of lime carbonate, and the burnt stone would still give 

 a hydraulic product. If, however, the clayey portion (silica, 

 alumina, and iron oxide) of the Rosendale rock could be re- 

 moved, leaving only the magnesium and lime carbonates, the 



