PACINI, METAMORPHISM OF PORTLAND CEMENT 189 



The effect of sudden chilling at a period when a large proportion of the 

 strength is already developed does not show any decided direction, both 

 the positive and negative variations from the normal averaging the same. 

 It may therefore be concluded that, for the temperatures studied, a chill- 

 ing of this kind has no significant effect. 



An explanation according to the crystallization theory of hardening 

 would fail to fit the facts so satisfactorily. In the specimens that were 

 chilled at first and allowed to return to normal temperature, there should 

 be under this hypothesis a more significant decrease of strength, owing 

 to the formation of small, non-cohesive crystals from the rapid tempera- 

 ture change. The return to normal conditions should not favor so nearly 

 complete a recuperation as has been noted; unless a re-solution of the 

 crystals and recrystallization were supposed, in which case it may be 

 argued that such a process would require an abnormal solubility of small 

 crystals when compared with large. In a normal specimen, re-solution 

 and recrystallization are undoubtedly going on, strengthening the struc- 

 ture, and the large crystals are growing at the expense of the small. If 

 small crystals preponderate at seven days' age, resulting in a weak mass, 

 it is necessary to postulate a comparatively high solubility of the small 

 crystals in order to arrive at a normal strength at 28 days. This, while 

 by no means impossible, is not probable. 



Turning to the specimens kept continuously in cold Avater, it would 

 seem that, although the first chilling should show severe effects, as it did, 

 there should not be such a falling off in the rate of hardening, if the 

 crystallization be progressive. It is quite possible, however, that crys- 

 tallization at this temperature is not favored, and that the total number 

 of binding crystals of calcium hydroxide is therefore less than at normal 

 temperatures. 



QUANTITY OP WATEE AT FIEST ADDED 



Size of cement particles. — Other factors being equal, the amount of 

 cement rendered inert by the action of water is proportional to the per- 

 centage of fine particles. This is an absolute condition and presupposes 

 free access of water to every particle. jSTeedless to say, in practice this 

 condition is seldom realized, except approximately in laying concrete 

 under water, or in the careless use of an excess of water in mixing, or in 

 protracted mixing. 



In the use of a very fine cement, then, if the proper proportion of 

 water is added, the mixing time carefully regulated and proper precau- 

 tions taken in depositing, the influence of texture upon the strength of 

 the mass occasioned by the action of water is reduced to a small quantity, 



