188 
(f) 
(9g) 
(7) 
(Kk) 
REIBLING AND REYES. 
Portland cements expand very little in water unless they contain 
free lime (or free magnesia). 
Within the limits of perfect elasticity, Portland cements may show 
a gain in strength in spite of considerable expansion. 
The strength can be increased by reducing the expansion. 
There is no direct relationship between tensile and compressive 
strengths during the time in which the cements show marked 
changes in volume. : 
There is a marked relationship between the strength in tension 
and in compression after the volume has become constant and 
apparently no internal stresses are operating. 
Fluctuations in strength are caused by internal strains resulting 
' from the hydration of free lime, and within the elastic limit 
these internal strains affect the strength in tension and in 
compression in a dissimilar manner. 
The early strength, as an indication of the ultimate strength, is 
the more reliable the less free lime the material contains. 
There is definite relationship between the durability of strength 
and the results of accelerated tests of hard-burned Portland 
cement. 
As the soundness test fails to measure and often even to detect 
free lime or to designate its condition, the microscopic test 
offers much more reliable information on this point. 
We have found this effect of free lime so characteristic that no 
28-day, or longer, test of strength is needed to determine 
the fitness of cements for use. The study of a large number 
of commercial products has convinced us that we can ascertain 
by this means the true quality of Portland cements without 
resorting to the prolonged tests for strength. 
(19) The only new consideration which the subject of fineness 
introduces is concerned with the permanency of the strength 
developed by the finest and most active particles. The results 
obtained proved the durable nature of the indurated impalpable 
powder, and considering that free lime hydrates more readily 
the finer its state of subdivision, the great benefits derived from 
commercial fine grinding are apparent. 
(20) Temporary and ultimate strength: 
(a) 
(0) 
(c) 
(d) 
For greatest temporary efficiency it is necessary to grind finely 
and to burn at a high temperature. 
The endurance of early strength, the increase in strength with 
age, and the constancy in volume are greater the less free lime 
(or magnesia) the indurated cement contains. 
Portland cement of the desired quality can be obtained by proper 
mixing, hard-burning, and fine grinding. 
Cements should be purchased on a basis of quality rather than 
upon a mere consideration of quantity. 
