180 REIBLING AND REYES. 
(c) Cement made entirely from underburned clinker seldom appears 
on the market except as hydraulic lime, because it would fail 
to pass the test for strength. Underburned cement usually 
comes to the consumer mixed with the harder-burned material 
from the same mill and the present specifications of the 
American Society for Testing Materials are such that a mixture 
of 45 per cent of disintegrated clinker and 55 per cent of sound 
clinker passed all requirements except the percentage loss by 
ignition. This emphasizes the great value of the test for 
volatile constituents. 
(d) The best set-kiln process yields a considerable amount of under- 
burned clinker which, if not removed, does not give a true 
Portland cement but a mixture of seasoned underburned and 
well-burned cement containing sintered, nonsintered, and hy- 
drated free lime, calcium carbonate, etc. 
(e) The rotary kiln is capable of producing a more uniformly burned 
clinker than the set kiln. Extreme fineness in the grinding of 
the raw material is necessary to produce a perfectly sintered 
product. Few manufacturers grind finely enough. The ma- 
jority produce a hard-burned clinker which still contains a 
considerable percentage of free lime some of which fails to 
slake before induration and causes the much-discussed charac- 
teristic drop in the strength of rotary cements. 
PART III. THE SETTING PROPERTIES OF PORTLAND CEMENT. 
(5) For determining the time of initial and final set, the 
method employing the Vicat needle was found to be reliable, 
impartial, and accurate, and abnormal results were obtained only 
with cements of very poor quality. 
(6) Preliminary experiments on the setting properties of com- 
mercial products demonstrated the following: 
(a) Although cements of different chemical composition have dif- 
ferent natural setting tendencies, changes in the setting proper- 
ties are due primarily to the absorption of water or water 
and carbon dioxide. 
(6) Changes in the setting properties were independent of the ulti- 
mate chemical composition, the fineness, the amount of retarder, 
the quantity of water and carbon dioxide absorbed, the nature 
of the raw materials used, or the burning process employed. 
(c) Changes in the rate of set are brought about largely by alterations 
in the condition of the free lime. 
(7) Large quantities of fresh, nonseasoned clinkers with char- 
acteristically different chemical and physical properties were 
studied and the following results were obtained: 
A. Nonseasoned, nonplastered cements: 
(a) Manipulated in the ordinary way, the plasticity is poor, and 
the set abnormal and apparently slow and erratic. 
