PACINI, METAMORPHISM OF PORTLAND CEMENT 219 



GENERAL CONCLUSIONS 



In general, the metamorphism of Portland cement represents on an 

 accelerated scale the processes which occur in natural rocks. The accel- 

 eration is of course due to the ease with which water has access to the 

 finely comminuted particles in the initial stages of metamorphism. Many 

 of the minerals found in natural rocks, when ground as finely as, or finer 

 than Portland cement, undergo vastly accelerated reactions in the pres- 

 ence of water; colloidal bodies are thereby produced, and the water is 

 rendered alkaline (18). 



The end product of prolonged water action on Portland cement bears 

 a striking qualitative similarity to the end product in the kaolinization 

 of feldspars. The same transformations evidently occur in both cases, 

 the alkalies and the lime are abstracted, and the water and alumina con- 

 tents increased. The exceeding fineness and high adsorptive power of 

 the resulting products are also similar. The action of water on nearly 

 all silicate minerals is, in effect, a repetition of this process. 



The peculiar adsorptive properties of colloidal bodies render these 

 liable to coagulation. As has been pointed out in preceding pages, much 

 of the cementing material of conglomerates and sandstones, except where 

 calcitic, may have its origin in a similar phenomenon. 



On a natural scale, the action of water is greatly retarded, because of 

 the larger size of the bodies acted upon, and the consequent paucity of 

 surface upon which water may exert its influence. When Portland ce- 

 ment has properly undergone its initial metamorphism, the setting 

 process being complete and the hardening process in great part so, it 

 approaches the condition of a natural metamorphic rock, and activities 

 towards its further change are katamorphic and vastly slower in their 

 results than the initial changes. The component particles have now 

 become consolidated and the surface offered to the action of water is 

 minimized. Of course, this is truer of neat cement than mortar and 

 truer of mortar than of concrete, these being in the order of increasing 

 porosity. 



The hypothesis that crystal formation is responsible for the strength 

 of hardened cement is not so complete and satisfactory as the colloidal 

 hypothesis just referred to. In a compact mass, the growth of crystals 

 can hardly be considered anything but an element of weakness. As has 

 been shown by the foregoing results, the effects of varying some of the 

 conditions of the action of water upon cement are best explained by 

 considering the hardening a coagulative process rather than a process of 

 crystallization. 



