eckel.] COMPOSITION OF PORTLAND CEMENTS. 23 
together the underburned and overburned materials formed during 
the burning of natural cements. Several of these brands contain from 
5 to 15 per cent of magnesia; and under no circumstances can they be 
considered true Portland .cements. 
In view of the conditions above noted the writer believes that the 
following definition will be found more satisfactory than the one above 
quoted: 
Portland cement is an artificial product, obtained by finely pulver- 
izing the clinker produced by burning to semifusion an intimate mix- 
ture of finely ground calcareous and argillaceous material, this mixture 
consisting, approximately, of one part of silica and alumina to three 
parts of carbonate of lime (or an equivalent amount of lime). 
COMPOSITION AND CONSTITUTION. 
During recent years much attention has been paid by various inves- 
tigators to the constitution of Portland cement. The chemical com- 
position of any particular sample can, of course, be readily determined 
by analysis, and by comparison of a number of such analyses general 
statements can be framed as to the range in composition of good 
Portland cements. 
Portland cements may be said to tend toward a composition approx- 
imating to pure tricalcic silicate (3CaO.Si0 2 ) which would nearly cor- 
respond to the proportion CaO, 73.6 per cent, Si0 3 , 26 A per cent. 
As can be seen, however, from commercial analyses, actual Portland 
cements differ in composition somewhat markedly from this. Alumina 
is always present in considerable quantity, forming, with part of the 
lime, the dicalcic aluminate (2CaO.Si0 2 ). This would give, as stated 
by Newberry, for the general formula of a pure Portland — 
x (3CaO.Si0 2 ) + y (2CaO.Al 3 3 ). 
The composition is still further complicated by the presence of acci- 
dental impurities or intentionally added ingredients. These last may 
be simply adulterants, or they may be added to serve some useful 
purpose. Calcium sulphate is a type of the latter class. It serves to 
retard the set of the cement and, in small quantities, appears to have 
no injurious effect which would prohibit its use for this purpose. In 
dome kilns sufficient sulphur trioxide is generally taken up by the 
cement from the fuel gases to obviate the necessity for later addi- 
tion of calcium sulphate, but in the rotary kiln its addition to the 
ground cement, in the form of either powdered gypsum or plaster of 
Paris, is a necessity. 
Iron oxide within reasonable limits seems to act as a substitute for 
alumina, and the two may be calculated together. Magnesium car- 
bonate is rarely entirely absent from limestones or clays, and mag- 
nesia is therefore almost invariably present in the finished cement. 
