166 AXXALS XEW YORK ACADEMY OF SCIENCES 



are found slow setting and slow hardening, while those high in alumina 

 are quick setting and quick hardening. An increase of lime in the latter 

 retards the setting (63). 



The calcium aluminates are probably the main factors in the setting 

 of cement, while the hardening is due to the calcium silicates. The mag- 

 nesium compounds are inessential to the hydraulic processes (105). 



Upon the addition of water to cement, the equilibrium in the system 

 of solid solutions and chemical compounds is destroyed, and a series of 

 changes is inaugurated tending towards the production of a system which 

 will be stable under the new conditions. The first effect resulting from 

 the solutions and reactions brought about by the presence of water is the 

 setting of the plastic mass. 



Under ordinary conditions of practise, the quantity of water used is 

 about 22 per cent in the case of a neat cement, being less in the case of a 

 mortar, and still less in the case of a concrete. When this proportion of 

 water is used, it is probable that the setting of cement is mechanically 

 analogous to the setting of plaster of Paris and is caused by the growth 

 throughout the mass of a network of crystals, deposited from the satu- 

 rated solution formed by the first stage of hydro-nietamorphism. 



Owing to the lo\v solubility in water of the original component sub- 

 stances, the attainment of final equilibrium is a matter of considerable 

 time, and is further delayed by the automatic protective action of films 

 of insoluble substances coating the active particles (23). These films in 

 some cases are semi-permeable, and exert a selective influence upon the 

 solutions osmotic-ally penetrating them. Under normal conditions, that 

 is under those conditions which have been found in practise to yield the 

 densest and strongest product, this attainment of equilibrium considered 

 apart from the setting process at first proceeds rapidly, but the rate of 

 increase of strength grows smaller, tending to a minimum. 



A. Erskine Smith has shown (90) that there has been no permanent 

 retrogression in the strength of cement in the case of specimens kept 

 under observation for 21 years. Of course, this relates to laboratory 

 specimens protected from weathering, but shows one of the directions 

 which this met amor phi sm may take. 



HATJDEXIXG PROCESS 



The hardening of cement has been ascribed variously (-8) 



1. To the fineness of grinding, 



2. To the increasing stability of calcium compounds due to combina- 

 tion of part of the silicic acid as the silicates grow less basic, 



3. To the action of free lime upon calcium compounds, 



