186 REIBLING AND REYES. 
(15) The effects of seasoning on the hardening properties of 
Portland cements: 
A. Cement produced from soft, decidedly underburned Portland clinker: 
(a) Nonseasoned, underburned Portland cement is unsound. 
(b) Underburned clinker seasons readily. 
(c) Seasoned to soundness, it hardens very slowly, but shows a 
steady increase for an apparently indefinite length of 
time. 
(d) Ordinarily, expansion due to free lime occurs before or soon 
after the set, and, therefore, if the percentage of mag- 
nesia is low, it is safe to assume the ultimate soundness 
if the neat mortar does not disintegrate after the usual 
tests. 
B. Hard-burned Portland cements: 
(a) The hard-burned clinker may, or may not, produce a cement 
which requires no curing to enable it to pass the accelerated 
test for soundness. 
(6) Seasoned or nonseasoned, provided the cement passes the 
hot tests, it may be used with reasonable certainty of its 
ultimate soundness; no reliance can be placed on the 
normal 28-day tests for strength or soundness—facts 
which ought to be sufficient cause for the rejection of all 
Portland cements which fail to pass the hot tests. 
(c) Under normal conditions the expansion due to free lime in 
indurated cement develops so slowly that it may not 
effect the early strength. 
(d) The cement hardens very rapidly and attains a great early 
strength, but unlike the soft-burned product the strength 
at the end of years is usually less than at the end of 
twenty-eight or sometimes even seven days. 
C. Sound commercial Portland cements: 
(a) The early strength of even hard-burned cements can be 
reduced to a low figure by thoroughly aérating the ground 
product. 
(b) Prolonged aération of the ground product has no marked 
effect on the ultimate strength of hard-burned cement. 
(c) Aération results in the conversion of slaked lime into cal- 
cium carbonate, and the tendency of this reaction to 
confine itself to the surfaces of the individual particles of 
cement accounts for the manner in which the hydraulic 
properties are retarded rather than eliminated. 
(d) Owing to this coating of carbonate, the rate of induration of 
the inner active constituents of aérated cement depends 
upon the permeability and exposure of the mortar. 
(e) The original hardening properties of an aérated, hard-burned 
cement are restored almost entirely by regrinding; but, 
although such treatment tends to increase the efficiency 
both in sand-carrying capacity and in constancy of 
strength and volume, it is too expensive to be practical. 
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