PACINI, METAM0RPHI8M OF PORTLAND CEMENT 179 



attention is drawn to the effect of the usual addition of ground gypsum 

 or of plaster of Paris to the ground clinker, for the purpose of retarding 

 the set. There are other salts whose retarding influence on the set of 

 ground clinker is comparable and probably superior to that of gypsum, 

 but their use is not so practical, consequently, it has been adopted as the 

 restrainer for general use. 



It has been shown by Eohland (83) that the salts which respectively 

 accelerate and retard the setting of cement are the same as those which 

 accelerate and retard the hydration of quicklime. From this it is con- 

 cluded that their influence is "catalytic." 



A detailed explanation of the mechanism of the action of gypsum has 

 been put forth (79), holding that the presence of calcium ions in the 

 mixing water, resulting from the solution of gypsum therein, decreases 

 the solution of other calcium ions, thus retarding the solution of lime 

 and the hydrolysis of the aluminates, which in turn retards the set. 



It seems probable, upon this basis, that the presence of certain elec- 

 trolytes in the mixing water acts upon the set by influencing the solu- 

 bility of calcium sulphate therein, and consequently increasing or dimin- 

 ishing the number of calcium ions present in the mixing water as a result 

 of the solution of calcium sulphate. 



For example, sea water has been found to retard the set of cement 

 (83). Gypsum, although a relatively insoluble salt, may be regarded as 

 fairly soluble in moderately strong solutions of sodium chloride or of 

 other salts having no common ion (14). In the presence of sodium 

 chloride, then, the calcium ion concentration in the mixing water is 

 raised, and the solution of the calcium aluminates diminished, with the 

 effect of retarding the set. Sulphates have been found, when dissolved 

 in the mixing water, to have the property of retarding the set, with the 

 exception of aluminum sulphate and calcium sulphate when in low con- 

 centration. In view of the latter fact, it is evident that the above expla- 

 nation is perhaps only a partial one. 



A large number of other electrolytes and miscellaneous compounds 

 have been investigated and the results are recorded (83). 



The effect of soluble constituents in the sand used for making concrete 

 is by no means negligible (4) and may offer an explanation for many 

 instances of puzzling behavior of the mixture. 



Sea water has been and is, in many instances, still used for mixing con- 

 crete, and to the best of our knowledge, no cases of failure can be attrib- 

 uted to this cause alone. Apart from the influence upon setting, the 

 presence of dissolved electrolytes in the mixing water seems to increase 

 the strength of cement in the early periods, as far as reported results have 



