PHYSICAL AND CHEMICAL PROPERTIES. PARTS IV, V. 187 
(f) The characteristic drop in strength may not be eliminated 
even by prolonged aération. 
(g) The strength and even the character of the curve of strength 
of any given cement may be affected to a considerable 
extent by the method of molding employed, the quality of 
water or of sand used, and the exposure or seasoning 
resorted to, but in spite of these variable factors certain 
properties influence the strength to such an extent as to 
be readily apparent. 
(h) A low early strength always results from premature partial 
regauging or caking; coarse grinding; the presence, in 
quantity, of foreign substances such as clay, sand, or slag; 
underburning; or excessive seasoning. 
(i) A low early strength, provided the cause is not due to coarse 
grinding, is always associated with a low specific gravity 
(dried at 110°) and a corresponding high loss on ignition. 
(16) The general character of the commercial cements ex- 
amined and the general nature of their curves of strength: 
(a) Adulterated, coarsely ground or caked cements were readily 
detected by means of chemical or physical examinations. Such 
cements showed poor early and ultimate strength. 
(6) Underburned cements when sound have a low gravity, and the 
microscopic and chemical examination in most instances proved 
that the low gravity was due to underburning rather than to 
adulteration or excessive aération. Such material hardened in 
the manner characteristic of seasoned, underburned Portland 
cement. 
(c) If the cement was sound, a gravity above 3.10 was positive proof 
of hard-burning, and such material always showed the char- 
acteristic hardening properties of a well-sintered Portland 
cement. 
(d) Intermediate products of the cement kilns were represented by a 
specific gravity which was neither high nor low. They con- 
sisted of a mixture of soft- and hard-burned cement and showed 
no definite hardening properties. 
(17) A résumé of the above facts and conclusions proved that 
hard-burning is essential to the greatest efficiency in early 
strength and that in order to secure the desired permanency 
in strength, the destructive force which apparently operates after 
hard-burned cements have become thoroughly indurated must 
be discovered and eliminated. 
(18) The effects of changes in volume on the strength of hard- 
burned Portland cements: 
(a) Internal strains result from the presence of free lime in in- 
durated cement. 
