164 ANNALS NEW YORK ACADEMY OF SCIENCES 



Hardening is the progressive increase in strength acquired by the mass, 

 and it attains the greater part of its ultimate value in about a year. 

 Even after this period it is subject to a small progressive increase (42 ) 2 . 

 In general the properties of setting cement are to be found both in 

 mortars, or mixtures of cement and sand, and in concretes, or mixtures 

 of cement, sand and broken stone, these chemically inert materials added 

 to the cement exerting a physical influnce on metamorphism. 



CHEMICAL COMPOSITION 



The chemical composition of normal Portland cement is shown in the 

 following tables : 



Avcrayc of 300 Normal American Portland Cements, Representing 20 Vraml* 



of AH Types 



(Analyses by the writer for the Board of Water Supply) 



A1,0 S 

 CaO 

 MgO 

 SO 3 



Average of 100 German Portland Cements 

 (Burchartz, (12) ) 



SiO 20.87 



Fe,O ; . 2 . 98 



ALO.," 7 . 63 



CaO 02 . 99 



MgO 1 .55 



SO 3 1 . 85 



The ultimate chemical composition of a cement is only, however, a 

 rather indirect clue to its hydraulic properties, just as the ultimate 

 analysis of a composite rock may only give a faint idea as to its con- 

 stituent minerals or possible products of metamorphism. For example, 

 it would be quite possible to synthesize a mixture which would, on analy- 

 sis,, correspond exactly to the chemical composition of an excellent Port- 

 land cement, yet which, when gaged with water in the ordinary way, 

 would develop practically no tensile strength, in fact would possibly fail 

 to set at all. 



Cement, therefore, must owe its hydraulic possibilities to a particular 

 grouping of its constituent compounds, quite analogous to a series of 



2 Numbers in parentheses refer to the bibliography at the end of this article. 



