180 CEMENTS, LIMES, AND PLASTERS. 



In the earlier days of hydraulic-lime manufacture in France (and 

 even at the present day in England) it was the practice to put the 

 hydraulic lime on the market in lumps, just as it is drawn from the 

 kiln, leaving the work of slaking it to the purchaser. At present, how- 

 ever, the slaking in the French works is done at the lime-plant. The 

 advantages of this method of procedure are that (1) the slaking is done 

 more uniformly and carefully, so that the value and reputation of the 

 lime is improved, and (2) the lime gains considerably in weight and bulk 

 during slaking, so that the cost of slaking is made up. 



Slaking should be done with as little water as is compatible with 

 thorough slaking. The lime as drawn from the kiln is therefore spread 

 out in thin layers and lightly sprinkled with water. It is then shoveled 

 up into heaps or into bins, where it is allowed to remain for ten days 

 or so. The slaking is completed, while the lime is thus heaped up, by 

 the aid of the steam which is generated. 



After slaking is completed, the lime remains as a fine powder inter- 

 spersed with lumps (grappiers) of harder material. These lumps con- 

 sist in part of lime silicate and in part of unburned or underburned 

 limestone. It would be desirable if practicable to remove the latter 

 material, as' it is, of course, valueless as a cement. The lumps of lime 

 silicate, on the contrary, will, if finely ground, make a good natural 

 cement. This separation is, however, commercially impracticable, 

 and therefore all the grappiers are treated together. 



The lime after slaking is passed over screens (of about 50-mesh). 

 These screens permit all the slaked lime to pass, but reject the grap- 

 piers. The lime is sent to the packers, while the grappiers are ground 

 finely under millstones. So far as can be learned, a certain percentage 

 of ground grappiers is always added to the lime, in order to increase 

 its hydraulicity. As later briefly noted (p. 185), the grappiers alone 

 are also sold as a cement. 



The analyses by Durand-Claye, given in Table 75, are quoted in 

 Spalding's "Hydraulic Cements", p. 20, and serve to illustrate the com- 

 position of the various products. 



In this series analysis No. 1 is of the lime which has completely 

 powdered during slaking and passed through the first sieve, while 

 analysis No. 3 is of the grappiers rejected by this sieve. It will be 

 seen that while the slaked lime has a Cementation Index of 0.992, the 

 grappiers are proportionately less rich in lime (CaO), having an index 

 of 1.63. In order to increase the hydraulic properties of the lime which 

 has passed the sieve, a certain proportion of ground grappiers is added 

 to it. This causes the lime as marketed to have a Cementation Index 



