406 REIBLING AND REYES. 



ground cement which is being aerated (Table A 7 II), but a study of com- 

 mercial clinkers gives an explanation for this difference and likewise 

 reveals the causes of others. 



The presence of free lime in hard-burned clinker has always been traced 

 by us to too coarse grinding or poor mixing of the raw material, but 

 never to separation from a previously combined state. 



The effect of coarse grinding on the fusion product was demonstrated by 

 Campbell and Ball. 36 They were unable to burn the raw material secured from 

 a Lehigh district plant into a clinker which would produce sound, unseasoned 

 cement, even though the temperature of their experimental rotary klin was 

 raised as high as 1,612 degrees. When the same raw meal had been ground finer, 

 so that 98.0 instead of 86.5 per cent passed a 100-mesh sieve, the sound product 

 was obtained at 1,475 degrees. They concluded that the coarse particles of 

 calcined limestone had failed to combine and atributed the unsoundness to free 

 lime. Later, White 3 ' confirmed the correctness of these conclusions. 



Campbell worked with a miniature furnace. Despite the longer time 

 during which the raw meal is confined in the clinkering zone of com- 

 mercial kilns, our microscopic, physical, and chemical examinations of 

 commercial products and investigations at factories, all confirm the 

 belief that in the majority of instances the raw meal is too coarse to 

 produce perfectly fused clinker. 



For instance, the product of one large Portland cement mill always showed 

 free lime in considerable quantities. We had suspected that this might be due 

 to improper mixing, or slight underburning of the raw meal, or to a possible 

 mixing of bad and good cement. When the plant was inspected, the greatest 

 care was being taken to maintain a good mixture of raw-material. The rotary 

 clinker was also hard-burned and well sintered at the mill. It showed sintered 

 lime as before. However, the clinker was full of white specks. Some of these 

 were identified as free lime, and an examination of the raw meal showed that 

 only 78.4 per cent passed the 100- and 65.0 per cent the 200-mesh sieve. The 

 raw material was hard mountain limestone and clay. This is more difficult 

 to sinter properly than cement rock such as Campbell worked with. Similar 

 conditions were found at other plants. In fact, the grinding was often so coarse 

 that even the hard-burned clinker from stationary kilns showed free lime, visible 

 to the naked eye. 



Figure 14 is a photograph of several clinkers representing different 

 brands of cement. The clinkers are unseasoned and the surfaces photo- 

 graphed were freshly exposed. The white specks show the free lime in 

 the black, hard magma surrounding them. All unaerated clinkers which 

 failed to contain white specks also failed to show more than a trace of free 

 lime. 



™Journ. Am. Chem. 8oc. (1903), 25, 1103. 

 37 Loc. cit. 



