174 ANNALS NEW YORK ACADEMY OF SCIENCES 



Hydrolysis theory. The formation of such a protective film upon the 

 surface of a coarse particle will so regulate the access of water to its 

 interior that the contents will be slowly and normally hydrated. If the 

 entire mass of the particle were at once accessible to an excess of water, 

 the weakly acid and basic compounds at first formed would soon be hy- 

 drolysed and shorn of their binding power, and instead of the normal 

 complex colloids described by Michaelis (64, 65), capable of adsorbing 

 electrolytes and so coagulating into a dense rigid mass, simpler colloids 

 such as hydrous silicic acid and aluminic hydroxide would form, which 

 have not these powers to so high a degree. 



Finally, the rate of setting and hardening of a cement may be con- 

 sidered a function of the proportion of fine particles present. Mortars 

 set and harden more slowly than neat cement, and concretes more slowly 

 than either. This is simply a development of the fact that coarsely 

 ground cement sets and hardens more slowly than that which is finely 

 ground. It may be considered., from another viewpoint, that the inactive 

 material interferes with the liberation of heat from the system, and that 

 chemical reaction is consequently delayed in proportion to the amount 

 of inert material present. 



Mechanical agitation when water is added. If cement in the state of 

 a plastic mass be worked and kneaded, the ultimate strength will benefit 

 thereby, up to a maximum time of working. It is legitimate, a priori, to 

 surmise that the setting is hastened, within limits, although no record of 

 this is found. 



After the maximum time referred to, which in experiments made at 

 the Board of Water Supply laboratory lias been found to correspond 

 roughly with the time of initial set, continued working will cause a fall- 

 ing off in the strength. Up to this time, mechanical agitation with the 

 proper amount of gaging water will cause an increase in the ultimate 

 strength. 



The formation of the crystalline network, which constitutes the setting 

 of cement, and which is responsible for the primary strength by holding 

 the plastic mass rigid and in place, while the more important elements 

 of hardening make their appearance, is unquestionably facilitated by 

 agitation. Stirring is a means of hastening chemical reactions by bring- 

 ing the agents into more intimate contact. The compounds that go to 

 make up this network, being sooner brought into solution, perform their 

 function more quickly, and the crystals begin to form. Instead, however, 

 of forming a continuous rigid network, the crystals will be smaller and 

 less cohesive than if undisturbed in their growth, and the set can be 

 delayed and even prevented by continuing the agitation long enough. 



