730 NEW YOKK STATE MUSEUM 



requires tliat tlie tests should also be applied to the cement a& 

 found in the market. Its quality may be so high that it will 

 stand the tests even if very coarse and granular, and, on the 

 other hand, it may be so low that no amount of pulverization 

 can redeem it. ' In other words, fineness is no sure indication of 

 the value of the cement, although all cements are improved by 

 fine grinding. Cement of the better grades is now usually ground 

 so fine that only from 5fc to lOfc is rejected by a sieve of 2500 

 meshes per square inch, and it has been made so fine that only 

 from 3/^ to 10^ is rejected by a sieve of 32,000 meshes per square 

 inch. The finer the cement, if otherwise good, the larger the 

 dose of sand it will take, and the greater its value. 



CHECKIXG OU CRACKING 



The test for checking or cracking is an important one, and, 

 though simple, should never be omitted. It is as follows: make 

 two cakes of neat cement, 2 or 3 inches in diameter, about i inch 

 thick, with thin edges. I^ote the time in minutes that these 

 cakes, when mixed with water to the consistency of a stiff plastic 

 mortar, take to set hard enough to stand the mre test recom- 

 mended by Gen. Gilmore, iV inch diameter wire loaded with ^ 

 of a pound, and -gV inch loaded with 1 pound. One of these cakes, 

 when hard enough, should be put in water and examined from 

 day to day to see if it becomes contorted, or if cracks show them- 

 selves at the edges, such contortions or cracks indicating that 

 the cement is unfit for use at that time. In some cases the 

 tendency to crack, if caused by the presence of too much unslaked 

 lime, will disappear with age. The remaining cake should be 

 kept in the air and its color observed, which, for a good cement, 

 should be uniform; the Portland cements being of a bluish gi^ay 

 throughout, yellowish blotches indicating poor quality; and the 

 natural cements being light or dark, according to the character 

 of the rock of which they are made. The color of the cements 

 when left in the air indicates the quality much better than when 

 they are put in water. 



TESTS RECOMMENDED 



It is recommended that tests for hydraulic cement be confined 

 to methods for determining fineness, liability to checking or crack- 

 ing, and tensile strength; and for the latter, for tests of seven 

 days and upward, that a mixture of one part of cement to one 

 part of sand for natural cements, and three parts of sand for 



