PACINI, METAMORPHWM OF PORTLAND CEMENT 169 



The final effects of geological processes do not differ in the main, 

 whether these operate upon natural substances or- upon the products of 

 human industry. The agent whose activity is responsible for the majority 

 of terrestrial changes, namely water, is also the main factor in the meta- 

 morphism of the artificial rock, cement. By intelligent control of the 

 action of water upon this rock, the desired results are obtained, and its 

 value as a material of construction is inestimable. Lacking this insight, 

 the action of water may result in catastrophe, or at least loss of time, 

 money or efficiency. Geology, then, through hydrology (59), is enabled 

 to give substantial aid to the engineer. 



TEMPEBATUEE OF THE WATEE AT FIEST ADDED 



In construction, the water at first added to cement, known as the 

 gaging or mixing water, is subject to the entire range of variation of 

 atmospheric temperature. The lower limit is far below the freezing 

 temperature of water and of course, in this phase, water is useless for 

 the purpose. 



Within the possible range of temperature under working conditions, it 

 has been established that as the temperature of the gaging water used is 

 higher, the set becomes more rapid. Considering the setting clue to the 

 deposition of a network of crystals from the supersaturated mixing 

 water, the beginning of this deposition would be sooner attained, if the 

 water reached its condition of supersaturation more quickly; and this 

 condition would be brought about by a higher original temperature, 

 provided, of course, that the solutes increased in solubility with the tem- 

 perature. With a higher temperature, the volume of the water would be 

 greater and the viscosity less, and consequently its range of activity would 

 be increased; that is, it would be enabled to reach a larger number of 

 cement particles and thereby more quickly arrive at its saturation point, 

 and the deposition of the crystalline network hastened in consequence. 

 If the temperature of the mixing water be above about 37° C, the setting, 

 instead of being hastened, begins to be delayed. If the deposition of this 

 network were a simple case of precipitation from a hot solution, it would 

 be logical to state that the solubility of the compounds concerned was so 

 high at this temperature that they were not deposited from solution. 

 The problem, however, seems chemical rather than physical, and it is 

 more probable that this effect is due to hydrolysis. 



Hydrolysis increases with the temperature. In the case of the weak 

 salts that must exist in the system we have under consideration, the ulti- 

 mate products of hydrolysis are the gelatinous materials — silica, in the 

 hydrated form, aluminic hydroxide and ferric hydroxide. The adsorp- 



