XI, A, 6 Witt: The Effect of Sulphide on Cement 287 



are given in terms of grams sulphur, as sulphide, per cubic 

 centimeter of sodium sulphide solution. 



Sulphur per cubic centimeter — Gram. 



In original solution 0.0460 



In blank after warming 0.0240 



After warming with iron oxide 0.0090 



The ferric oxide, therefore, removed 0.0150 gram sulphur from 

 each cubic centimeter of solution. The total iron present was 

 0.3500 gram. The total sulphur removed w^as 0.1500 gram. 



It was apparent that not all of the ferric oxide had been at- 

 tached by the sulphide. The film which formed around each 

 small particle protected the interior, and the action stopped 

 as soon as mechanical equilibrium {Mechanisch Labil ^^) was 

 established. 



The black substance was insoluble in the sulphide solution 

 and was filtered out with the unconverted oxide for the purpose 

 of examining its composition. However, on attempting to wash 

 it with water, it produced a dark green, slightly turbid solution. 

 More of the substance has been prepared, and its behavior has 

 been studied. The results indicate that it is a colloid. The sub- 

 stance may be precipitated by a number of electrolytes, among 

 which are sodium sulphide, calcium hydroxide, and sodium sul- 

 phate. On this basis both chemical and physical tests point to 

 the following explanation : 



When cement is gauged with a sodium sulphide solution, the 

 sulphide acts with the iron in the cement and forms a colloid, 

 which may be percipitated by dissolved calcium hydroxide or 

 by excess sodium sulphide present. Some of the calcium hy- 

 droxide is removed from solution by adsorption, and possibly 

 some is directly precipitated by the sodium sulphide. This is 

 no doubt largely responsible for the decrease in tensile strength. 

 It is in harmony with the work of Renezeder ^^ previously 

 mentioned. After a cement briquette has fully hardened, the 

 sulphide solution cannot reach a sufficient amount of the iron 

 to cause any noticeable effect, except after a long period of 

 time, although it does react with the iron at the surface. 



CONCLUSIONS 



While the results of this investigation are not directly ap- 

 plicable to every cement on the market, they may serve as a 



"Vide Cox, Alvin J., Zeitschr. f. anorg. Chem. (1904), 40, 174. 

 " Loc. cit. 



