PACINI, METAMORPHISM OF PORTLAND CEMENT 



197 



Table 9 

 Compressive Strength of Grouts Mixed with Vaiying Proportions of Water, 



Over Extended Period 

 (Each result is the average strength of three specimens.) 



Percentage of 

 water 



Compressive strength, pounds per 

 square inch 



Per cent gain in 



strength over 



28 days 



28 days 



3 months 



22 



25 



30 



38 



50 



75 



100 



150 



200 



500 



7076 

 6174 

 4563 

 3992 

 2991 

 2113 

 1609 

 1270 

 1306 

 399 



7504 

 5402 

 6030 

 5059 

 5312 

 4078 

 3544 

 2379 

 2579 

 1141 



6 



—13 

 32 



27 



77 



93 

 120 



87 



97 

 186 



It is apparent from these figures that the effect of hydrolysis upon the 

 strength of cement is a reversible one, at least to a certain extent, since 

 the specimens in which an excess of water was used in mixing showed a 

 greater recuperative ability at the longer period than the cement in 

 which the normal amount of mixing water, in this case 22 per cent, was 

 nsed. 



Upon inspection, it was observed that the three months' specimens 

 ■showed in each case much less laitance than the similar 28 days' speci- 

 mens had shown, and it was considered probable that the laitance, in 

 standing, had adsorbed free lime from the remainder of the cement, 

 through the activity of the water permeating the mass, and thus reverted 

 to the original condition of the cement, or an approach thereto. An 

 analysis was accordingly made of laitance scraped off from the top of one 

 of the 500 per cent water specimens and thoroughly washed by decanta- 

 tion. It probably represents a maximum condition in the hydrolysis of 



cement. 



Table 10 



Analysis of Laitance from 500 per cent Specimen 



Si0 2 



Fe 2 O s 



A1A 



CaO 



MgO 



S0 3 



C0 2 , H 2 0, etc 



As obtained from 

 specimen 



15.28 

 2.28 

 3.98 



26.96 

 2.86 

 6.47 



42.17 



Treated with lime 

 water 



15.91 

 2.42 

 5.82 



36.67 

 1.28 

 2.72 



35.18 



