586 



CEMENTS, LIMES, AND PLASTERS. 



3.00. Some few American natural cements do, however, show a higher 

 specific gravity, as can be seen from the table on page 263. 



Setting properties. A certain minimum time of initial and final set 

 is usually specified, for the convenience of the workmen. This is regu- 

 lated by the use of gypsum or plaster at the plant, a practice whose 

 effects have been discussed in detail in Chapter XXXVI. 



\ 



CURVE OF TIME BEFORE 

 SETTING IS COMPLETED. 





5 10 15 20 25 



40 45 50 C. 



\ 



CURVE OF TIME BEFORE 

 SETTING BEGINS. 



to*\ 



t= 



4.0+C" 



1.5 IN MIN 



5 10 15 20 25 30 35 40 45 50C. 



FIG. 145.* Effect of temperature on setting time. 



The effect of temperature on the setting of Portland cement is well 

 shown in Fig. 145. It will be noted that the setting is much slower at 

 low than at high temperatures, within the limits of the experiments. 



Tensile strength. The tensile strength of a cement is of very little 

 importance or interest of itself, because cements are rarely subjected 

 intentionally to tensile strains. But in practice the tensile test is the 



* From Johnson's "Materials of Construction," p, 616. 



