‘140 REIBLING AND REYES. 
cements recorded in Table A, Appendix 1,7 developed the up 
and down values in the tests for strength. Sometimes a drop 
was met with soon after the first twenty-four hours, sometimes 
not until after six months, and the retrogression was apparently 
independent of the ultimate chemical composition, and bore no 
quantitative relation to the early strength developments. 
It also became evident that the development and the main- 
tenance of the early strength necessarily did not depend upon a 
narrow limitation of the percentage of lime, the value of the 
silica alumina ratio, the hydraulic index, or any other available 
chemical information. In most instances it was found possible 
to change the influence of all of these factors by altering the 
degree of burning, the fineness of either the raw material or the 
cement, and by seasoning the cement or the clinker to a different 
degree. This was true both for well-burned and under-burned 
cements, but more pronounced in the latter. 
Our work showed that, as in the setting phenomena, changes 
in the condition of the free lime influenced the results of deter- 
minations of strength. As anticipated by EH. B. McCready *— 
no cements are perfect as regards to the chemical composition and the 
rate at which the disruptive strain due to free lime increases in different 
samples under various conditions of burning, grinding and testing, is the 
kernel of the nut which we ought to crack before placing too much reliance 
on rules deduced simply from analyses or percentages of gain. 
The microscopical physical and chemical observations pub- 
lished in Parts I, II,*° and III ® of this paper have thoroughly 
demonstrated the existance of free lime in Portland cement, and 
a consideration of the chemical and physical properties of lime 
is in itself sufficient proof that it necessarily must exert an im- 
portant influence on the development and maintenance of the 
strength. Our previous work has demonstrated that different 
methods of burning, grinding, and seasoning produce alter- 
ations in the physical and chemical properties of this free lime. 
THE EFFECTS OF ANHYDROUS FREE LIME. 
Although the presence of uncombined magnesia in a thor- 
oughly burned cement may produce a gradual expansion which 
will cause mortars and concrete to disintegrate after several 
years, the usual cause of unsoundness is latent expansion due 
* This Journal, Sec. A (1910), 5, 415. 
Cement Age (1905), 5, 339. 
° This Journal, Sec. A (1910), 6, 367 to 418. 
™Tbid. (1911), 3, 207 to 252. 
0 ee ee ee a ee 
