THE THEORY OF HYDRAULIC LIMES. 173 



sufficient free lime to slake the clinker. Hydraulic limes produced 

 in this fashion are the typical hydraulic limes, and the following chapters 

 will have reference to such materials only. It is possible, however, to 

 produce a hydraulic lime by another method, as above noted. This 

 second and much less satisfactory method is 



(2) By the calcination, at temperatures too low to permit perfect 

 combination of the silica, alumina, and iron oxide with the lime, of a 

 siliceous or argillaceous limestone (less rich in lime than those employed 

 in the first method) having a Cementation Index of 1.10 to 1.60 or over. 

 In other words, a rock is used which would, under proper conditions of 

 burning, give a good natural cement. If it is burned at too low a tem- 

 perature to effect this, however, the result will be a hydraulic lime, for 

 the clinker will consist partly of silicate and aluminate of lime, together 

 with notable amounts of free lime, free silica, and free alumina. 

 Hydraulic limes produced in this way necessarily carry a very large 

 proportion of absolutely inert material. They are, in fact, simple 

 imperfectly burned natural cements and will not be discussed further 

 in this connection. 



Reverting to the true hydraulic limes, it has been said above that 

 their Cementation Index may range from 0.30 to 1.10; and it will be 

 seen later that commercial hydraulic limes do occur with indexes as 

 low as 0.331, while others are as high as 1.06. 



There is, however, considerable reason for dividing the true hydraulic 

 limes into two groups, the first or eminently hydraulic limes containing 

 those products whose index lies between 0.70 and 1.10; while the second 

 group, or feebly hydraulic limes, contains products whose Cementation 

 Index ranges from 0.70 down as low as 0.30. Commercial as well as 

 theoretical differences serve to separate the two groups, and for that 

 reason they will be discussed in separate chapters. Curiously enough, 

 each of the two classes has an attendant secondary product to be con- 

 sidered. The eminently hydraulic limes during their calcination pro- 

 duce a by-product (grappiers) which is usually marketed separately 

 as a " grappier cement ". The feebly hydraulic limes on the other hand 

 are often treated with sulphuric acid in such a way as to develop new 

 properties, and are then marketed as selenitic limes. In further dis- 

 cussion of the hydraulic limes, therefore, they will be treated as two 

 groups in two separate chapters, covering respectively 



Chapter XIV. Eminently Hydraulic Limes : Grappier Cements. 



Chapter XV. Feebly Hydraulic Limes : Selenitic Limes. 



