222 



TESTS ON CEMENT 



change of form, called initial set, and (2) when the setting is 

 complete, or when the mass cannot be appreciably distorted 

 without rupture, called hard set. The time-of-setting test 

 consists, therefore, in determining the time required for the 

 cement to reach these two critical points. 



The test is made by mixing cement with the amount of 

 water required to produce normal consistency, in the same 

 manner as for neat tensile briquets, forming specimens, 

 placing them under one of the forms of apparatus, and observ- 

 ing the time that elapses between the moment the mixing 

 water is added and the mo- 

 ments when the paste acquires 

 initial set and hard set. 



The Vicat needle, shown in 

 Fig. 6, consists of a frame k, 

 holding a movable rod /, which 

 carries a cap d at the upper 

 end and a needle h at the 

 lower. A screw f holds the 

 rod in any desired place. 

 The position of the needle is 

 shown by a pointer moving 

 over a graduated scale. The 

 rod with needle and cap weighs 

 exactly 300 gr. and the needle 

 is 1 mm. in diameter with 

 the end cut off square. When 

 making tests of normal con- 

 sistency, the plunger b is sub- 



FIG. 



stituted for the needle h, and the cap a for the cap d, the dif- 

 ference in weight between the needle and plunger being com- 

 pensated by the difference in the weight of the caps. The 

 mold i for holding the cement paste is in the form of a trun- 

 cated cone. It has an upper diameter of 6 cm., a lower dia- 

 meter of 7 cm., and a height of 4 cm., and rests on a 4"X4" 

 X i" glass plate j. 



After the cement paste is mixed, the mold is filled by 

 forcing the cement through the large end; then, after turning 

 it over and smoothing the top, it is placed on the glass plate 



