PACINI, METAMORPHISM OF PORTLAND CEMENT jr? 



crete. He found that by using concrete that had attained its initial set 

 and exposing it to steam at 80 pounds pressure the six months' strength 

 could be obtained in two days, a tremendous accelerating of the harden- 

 ing process. 



This state of affairs is not very satisfactorily explained, if the harden- 

 ing of cement is supposed to be due to the progressive crystallization of 

 calcium hydroxide, since it is somewhat at variance with our knowledge 

 of the conditions of crystallization to assert that continuous exposure to 

 a high temperature, presumably constant, should accelerate crystalliza- 

 tion; particularly since in this case the amount of water present in the 

 system remains the same. On the basis of the colloid theory, however, it 

 is simply explained by supposing that adsorption of calcium hydroxide 

 by the complex hydrogel is accelerated by higher temperatures. 



Cold storage. — A series of tests, embracing neat cements and mortars, 

 was made upon tensile test specimens exposed, after the age of 24 hours, 

 to low temperatures under diverse conditions. The following conditions 

 were observed : 



1. Chilling the briquettes at 24 hours' age by filling the storage tank 

 with water at the lowest winter temperature as it came from the tap. 

 The water was then allowed to come slowly to normal winter temperature 

 for the tank, about 60° F. 



2. Chilling another set of specimens, otherwise normally treated, by 

 filling the tank with cold water as before, 24 hours before breaking. 



3. Storing another set in ice water for the entire period after remov- 

 ing from the damp closet at 24 hours' age. 



4. Normal treatment. 



Two brands of well-known cement of high quality were run in parallel. 

 The mortars were of proportions 1 :3, Ottawa sand being used. The 

 results obtained are summarized below : 



