SLAG CEMENT: LIME, MIXING AND GRINDING. 655 



Sieving and grinding the lime. If lime has been thoroughly burned 

 and carefully slaked it will all be in the form of a very fine powder, much 

 finer than can be obtained by any economically practicable grinding- 

 machinery. In practice, however, it will be found that after slaking 

 the lime has not all fallen to powder, but still contains a certain pro- 

 portion of hard lumps. The degree of carefulness with which the burn- 

 ing and slaking have been conducted may be roughly judged by observ- 

 ing the relative proportions of lumps and powder. 



The material remaining as lumps is of three different kinds. First, 

 and in greatest proportion, are fragments of limestone which have not 

 been thoroughly burned in the kiln. Such unburned pieces would 

 be inert if used in the cement. Second, part of the lumps represent 

 fragments of limestone which have been overburned in the kUn and 

 have, therefore, partly clinkered. This is particularly likely to happen 

 if the limestone contained any large proportion of silica or alumina. These 

 partly clinkered lumps, being really poor-grade natural cements, can 

 if pulverized do no particular harm to the slag cement, but on the other 

 hand they cannot do as much good as an equal amount of lime. The 

 third kind of material that may be present in lump form consists of 

 fragments of well-burned lime, which, through accident or carelessness, 

 have not been well slaked. These lumps of quicklime would, if incor- 

 porated in the cement, be actively injurious. 



The preceding description and discussion of the three classes of mate- 

 rial which are likely to remain as lumps in the slaked lime have been 

 intentionally made detailed in order to point out an error in practice 

 committed occasionally at slag-cement plants. It has been seen that 

 the materials composing these lumps are of such a character as to be 

 either useless or actively injurious if used in a slag cement. It should 

 be obvious, therefore, that the only rational method of treatment is 

 to sieve the slaked lime and to reject entirely all the material failing 

 to pass through the sieve. This is the best practice and the method 

 usually followed. Occasionally, however, urged by a false idea of 

 economy or by inaccurate reasoning, the manufacturer saves the mate- 

 rial failing to pass the sieve, crushes it, and adds it to the cement at 

 a later stage in the manufacture. 



Proportions of lime and slag. Prost, in consequence of his experi- 

 ments with various proportions of lime, advocated the proportion, to 

 secure the best results, of from 35 to 40 parts of lime to 100 parts of 

 slag. He also stated that the amounts of lime used in actual practice, 

 for each 100 Ibs. of slag were: at Choindez, 40 to 45 Ibs.; at Donjeux, 

 40 Ibs. ; at Brunswick, 33 Ibs., and at Cleveland, 33 Ibs. Mahon, in report- 



