PACINI. METAMORPHISM OF PORTLAND CEMENT 165 



minerals; and looking to the identification and classification of these 

 minerals, a great deal of investigation has. been done. 



By trial burnings of simplified mixtures, such as lime-silica melts, and 

 by microscopical examination of sections of the resulting clinker, the 

 problem is gradually being clarified, but, owing to its great complexity, 

 much controversial literature thereon has been issued on both sides of 

 the Atlantic (52, 69, 80, 64, 65, 88). The theories put forth have so far 

 had little practical effect upon the manufacture and composition of the 

 commercial product ( 63 ) . 



No complete and final enumeration of the chemical compounds result- 

 ing from the burning of such a mixture of clay and limestone has yet 

 been accepted as authoritative. The microscopical identification of the 

 individual chemical compounds which go to make up the mineralogical 

 entities is at best somewhat unsatisfactory, especially because of the 

 minuteness of the particles of raw materials necessary to secure thorough 

 and uniform calcination, and consequently the extremely small size of 

 the resulting crystals and aggregates. It has been proposed, in this con- 

 nection, to secure these of a size available for study by the expedient of 

 fusing the clinker in an electric furnace; and, by this means, a partial 

 clarification of the system has been obtained (103). 



Mineralogical Constitution 



The minerals which are recognized in cement clinker have been named 

 alit, belit, felit and celit (101), and a metamorphism 3 of these occasioned 

 by the action of water is the cause of the setting and hardening of Port- 

 land cement. 



Alit has been reported a solid solution of tri-calcic silicate in tri-calcic 

 aluminate, and celit a solution of di-calcic aluminate in di-calcic silicate 

 (61). Other investigators have reported alit and celit to be silicates of 

 different silicic acids (26). 



Belit is probably a calcium aluminum silicate of the composition 

 Ca 3 Al 2 Si 2 O 10 , a form found in nature as the mineral gehlenite (27). 



Setting Process 



Precisely what chemical reactions and physical transformations take 

 place in the setting and hardening processes is not yet definitely settled. 

 It may, however, be stated that by modifying the proportions of clay to 

 limestone through a certain range, we obtain a product which varies in 

 its speed of setting and of hardening. In general, cements high in silica 



3 Metamorphism : Any change in the constitution of any kind of rock, Van Hise (104). 



