156 REIBLING AND REYES. 
Hard-burned Portland cement.—The results obtained with 
these samples show that the hard-burned clinker may or may not 
produce a cement which requires no curing to enable it to pass 
the accelerated tests for soundness. Seasoned or nonseasoned 
cement, provided it passes the hot tests, may be used with reason- 
able certainty of its ultimate soundness. However, as is shown 
by sample 1, no reliance can be placed on the 28-day tests. 
Such material frequently attains a very high and increasing 
early strength, but afterward expands so much that eventually 
even lean sand mortars disintegrate. This fact is sufficient 
cause for the rejection of all Portland cements which fail to 
pass the hot test. 
Sample 1 also shows the effect of high temperature on the 
slaking properties of free lime. The expansion due to this 
cause develops very slowly; so slowly, in fact, that it does not 
interfere appreciably with the early strength. The high tem- 
perature also induces the formation of the quick-hardening 
calcium silicates and aluminates, which accounts for the charac- 
teristic great early strength of the hard-burned cement. 
We find that whereas the soft-burned cement continues to 
harden indefinitely, the strength of sintered products may be 
less at the end of years than it was at the age of twenty-eight 
or sometimes even seven days. The practical importance and 
the real significance of this great difference in the hardening 
properties of soft- and hard-burned cement is apparent from a 
comparison of the time strength tests of typical natural and 
Portland cements such as are given in Table XLIV. 
The most efficient natural, or Roman, cements are composed 
mostly of soft-burned material while the hard-burned product 
very largely predominates in the Portland cements of good 
quality. Accordingly, as is shown in Table XLIV, it is charac- 
teristic of the natural cement sand mortar to show such a 
steady increase in strength that it eventually becomes stronger 
than similar mortars of Portland cement. This, and the relative 
cheapness of natural cement, have caused considerable discus- 
sion concerning the relative merits of the two products, and 
there are many who advocate the general use of natural cement 
in preference to Portland. However, the slowness with which 
natural cements harden hinders rapid construction work to such 
an extent that the use of Portland cement is usually advisable 
and very often actually necessary. Consequently, the greatest 
