z 5 6 THE POPULAR SCIENCE MONTHLY, 



the same way have shown that these were unreliable as final 

 tests of strength, because the briquettes had not hardened suffi- 

 ciently, and the table would place inferior cements above those 

 of much greater strength because the inferior develops its ulti- 

 mate strength much sooner. But a comparative test of the same 

 cements when mixed with sand in equal parts was also made, 

 and is of very great value and probably perfectly reliable, for 

 the tests were then made at the end of ninety days, so giving the 

 slow-setting cements time to develop their strength. 



Thus the one which in the test applied to clear cement broke 

 at 38 pounds now sustained 152i pounds, an increase of four hun- 

 dred per cent ; while the one that was the strongest at the end of 

 seven days now broke at 204|- pounds, an increase of only fifty-four 

 per cent. The one that showed the greatest tensile strength of all, 

 at the end of ninety days, the Milwaukee cement, 290 pounds, broke 

 at only 96 at the end of seven days. An experiment made with a 

 briquette taken at random, that had been made six months and 

 exposed to the air at least half that time, strikingly showed the 

 same fact, for it broke only under a strain of 636 pounds. This 

 test was made simply to show the writer the method of using the 

 testing machine. 



The United States Government had a series of tests made a 

 few years ago, using the cements commonly sold in the West, and 

 giving in each case the mean result of seventy -five tests. The 

 table is so interesting that we give it entire. 



Tensile Strength of Pur.e Cements, each Test given "being the Mean Result from 

 Seventy-five Specimens, Thirty and Sixty Days. 



Cement is far more often called upon to resist a crushing than 

 a tensile strain. A large number of tests has been made to de- 

 termine the weight required to crush a cube one inch in each 

 dimension. When mixed with sand in equal proportions, the best 

 cements will sustain a crushing weight of upward of a ton, the 

 specimen having been allowed to harden for ninety days, while the 

 poorest do not sustain quite half a ton, and even when mixed with 

 three parts sand to one of the cement, the Milwaukee, which tests 

 have shown the best, sustains over eleven hundred pounds. These 

 tests show conclusively that structures well built of mixed cement 



