158 RBIBONG AND SALINGER. 



However, it lias been our experience that, as Spalding ^° states, "the 

 I'ate of setting of neat paste gives but little inclieaton of what the action 

 ma}' be with sand." Several instances of satisfactory neat and unsatis- 

 factory mortar tensile strength have been encountered in this laboratory, 

 because of the more rapid setting of the cement when combined with 

 sand. It is deemed sufficient to state here that the mortar and neat set 

 must vary because of the differing jjercentage of water which is used, the 

 difference in physical manipulation, in the air exposure, in mixing, the 

 physical and possibly also the chemical influence of the sand. 



For the thorough study of the nature of some cements the determina- 

 tion of neat and mortar setting qualities may he essential. A simple 

 method to determine the setting time of a mortar is here suggested. The 

 beginning of setting when sufficiently rajjid appreciably to influence the 

 briquette manipulation is characterized by sudden drying and a slight 

 stiffening of the mortar. If a mixture is made as if for briquettes and 

 the mortar then placed on a glass plate and divided into cubes with the 

 trowel, a slight set may readily be detected when a cube, upon being 

 crushed between the finger and thumb, feels dry, crumbles apart and offers 

 a slight resistance to the crushing force. A harder set may be arbitrarily 

 fixed and determined when the setting has progressed to the extent that 

 a one inch cube dropped from a height of one foot will not crack. 



However skilled the operator may be, or however accurate his method, 

 uniform results even by the same operator and on the same cement can 

 not be insured unless the precautions described under the iDrevious head- 

 ings of "Effects of aeration" and "The moist-air closet" are heeded. 

 Thus, the first sample taken from near the surface of an exposed package 

 not previously mixed, may set in twenty minutes, while succeeding sam- 

 ples taken at a lower depth may not set for hours. When it is desired 

 to make a series of comparative setting tests on the same"^ cement it is 

 advisable to remix the sample thoroughly before weighing and then store 

 the cement in tightly stoppered, wide-mouthed bottles rmtil it is used. 

 The pats should be stored in-insulated compartments of the moist-air 

 closet to avoid the influence of the heat liable to be generated by other 

 pats during setting. 



SOUNDNESS. 



Tests for soundness, like setting pats, should be made with a paste 

 of the correct p)lasticity. If too little water is used in gauging, the 

 cement will not adhere jjroperly to the plate, and lack of cohesion in the 

 cement itself may result in cracks not due to its subsequent expansion 

 or contraction. If too much water is used, shrinkage cracks of such a 

 nature as to be easilj^ mistaken for evidence of imsoundness, may occur. 



"/6id., 111. 



