DEFINITION AND RELATIONS OF NATURAL CEMENTS. 195 



sition and properties of the various natural cements will be discussed 

 in considerable detail. 



In the present volume the term " natural cements " will be used to 

 include all those cements which are produced by burning, without pre- 

 vious mixing or grinding, a naturally impure limestone rock, i.e., a 

 clayey or argillaceous limestone. As so used the term will include 

 the class of doubtful products commonly known as " natural Portland 

 cements ", a class which is quite largely manufactured in Belgium 

 and France. The reasons for including these " natural Portlands " 

 with the natural cements instead of with the true Portlands are stated 

 in detail in a later section of this volume. 



The definition of natural cements given on a previous page can 

 be restated here to advantage. 



Definition. Natural cements are produced by burning a natural 

 clayey limestone containing 15 to 40 per cent of silica, alumina, and 

 iron oxide without preliminary mixing and grinding. This burning 

 takes place at a temperature that is usually little, if any, above that 

 of an ordinary lime-kiln. During the burning the carbon dioxide of 

 the limestone is almost entirely driven off, and the lime combines with 

 the -silica, alumina, and iron oxide, forming a mass containing silicates, 

 aluminates, and ferrites of lime. In case the original limestone con- 

 tained any magnesium carbonate the burned rock will contain a corre- 

 sponding amount of magnesian compounds. 



After burning, the burned mass will not slake if water be poured 

 on it. It is necessary, therefore, to grind it quite fine, after which, 

 if the resulting powder (natural cement) be mixed with water, it will 

 harden rapidly. This hardening, or setting, will take place either in 

 air or under water. 



Relations of natural cements to others. Natural cements differ 

 from ordinary limes in two very noticeable ways. These are: 



(1) The burned mass does not slake when water is poured on it. 



(2) After grinding, natural-cement powder has hydraulic proper- 

 ties, i.e., if properly prepared it will set under water. 



Natural cements are quite closely related to both hydraulic limes, 

 on the one hand, and Portland cement, on the other, agreeing with 

 both in the possession of hydraulic properties. They differ from 

 hydraulic limes, however, in that the burned natural-cement rock will 

 not slake when water is poured on it. 



The natural cements differ from Portland cements in the following 

 important particulars: 



(1) Natural cements are not made by burning carefully prepared 



