172 AyyALB XEW YORK ACADEMY OF SCIEyCES 



mass, upon the total volume of the voids and upon the ratio of capillary 

 to subcapillary voids. The disruptive effect is, of course, due to the ex- 

 pansion of the water during freezing. Consequently there is a possibility 

 that during the earlier stages of the history of the mass this effect may 

 be to a great extent neutralized by the presence of soft colloidal material 

 (45), because of its lack of rigidity. 



Voids are undoubtedly present even in neat cement mixes, and they 

 are more common in mortars and in concretes; when, therefore, these 

 have attained a sufficient hardness, they are in all respects similar to a 

 natural rock and subject to the same katamorphic processes. The effect 

 of frost increases in intensity as the mass ages and loses elasticity. 



As water permeates the cement, even after hardening has progressed 

 to a considerable extent, it becomes charged with various electrolytes, and 

 its freezing point is consequently lowered. To some extent this immu- 

 nizes the mass from frost action. On the other hand, as we have seen 

 before, cryohydric compounds may be formed at these low temperatures, 

 and the separation of these from solution is a factor in the opposite 

 direction. 



QUANTITY OF WATER AT FIRST ADDED 



Size of cement particles. The finest particles in cement, provided that 

 they are chemically identical with the remainder, are the most active 

 oementitiously, because of the ease of reaction and of the greater proba- 

 bility of this action being uniform throughout the mass of each particle. 

 This is recognized under the microscope by the ultimate disappearance 

 of these particles as individuals upon the addition of water. Owing to 

 the relative insolubility of the constituents of cement, both before and 

 after metamorphisni, each particle becomes covered to a certain depth 

 with the reaction products, which in this case take the shape of gelatinous 

 films (2) in such manner as to offer hindrance to the further action of 

 water. 



The particles whose diameter is smaller than or equal to the thickness 

 of this zone evidently are the most efficient chemically. The larger par- 

 ticles are less so. as the passage of \vater through the enveloping film is 

 a slow matter, and some particles may be so large as to remain internally 

 unchanged. It is probably this fact that gives a hydraulic quality to 

 previously set cement that has been reground and retempered with 

 water; in fact, this process may be repeated a number of times with the 

 same sample of cement. 



Xot all of each particle, therefore, can take part in the setting and 

 hardening, and sometimes this proportion of inert material is consider- 



