46 The Philippine Jouimal of Science i9i8 



DISCUSSION OF RESULTS 



Tensile strength. — The general tendency of all the solutions 

 is to decrease the tensile strength. Of the 352 results here 

 recorded (which represent 1,056 briquettes), only 44 show an 

 increase in strength, and as a rule, this increase is small. The 

 greatest number of such cases occurs with the sulphates. The 

 increases occur mostly with cements I and II and with the 1.0 

 and the 0.5 normal solution. 



The decrease in tensile strength is most prominent with the 

 7-day briquettes, and the principal ones are with the maximum 

 concentration of each solution employed. Cement IV is the 

 most sensitive to the effect of the solutions in lowering the 

 tensile strength. With this cement, every solution used causes 

 one or more series of briquettes to fall below the specified limits. 

 This cement is highest in calcium content. 



Setting time. — Apparently there is no relation between the 

 effect of the solutions on' the set and on the tensile strength. 

 On the basis of their effect on the set, the salts may be divided 

 into two groups: 



1. Sodium chloride, sodium nitrate, potassium nitrate, ammo- 

 nium nitrate, sodium sulphate, sodium bicarbonate, and potas- 

 sium bicarbonate. 



> 2. Zinc chloride, copper chloride, zinc sulphate, and copper 

 sulphate. 



When a member of the first group is added to a cement, a 

 small amount of the salt causes a retardation of the set. After 

 a maximum point is reached, the set is accelerated by further 

 additions of the substance, until the original setting time is 

 reached or even passed. Within the limits investigated, the 

 behavior of these electrolytes is, in general, similar to many 

 others that have been investigated from time to time, such as 

 sodium sulphide and calcium sulphate. ^^ The second group 

 shows a retardation with the lowest concentration of each 

 solution employed. The retardation increases with increasing 

 concentration. For the ranges studied the time-concentration 

 curves of these salts rise indefinitely. This is contrary to the 

 corresponding curves of group 1, which pass thi'ough maximum 

 points. 



Though it was expected that the results would tend to. divide 

 the salts into groups, the basis of division indicated by this 

 work was somewhat surprising. This basis is not the solubility 



" Witt, J. C, and Reyes, F. D., loc. cit. 



