902 Transactions of the American Institute. 



Strength. 



Portland cement, and in fact all mixtures of this cement and sand, 

 according to Major-General Gillmore, acquire, during the first two 

 years, fully nine-tenths of the strength and hardness which, in course 

 of time, they finally attain. Both the tensile and crushing strength 

 vary, of course, according to the quality of the cement ; but they are 

 generally the greater the denser the mortar. The tensile streagth 

 per square inch of blocks seven days old was found by Mr. Grant to 

 be 236 pounds for English cement powder weighing 103 pounds per 

 United States bushel, while it was 406 pounds for cement weighing 

 226 pounds, showing a difference of 170 pounds per square inch. 

 The strength of Boulogne cement, for blocks fifteen months old, was 

 found by General Gillmore to be 496 pounds. At the end of one 

 month, according to this authority, the tensile strength of neat Port- 

 land cement is equal to about two-thirds of what it attains during the 

 first two years. With regard to its crushing weight, it does not reach 

 its maximum limit within a period of less than two, or perhaps three, 

 years. The crushing weight of English Portland cement was found 

 by Mr. Grant to be 3,S06 pounds per square inch for blocks three 

 months old ; 5,388 pounds at the end of six, and 5,973 pounds at the 

 end of nine months. The tensile strength of the best Portland 

 cement, when seven days old, is about six times as great as that of the 

 best natural American cements. This holds true whether it is mixed 

 with a little or a good deal of water. The crushing weight of the 

 Portland cement, after the lapse of seven days, is, even when made 

 plastic like mortar, nearly twenty times as great as that of Iiosendale 

 cement. 



Neat cement finds but a very limited application, firstly because it 

 would be too expensive, and secondly from the fact that even if mixed 

 with sandy materials it is yet sufficiently strong for most practical pur- 

 poses. • Portland cement, mixed with three times its quantity of sand, 

 becomes, in a few months, superior to ordinary mortar more than a 

 hundred years old, while the cost of the material is in no small degree 

 lessened. 



With an admixture of from two to four volumes of sand, the cost 

 of the material is considerably lessened ; but above that figure, as 

 held by Michaelis, a decided disproportion between price and solidity 

 ensues. With regard to the tensile strength and crushing weight of 

 various mixtures of sand and cement, reference is made to Plate I, of 

 the Practical Treatise on Coignet Beton and other artificial stone, 

 by Major-General Gillmore. For foundations, flooring, houses, con- 



