PROPERTIES OF PORTLAND .CEMENT. 



Chemical analyses of ignited samples. 



413 



Disinte- 

 grated ! 

 clinker, i 



Sound 

 clinker. 



i Per cent. 



Silica, Si0. 2 (total) J 20.66 



Alumina, A1 2 3 6.95 



Iron oxide, Fe 2 O s 4.63 



Lime, CaO , 64.18 



Magnesia, MgO ' 2.23 



Insoluble residue (sand) 0.81 



Per cent. 



21. 75 

 7.53 

 4.24 



63.44 

 2.00 

 1.00 



A mixture of 45 per cent of this disintegrated, and 55 per cent of this sound 

 clinker passes our cement specifications in all respects except in the percentage 

 loss by ignition. 



The records at this Bureau all indicate that just such mixtures were being 

 imported and accepted here four and five years ago. The erratic, unsatisfactory 

 nature of some of these shipments were fully discussed in the previous paper. 39 



Since local officials have insisted upon a higher specific gravity (at least 3.08, 

 dried at 110° C, but not ignited) and a loss by ignition of not more than 4.0 

 per cent, a sound cement made from hard-burned clinker is assured. 



The rotary-kiln process is capable of producing a more uniformly 

 burned clinker than the set kiln. Owing to the short time during which 

 the clinker is in the clinkering zone, extreme fineness in the grinding of 

 the raw-material is necessary to produce a perfectly sintered product. 

 Some manufacturers turn out an almost homogeneous, thoroughly sintered 

 rotary clinker. Others save coal and grinding expenses and are satisfied 

 to- produce a clinker which will season sound. As far as our knowledge 

 extends, by far the larger majority produce a hard-burned clinker, but one 

 which still contains a considerable percentage of free lime, and to this fact 

 alone we attributed the much-discussed cause of the characteristic drop 

 in strength of rotary cement. The observations and experiences which 

 have brought us to this conclusion are presented in another part of this 

 paper in a discussion on the significance and causes of the erratic develop- 

 ment of strength, characteristic of commercial Portland cements. How- 

 ever, in order to present the discussion in logical sequence the subject of 

 the setting properties of Portland cement will first be discussed. 



39 Loc. cit. 



