568 CEMENTS, LIMES, AND PLASTERS. 



therefore, iso tropic. This differentiates this aluminate very sharply 

 from the preceding one and makes the identification of the two materials 

 very easy. Specific gravity 2.79. 



"Tricalcic aluminate (A^OsSCaO): This aluminate crystallizes from 

 the fused condition in elongated octahedra. It is isotropic and it might 

 at first be assumed that it was not a definite compound, but merely the 

 dicalcic aluminate crystallizing out of a magma of indefinite composi- 

 tion. It has been shown, however, by further investigations too lengthy 

 to go into at this point, to be undoubtedly a definite aluminate. Specific 

 gravity 2.91. 



" Definite compounds of iron and lime and alumina and magnesia 

 have also been shown to exist, but their consideration here is unneces- 

 sary, as the constitution of Portland cement can be better discussed, 

 theoretically, by a study of clinker, into which these elements do not 

 enter. 



" Among the theories advanced as to the constitution of Portland 

 cement there are those which assume the presence of certain so-called 

 silico aluminates, such as 2SiC>2, 2A1 2 03, 6CaO, and others of less basic 

 form. All of these proposed compounds have been prepared by the 

 writer and found not to be definite chemical compounds nor to corre- 

 spond in any way with any of the mineral entities found in industrial 

 clinker. They are in fact only solid solutions, of aluminates in silicates, 

 of indefinite structure." 



Microscopic investigations. Le Chatelier, Tornebohm, and Richard- 

 son have studied both industrial and synthetic clinker under the micro- 

 scope, and the results of these preliminary studies have thus been sum- 

 marized by the last-named investigator: 



"By this method of study Le Chatelier, and, at the same time inde- 

 pendently of him, Tornebohm identified in Portland-cement clinker 

 four distinct mineral constituents which Tornebohm described as fol- 

 lows, naming them Alit, Belit, Celit, and Felit: 



"Alit is the preponderating element and consists of colorless crys- 

 tals of rather strong refractive power, but of weak double refraction. 

 By this he means that alit in polarized light between crossed nicol prisms 

 has insufficient optical activity to produce more than weak bluish gray 

 interference colors. 



" Celit is recognized by its deep color, brownish orange. It fills the 

 interstices between the other constituents, being the magma or liquid 

 of lowest freezing-point out of which the alit is separated. It is strongly 

 double refractive, that is to say, gives brilliant colors when examined 

 between crossed nicol prisms. 



