SLAG CEMENTS: COMPOSITION AND PROPERTIES. 673 



(7) Fineness. Ninety-seven per cent of the cement must pass 

 through a sieve made of No. 40 wire, Stubb's gauge, having 10,000 

 openings per square inch. 



(8) Specific gravity. The specific gravity of the cement, as deter- 

 mined from a sample which has been carefully dried, shall be between 

 2.7 and 2.8. 



(9) Soundness. To test the soundness of cement, pats of neat 

 cement mixed for five minutes with 18 per cent of water by weight 

 shall be made on glass, each pat about 3 inches in diameter and inch 

 thick at the center, tapering thence to a thin edge. The pats are to be 

 kept under wet cloths until finally set, when they are to be placed in 

 fresh water. They should not show distortion or cracks at the end 

 of twenty-eight days. 



(10) Time of setting. The cement shall not acquire its initial set 

 in less than forty-five minutes and shall acquire its final set in ten hours. 

 The pats made to test the soundness may be used in determining the 

 time of setting. The cement is considered to have acquired its initial 

 set when the pat will bear, without being appreciably indented, a wire 

 T *2 inch in diameter loaded to \ Ib. weight. The final set has been 

 acquired when the pat will bear, without being appreciably indented, 

 a wire ^ inch in diameter loaded to 1 Ib. weight. 



(11) Tensile strength. Briquettes made of neat cement, after being 

 kept in air under a wet cloth for twenty-four hours and the balance 

 of the time in water, shall develop tensile strengths per square inch as 

 follows : 



After seven days, 350 Ibs.; after twenty-eight days, 500 Ibs. 

 Briquettes made of one part cement and three parts standard sand 

 by w r eight shall develop tensile strength per square inch as follows: 

 After seven days, 140 Ibs.; after twenty-eight days, 220 Ibs. 



(12) The highest result from each set of briquettes made at any 

 one time is to be considered the governing test. Any cement not show- 

 ing an increase of strength in the twenty-eight-day tests over the seven- 

 day tests will be rejected. 



(13) When making briquettes neat cement will be mixed with 18 

 per cent of water by weight, and sand and cement with 10 per cent of 

 water by weight. After being thoroughly mixed and worked for five 

 minutes the cement or mortar will be placed in the briquette mould 

 in four equal layers and each layer rammed and compressed by thirty 

 blows of a soft brass or copper rammer, f of an inch in diameter or 

 T \ of an inch square, with rounded corners, weighing 1 Ib. It is to 

 be allowed to drop on the mixture from a height of about half an inch. 



