608 CEMENTS, LIMES, AND PLASTERS. 



33. The trial pastes are made with varying percentages of water 

 until the correct consistency is obtained. 



34. The Committee has recommended, as normal, a paste, the con- 

 sistency of which is rather wet, because it believes that variations in 

 the amount of compression to which the briquette is subjected in mould- 

 ing are likely to be less with such a paste. 



35. Having determined in this manner the proper percentage of 

 water required to produce a neat paste of normal consistency, the proper 

 percentage required for the sand mortars is obtained from an empirical 

 formula. 



36. The Committee hopes to devise such a formula. The subject 

 proves to be a very difficult one, and, although the Committee has 

 given it much study, it is not yet prepared to make a definite recom- 

 mendation. 



TIME OF SETTING. 



37. Significance. The object of this test is to determine the time 

 which elapses from the moment water is added until the paste ceases 

 to be fluid and plastic (called the " initial set"), and also the time 

 required for it to acquire a certain degree of hardness (called the 

 "final" or "hard set"). The former of these is the more important, 

 since, with the commencement of setting, the process of crystallization 

 or hardening is said to begin. As a disturbance of this process may 

 produce a loss of strength, it is desirable to complete the operation of 

 mixing and moulding or incorporating the mortar into the work before 

 the cement begins to set. 



38. It is usual to measure arbitrarily the beginning and end of the 

 setting by the penetration of weighted wires of given diameters. 



39. Method. For this purpose the Vicat needle, which has already 

 been described in Paragraph 30, should be used. 



40. In making the test, a paste of normal consistency is moulded 

 and placed under the rod, as described in Paragraph 31 ; this rod, bear- 

 ing the cap at one end and the needle, 1 mm. (0.039 inch) in diameter, 

 at the other, weighing 300 gr. (10.58 oz.). The needle is then carefully 

 brought in contact with the surface of the paste and quickly released. 



41. The setting is said to have commenced when the needle ceases 

 to pass a point 5 mm, (0.20 inch) above the upper surface of the glass 

 plate, and is said to have terminated the moment the needle does not 

 sink visibly into the mass. 



42. The test pieces should be stored in moist air during the test; 

 this is accomplished by placing them on a rack over water contained 



