xiii, A, 4 Witt and Reyes: Solubility of Portland Cement 157 



tions which compose Portland cement, a new component is introduced, which 

 immediately results in lack of equilibrium, which is only brought about 

 again by the liberation of free lime. This free lime the moment that it 

 is liberated is in solution in the water, but owing to the rapidity with 

 which it is liberated from the aluminate, the water soon becomes supersa- 

 turated with calcic hydrate and the latter crystalizes out in a network 

 of crystals, which binds the particles of undecomposed Portland cement 

 together. 



The results of the present work show that one of the products 

 of hydration is undoubtedly calcium hydroxide and that the 

 water present contains some of it in solution. Further it is 

 well established that crystals of this compound are found in 

 hardened cement. There is a question, however, whether or 

 not these crystals are as important as the writer intimates and 

 also as to the mechanism of their formation. Considering the 

 amount of water that is ordinarily mixed with cement and the 

 low solubility of calcium hydroxide, it is evident that if at a 

 given instant all the water were saturated with this compound, 

 and then all the calcium hydroxide should crystallize out, the 

 percentage of the total calcium in the cement so affected would 

 be small. For example, let us consider that one kilogram of 

 cement is mixed with sufficient water to produce a paste of nor- 

 mal consistency. The average amount of water required is from 

 20 to 25 per cent of the weight, or say 250 cubic centimeters. 

 Now cement will contain on an average of 62 per cent calcium 

 oxide, or 620 grams — which is equivalent to approximately 819 

 grams of calcium hydroxide — per kilogram. Disregarding the 

 portion of the water that enters into combination with calcium 

 oxide and is unavailable for other reasons, the 250 cubic centi- 

 meters present would dissolve only 0.41 gram of the solid at 

 a temperature of 20° C. according to Seidel.^* This is, of 

 course, on the assumption made previously that the relative 

 amounts of other substances in solution are not sufficient mate- 

 rially to affect the solubility of calcium hydroxide. It can be 

 seen that this amount of the substance is not sufficient to bind 

 the cement together. 



It is possible to consider that the crystallization occurs pro- 

 gressively, that is, when the solution becomes saturated, some 

 of the dissolved hydroxide crystallizes out, more goes into solu- 

 tion as a result of further hydration, and so on. However, a 

 number of facts are opposed to such a view. If the water is not 

 saturated at a given time, and crystals of calcium hydroxide are 

 present, it is more likely that some of these would dissolve than 



" Loc. cit. 



