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ILLINOIS ACADEMY OF SCIENCE 



of the dry specimen, but remaining in the water after this for 

 an indefinite time (up to 176 days) caused little further loss in 

 strength. Obviously this is more severe treatment than gypsum 

 in actual service may be expected to receive. Fig. 5 shows the 

 effect of standing in water for various lengths of time. 



The time of set of gypsum may be controlled by the addi- 

 tion of a retardant. Fig. 6 shows the variation in strength 

 due to the addition of varying amounts of retarder. In the 

 study of this diagram it is of interest to note that for 4 first 



Second Settfy Gypsum 

 01 % Retarder 



40 60 60 , J 00 JZO 



Days immersed in water 



160 



160 



r-vy .-> 



settle gypsum stored after hydration at about 70° F, 0.1 per 

 cent retarder caused a material increase in strength, but that 

 further addition of retarder caused a decrease in strength. 

 The same effect was observed in a previous series of tests. At 

 the same time it is important that with either first settle or 

 second settle gypsum specimens dried at 100° F or less, as 

 shown by this diagram, the addition of any amount of re- 

 tarder caused a loss in strength. 



All the strengths given heretofore were obtained from gyp- 

 sum calcined at one of the mills of the United States Gypsum 

 Company. Attention was given to carrying out the calcining 



4 When a charge of ground gypsum is subjected to a steadily increasing temper- 

 ature, it goes through alternate stages of quiescence and of boiling due to the more 

 rapid ejection of water as steam at certain stages than at others. Gypsum whose 

 calcination stops with the end of the first boiling stage is termed here first settle 

 gypsum and gypsum whose calcination stops with the end of the second boiling stage 

 is termed second settle gypsum. See pp. 55 and 59, Bulletin No. 11, Oklahoma 

 Geol. Survey, "Gypsum and Salt of Oklahoma," by L. C. Snider, and p. 107, Mines 

 Branch Bulletin No. 84, "Gypsum Deposits of the Maritime Provinces," Mines De- 

 partment, Ottawa, Canada. 



