PHYSICAL AND CHEMICAL PROPERTIES. PARTS IV, V. 175 
even 3-month tests for strength. The study of a large number 
of commercial products has convinced us that in a few days we 
can positively identify those commercial Portland cements 
which are best suited to our needs. 
The effect of the degree of pulverizationtThe grains of true 
Portland cement too coarse to pass a standard 150-mesh sieve are 
practically inert. The fine impalpable flour, too fine to be meas- 
ured on any sieve, gives the cement its real adhesive and hard- 
ening properties. The coarse, inactive particles may be con- 
sidered as clinker the finer grinding of which produces a cement 
whose efficiency depends upon the same conditions of compo- 
sition, burning, seasoning, etc., as that of the impalpable powder 
produced from larger clinker in the ordinary process of man- 
ufacture. 
The only new consideration which the subject of fineness in- 
troduces into this investigation is the permanency of the strength 
developed by the finest and most active particles. A sample 
of the hard-burned cement L, the physical properties of which 
are recorded in Table LII, was reground until less than 3 per 
cent remained on the 200-mesh sieve, and then tested. The 
results obtained with the original and the reground products are 
recorded in Tables LIV and LV. 
From the data recorded in Table LV the effect of fineness 
upon the hydration of the hard-burned free lime is noticeable; 
the characteristic slight decrease in strength after thorough 
induration occurs more rapidly and disappears more quickly in 
the reground material. Both the commercial and reground prod- 
ucts show the same general characteristics in hardening proper- 
ties ; the ultimate strength of the reground material is so decidedly 
better than that of the coarser cement that the greater efficiency 
in the early strength of the former is maintained. 
These and other results proved the durable nature of the 
products formed by the induration of the finest and most active 
particles of hard-burned Portland cement, and considering the 
fact that free lime hydrates the more readily and thoroughly the 
finer its state of subdivision, one can readily explain the great 
improvement in the commercial product due to the improved 
degree of final pulverization. 
Improved grinding machinery has enabled manufacturers to 
turn out a finer cement which seasons more efficiently, is more 
reliable in the tests for constancy in volume and strength, and 
has increased sand-carrying capacity and better troweling prop- 
erties. The importance of demanding best commercial grinding 
