162 REIBLING AND REYES. 
the microscopic test revealed an almost transparent field in polar- 
ized light due to the presence of a large quantity of calcium 
hydroxide phenol crystals (Table VIII, Part 1). On the other 
hand cement G which has the highest specific gravity and shows 
the greatest strength in both sand and neat, 7- and 28-day 
mortar tests, was obtained from hard-burned clinker because it 
requires a well-sintered product to produce a sound Portland 
cement with a specific gravity as high as 3.11. The compar- 
atively great early strength is the more marked because this 
cement was rather coarsely ground. 
CHARACTERISTICS OF COMMERCIAL PORTLAND CEMENT. 
General classification—These facts concerning the hardening 
properties of underburned and hard-burned Portland cements 
enable us fully to designate the character of all commercial 
cements and to account for the general nature of their hardening 
properties. 
Adulterated, coarsely ground, or caked cements are readily 
detected by means of chemical or physical examinations. Such 
products show poor early and ultimate strength. 
Underburned cements, when sound, have a low gravity, and 
microscopic and chemical tests prove whether the low gravity 
is due to underburning or to excessive seasoning. 
If the cement is sound, a high gravity (above 3.10) is positive 
proof of hard-burning ; and we have always found that such high- 
gravity cements show the characteristic hardening properties 
of well-sintered Portland cement. 
Intermediate products of cement kilns are represented hy a 
specific gravity which is neither high nor low. Such materials 
usually consist of a mixture of soft- and hard-burned cement, 
and show no definite character in hardening properties. 
Early strength.—The high early strength of Portland cement 
depends primarily upon the thorough sintering or fusing of the 
raw materials into desirable compounds of base- and acid-forming 
metallic oxides. The best commercial practise produces a sound, 
finely ground product with a low content of volatile constituents 
and a specific gravity above 3.10. Well-burned cements, if un- 
duly exposed to moisture or atmospheric influences, will deterio- 
rate; not, however, without a lowering of the specific gravity 
and a correspondingly increased loss by ignition. Therefore, 
in order to obtain the most desirable material it is necessary to 
demand perfect soundness, extreme fineness, and a high specific 
gravity. 
