158 REIBLING AND REYES. 
The specific gravity of even a hard-burned well-manufactured 
cement can be reduced to a low figure by thoroughly aérating 
the ground product, but this process reduces the early strength 
of a sound cement. The actual value of the change which 
excessive aération produces is measured best by the results 
obtained from mortars of normal consistency, but as the aérated 
cement usually requires the most water it may be argued that 
the low strength is the natural result of a leaner mixture. 
However, the data recorded in Table XLVI show conclusively 
that the aérated cement is weaker than the original product. 
TABLE XLVI.—Tests of the tensile strength of cement M which show that 
the weakness produced by excessive aération is inherent in the cement 
itself. 
Tensile strength of 1:3 Tarlac | 
sand mortars in pounds per 
Method used for Water square inch. 
used. 
Condition of the cement. molding the 
briquettes. 
7 days. |28days.\3 miontetl lyear. 
Per cent. 
As received ___-_-----_---------- Pa SpA Meese 12.5 195 218 274 342 
Aérated 3.5 months ___-----------|----- Go) 2-28-3223 13.5 96 155 215 347 
1D a a ee Seen acral GO ape tee ease 12.5 101 168 230 316 
ND Ye essay re ee Pe oj=22=2- 2-5-3 10.0 128 171 201 | 282 
D DY ee Se eae eras A.S. T. M-__------ 12.5 135 172 236 329 
Do sit wes ee Ps dos. =22 25225 10.0 131 175 238 266 | 
The weakness of the aérated cement is the result of the con- 
version of slaked lime into calcium carbonate, and the tendency 
of this reaction to confine itself to the surfaces of the individual 
particles of cement shows that the strength is affected by re- 
tarding rather than by eliminating the activity of the hydraulic 
properties. 
For instance, it is the finest particles of cement that are affected 
the most by aération and in several instances we have succeeded 
in aérating very finely ground cement until it had apparently 
lost all power of hardening. However, it was found that if 
this material was left in the molds and later submerged in water 
it eventually hardened and in time gained great strength. Two 
instances of such an experience are given in Table XLVII. 
