236 



CEMENTS, LIMES, AND PLASTERS. 



low and the alumina and iron oxide high. A rock of this general 

 type can therefore vary quite widely in composition and degree of burn- 

 ing without running any risk of turning out an unsound product. 



As the percentages of lime and magnesia rise, however, the prob- 

 lem becomes more difficult, and when the index falls below 1.20 a com- 

 paratively slight variation in the composition of the rock or in the 

 degree to which it is burned is apt to give a product containing too 

 much free lime for safety. 



When this condition the presence of free lime or magnesia in the 

 product occurs frequently, the manufacturer has three options. He 

 may (1) burn at a higher temperature, (2) look for a lower-lime rock, 

 or (3) slake the free lime in some way. The first choice would usually 

 be the best, but in nine cases out of ten the manufacturer will take 

 the third, as being apparently the cheapest. 



Free lime in a natural cement may be neutralized either by aerating 

 the ground cement or by sprinkling or steaming the unground clinker. 

 When aeration is practiced, the ground cement must be exposed to 

 the air as freely as possible. This implies that it should be spread 

 out, rather than placed in deep bins, and consequently requires con- 

 siderable floor-space and manual labor. Steaming or sprinkling the 

 unground clinker requires less space and labor, but care must be taken 

 that excess of steam or water is not allowed to reach the cement. The 

 ideal aimed at is to supply sufficient moisture to slake the free lime, 

 but to leave the aluminates and silicates untouched. Simple storage 

 of the clinker, with free access to the air, will often accomplish this 

 result. An incidental benefit to the manufacturer which comes from 

 slaking the clinker (either by steaming, sprinkling, or storage) lies in 

 the fact that the lime in slaking helps to disintegrate the> clinker and 

 thus reduce the cost of grinding. 



Grinding the Clinker. 



When natural cement was first manufactured in this country, the 

 millstones used at flour-mills were the only available fine grinders. 

 Grinding practice at natural-cement plants was therefore soon estab- 

 lished in a form which has persisted at many plants to the present 

 day. Even now a few small plants, I believe, grind natural cement 

 and flour in the same building at different times of the year. 



Until quite recently, grinding practice was almost uniform. The 

 burned rock was sledged if necessary, fed through a cracker or other 

 comparatively coarse reducer, and finished on millstones of one type 



