172 CEMENTS, LIMES, AND PLASTERS. 



In later chapters, when the separate products are under discussion, 

 their respective Cementation Indexes will be determined and stated. 

 At present we are only concerned with determining the limiting values 

 of this index for the hydraulic limes. As will be seen from following 

 paragraphs, these limits are theoretically very wide, but in actual prac- 

 tice very narrow. 



Definition of hydraulic limes. The hydraulic limes include all 

 those cementing materials (made by burning siliceous or argillaceous 

 limestones) whose clinker after calcination contains so large a per- 

 centage of lime silicate (with or without lime aluminates and ferrites) 

 as to give hydraulic properties to the product, but which at the same 

 time contains normally so much free lime (CaO) that the mass of clinker 

 will slake on the addition of water. 



The commercial advantage of manufacturing a material of this 

 kind is that, while the product has hydraulic properties, yet its clinker 

 will slake and pulverize itself on the simple addition of water, thus avoid- 

 ing the expensive mechanical grinding required by the clinker of natural 

 and Portland cements. 



The definition, therefore, requires that a material to be called a 

 hydraulic lime must satisfy two conditions: (1) its clinker must con- 

 tain enough free lime to slake with water, and (2) the resulting powder 

 must be capable of setting or hardening under water. These two 

 requisite conditions, in their turn, fix the limits of lime that the clinker 

 may contain. The minimum amount of lime present is obviously 

 determined by the consideration that, after burning, enough free lime 

 (in addition to that combined with the silica, alumina, and iron) must 

 be present in the clinker to reduce the entire mass to powder by the 

 force of its own slaking. The maximum amount of lime, on the other 

 hand, is determined by the commercial condition that no more free 

 lime should be present than is absolutely necessary to accomplish this 

 pulverization, for the free lime, whose slaking powders the mass, is by 

 that same slaking made into an inert, or at least non-hydraulic, material. 



The desired result the formation of a clinker consisting largely of 

 lime silicates, etc., but also containing sufficient free lime to slake readily 

 can be attained in two different ways, which yield products very 

 different in quality. These two methods are: 



(1) By the calcination, at a medium temperature, of a siliceous or 

 argillaceous limestone having a Cementation Index lying between 0.30 

 and 1.10. Such a limestone will carry so high a percentage of calcium 

 carbonate (relative to its content of silica, alumina, and iron oxide) 

 as to leave, after most of its silica, etc., have been combined with lime, 



