PACINI, METAMORPHISM OF PORTLAND CEMENT 175 



The ultimate resistance of cement which has been thus treated is 

 •decreased as well. The formation of the coagulated colloid, or of the 

 interlocking crystal units, whichever may be the cause of hardening, is 

 rendered imperfect and discontinuous, and the structure reflects the 

 weakness of its component units. 



It may moreover be supposed that more cement has been brought 

 within the range of hydrolysis by this agitation, and so converted into 

 laitance, even the larger particles being stripped of their protecting films 

 by the attrition. Tests made at the Watertown Arsenal (36) showed that 

 .after one hour's Avorking, cement had gained 4 per cent over the normal 

 strength, but that after 10 hours' working, it had lost 24 per cent from 

 the normal, in 20 hours 38 per cent, in 50 hours 56 per cent and in 100 

 hours 69 per cent. 



Total quantity of water at first added. — Under certain conditions, the 

 ^entire range of particles of a cement might be destructively hydrolysed, 

 resulting in what is termed "drowned" cement. The effect of an increase 

 in the quantity of mixing water is known to result in a diminution of 

 strength, and, bearing in mind what has been previously said regarding 

 hydrolysis, the reason is clear. If, before the cementing of contiguous 

 particles, an excessive amount of water is admitted to contact with the 

 •cement, colloidal material will form in increased amount. It has been 

 shown that an increased amount of mixing water results in an increased 

 volume of the paste produced (39). This indicates that a larger amount 

 of the products of hydrolysis is formed. 



Owing to difference in composition between these hydrogels and those 

 formed under normal conditions, they are incapable, as has been before 

 observed, of adsorbing electrolytes in such degree as to attain to the 

 density and rigidity of the latter. Admitting, on the other hand, that 

 colloids so formed do not differ in composition from those formed in the 

 normal hardening of cement, there still remains the abnormality of the 

 structure formed in this way. Being discontinuous, it would not offer 

 the same total resistance, in the form of connected films, to the passage 

 of water. Moreover, in the presence of an excess of water the working 

 ratio of electrolytes to colloids would be less because of the greater dilu- 

 tion in proportion to the volume of colloid. 



QUANTITY OF WATER THAT MAY SUBSEQUENTLY COME INTO CONTACT 

 WITH THE SYSTEM 



The effect of water upon cement after it has completely set rapidly 

 ■diminishes to a negligible quantity at ordinary temperatures, if the water 

 is reasonably free from dissolved or suspended impurities. There is a 



