PACINI, METAMORPHISM OF PORTLAND CEMENT 163 



to the petrologist and hydrologist; and this paper is an attempt to treat 

 cement as a rock, differing from other rocks only in being artificial, but 

 subject to the same internal and external influences as other components 

 of the earth's crust. 



A training in geophysics and geochemistry is, perhaps, the most valu- 

 able asset in surveying the field of Portland cement. If no other end is 

 achieved by the following pages, the mere representation of the question 

 as a problem in applied petrology will, it is hoped, help future investi- 

 gators in a more systematic inquiry. 



Part I of this paper is devoted to a necessarily brief review of the 

 present status of the subject, and no attempt is made to discuss data 

 quantitatively. Experimental results in elaboration of the various points 

 discussed are presented in Part II. 



The experiments described in Part II were made, at the laboratory of 

 the New York Board of Water Supply by the writer, and in part by his 

 associates, in the course of the investigations of the Board. The most 

 modern and complete equipment was available, thanks to the prudent 

 foresight of the gentlemen at the head of this great engineering enter- 

 prise. The data are reproduced by permission from the periodical bulle- 

 tins of the Inspection division and from the annual report of the Board 

 for 1911. 



Nature of the Problem 



Portland cement is a finely ground artificial rock, whose essential 

 constituents are silica, alumina and lime. In it are found a number of 

 component minerals recognizable by definite optical properties, but the 

 individual constitution of which is not yet clear. The percentages of 

 these minerals vary somewhat according to the method of manufacture 

 and the purity of the raw materials, but there is, on the whole, a fairly 

 stable proportion in a series of normal cements. 



The method of manufacture of Portland cement will not be discussed 

 here further than to state that it consists essentially of the calcination 

 of a mixture of calcareous and argillaceous rocks at high temperatures. 

 Usually, about 2 per cent of gypsum or of plaster of Paris is afterwards 

 added to retard the set. By varying the proportion of these rocks, the 

 temperature and duration of calcination, the fineness of grinding, and 

 also by the addition of foreign substances products are obtained having a 

 wide range of hydraulic properties. 



The hydraulic properties are setting and hardening. Setting is the 

 attainment of rigidity by the plastic mixture of cement and water and 

 begins immediately after mixing, requiring several hours for completion. 



