CONSTITUTION, ETC., OF PORTLAND CEMENT. 569 



" Belit is recognized by its dirty green and somewhat muddy color 

 and by its brilliant interference colors. It is biaxial and of high index 

 of refraction. It forms small round grains of no recognized crystalline 

 character. 



" Felit is colorless. Its index of refraction is nearly the same as that 

 of belit and it is strongly double refractive. It occurs in the form of 

 round grains, often in elongated form, but without crystalline outline. 

 Felit may be entirely wanting. 



" Besides these minerals an amorphous isotropic mass was detected 

 by Tornebohm and Le Chatelier. It has a very high refractive index. 



" Tornebohm adds the important fact that a cement 4 per cent richer 

 in lime than usual consists almost entirely of alit and celit." 



Theories of constitution. Until recently Portland-cement clinker 

 was commonly assumed to be a mixture of two or more definite chemi- 

 cal compounds, and the principal points at issue between various inves- 

 tigators were: (1) the exact formulas for these compounds, and (2) the 

 proportion in which they must exist to give a good Portland cement. 

 The two theories, in this regard, that have made the most impression 

 upon modern cement practice are those presented respectively by Le 

 Chatelier and Newberry. Recently, however, Richardson has formu- 

 lated a theory of entirely different type. These three explanations of 

 the constitution of Portland clinker will, therefore, be described briefly. 



Le Chatelier, speaking of Portland-cement clinkers, states * that: 



"Examined in thin plates under the microscope they are formed 

 of tricalcic silicate in crystals, with very feeble double refraction, 

 embedded in a crystalline ground-mass of silico-alumina ferrites of 

 lime. These are the two essential elements of Portland cement. If 

 the lime is in excess, aluminate of lime is first formed; then for a still 

 greater excess, ferrite of lime, and finally free lime. If, on the other 

 hand, the lime is deficient in quantity, a dicalcic silicate is formed, 

 recognizable by the spontaneous crumbling of the burnt pieces of cement. 

 When the mixture is imperfect or the burning insufficient, the reactions 

 remain incomplete, and although the average composition may be 

 suitable, there is a simultaneous production of free lime and aluminate 

 of calcium with dicalcic silicate. In a Portland cement of normal com- 

 position the proportion of lime, according to the chemical formulas 

 of the compound, should be greater than that determined by the follow- 

 ing formula: 



Si0 2 -Al 2 3 -Fe 2 03^ 



CaO >3 ' (1) 



* Trans. Amer. Inst. Mining Engineers, vol. 22, pp. 3-52. 



