160 REIBLING AND REYES. 
TABLE XLIX.—Compressive strength of weathered 6-inch concrete cubes 
made with aérated and nonaérated cement J. 
L Compressive strength. 
Age in days. + 
“ Propor- Total in pounds. ounds per square | 
Cement J. Specine tions by inch. | 
3 oe ———————— ee 
-_, |Weather- 
In moist 1 year lyear 
air. ont 60 days. |3 months.) ©9 4a¥8- |3 months, 
As received_______- 3.18 | 1:2:5 21 435 82,630 | 108,300 2, 290 3,010 
Aérated 90 days --- 3.00} 1:2:5 21 435 45,115 84, 450 1, 250 2,350 
It was to be expected that the original hardening properties 
of a normal Portland cement would be restored almost entirely 
if the impermeable coatings of carbonate surrounding the aérated 
particles were destroyed by regrinding. The instances given in 
Table L show the large extent to which this really occurs. 
TaBLeE L.—Tensile tests of hard-burned cements N and O showing the 
characteristic effect produced by regrinding after through aération. 
Tensile strength of 1:3 Ottawa 
Fineness. sand mortars in pounds per 
g Z Cement. Ser eee | Specie __. Savane inch. *_ (jae 
BE] ¢ 200 | 100 ; 3 
a ra] mesh. | mesh. days. | 28 days. months. 1 year. 
Per ct. | Per ct. 
1 | N | As received ____.--.----- 76.4) 95.6| 3.11 2D | ae S85 5 |uaSos 
U2, 400) | (3,160) | (8,510) | (2, 990) 
2 | N | Sample No.1reground__| 87.6] 100.0] 3.09 | é a i By , a @ ee 
3 N | SampleNo. 1thorough-j| | 3.01 240 291 357 329 
ly aérated 20days. _|| (a, 670) | (2,350) | (8,290) | (8, 600) 
4 N | Sample No. 3reground__| 91.8} 100.0 3.01 le a G a eG <, sank 
5 | O | Asreceived_.....-------- m4] 97.1] 3.100}! 27 He Be $e 
le 820) | (2,460) | (8,130) | (8,260) 
6 | O | Sample No. 5 thorough- |] 77 ¢ 98.0 2.955 { 194 284 861 338 
ly aérated 100 days. || (1,730) | (2,050) | (2,190) | (2, 750) 
7 O | Sample No. 6 reground_- 89.4 98.8 2.950 le a @ BA @ as 4 ia 
® The figures inclosed in parentheses give the values obtained by compressing the broken 
briquettes. 
A low specific gravity (dried at 110°) is always associated 
with a correspondingly low early tensile strength.** Of course, 
"The strength developed by the aérated samples (numbers 4 and 7) 
after they had been reground offers an apparent contradiction to this 
statement. However, the assertion is made with reference to the com- 
mercial product, and as such we believe it true because no manufacturer 
goes to the great expense of thoroughly aérating his cement (a process 
a 
