PHYSICAL AND CHEMICAL PROPERTIES. PARTS IV, V. 17] 
TABLE LI.—Relative changes in tensile and compressive strength produced 
by aération. Cement O, reground. 
Strength of 1:3 Ottawa sand mortar in pounds per 
square inch. 
ear Description Specific Tensile strength Compressive strength 
No P P gravity. ful P Ee 
7 28 3 1 7 28 3 1 
days. |days. months. | year. | days.|days.|months.| year. 
1 | Nonseasoned --____---_--- 3.145 | 357] 399 364 | 350 /3, 130 |3, 160 3, 620 | 3,600 
2| Aérated 3 days -_-___-_-- 3.09 379 | 417 431 | 402 |2,970 |3, 265 8,565 | 4,200 
3 | Aérated 7 days ____- paces] 8b 358 | 454 462 | 451 |2,635 |3, 765 3,300 | 4,560 
The microscopic test of the cement aérated three days showed 
very little nonslaked free lime. Owing to the preliminary hydra- 
tion, its expansion, especially during the early stages of indura- 
tion, was much less than that of the nonseasoned material, and 
the early tensile and compressive strengths were increased. 
Sample 3 had aérated until most of the slaked lime had 
been converted into calcium carbonate which accounts for the 
low strength at seven days. The beneficial results obtained 
from greater constancy in volume are apparent from the strength 
developed later. 
Apparently there is no direct relationship between the cor- 
responding tensile and compressive strengths during the period 
of time in which the cements show marked changes in volume. 
However, at the end of one year, when the volume has become 
constant and apparently no internal stresses are operating, there 
is a more or less direct relationship between these factors. 
Hence, it appears that fluctuations in strength are caused by 
internal strains which tend to produce changes in volume, and 
that, within the limits of perfect elasticity, these internal strains 
affect the strength in tension and in compression in unlike 
manners. 
Numerous instances proved that the internal stresses developed 
in a contracting mortar do not favor the compressive strength, 
and that a decrease in volume after an abnormal expansion is 
very apt to be accompanied by a corresponding decrease in 
compressive strength even though the tension test shows at the 
same time an increase in strength. 
It has been stated that the beneficial results derived from 
aération, as recorded in Table LI, are due to the removal of 
nonslaked free lime. If this is true, then the early strength 
