120 
Philippine Journal of Science 
calcium present must be based on the determination of the cal- 
cium itself. The reason for this is shown in fig. 2, which is 
based on the analysis of a number of samples of raw mixture 
at various stages of conversion into clinker. Curve a shows total 
calcium; b, total alkalinity; and c, carbon dioxide — all calculated 
to percentage of calcium oxide. The raw mixture as it entered 
the kiln contained almost 44 per cent calcium oxide as indicated 
by the three methods of analysis, taking into account the errors, 
which have been explained. 
The first part of all three curves rises, because during the first 
period no chemical change in the constituents of the mixture is 
taking place, and the percentage of calcium oxide is increasing 
because of the loss in moisture, organic matter, and the like. 
However, the change at once becomes pronounced, when the tem- 
perature becomes sufficiently high to decompose the limestone. 
Due to the loss in weight caused by the evolution of carbon 
dioxide, the percentage of calcium oxide present continually in- 
creases and the percentage of carbon dioxide decreases. The 
curve representing the alkalinity, or the acid-consuming value, 
of the mixture does not change so much as the other two. The 
factors that tend to make the curve rise and those having the 
reverse effect largely neutralize each other. 38 
SUMMARY 
In determining the calcium carbonate content of Portland ce- 
ment raw mixture the methods available may be divided into two 
classes; namely, those in which the calcium is determined and 
those in which some other constituent is determined. Although 
some methods of the first class have been greatly shortened by 
several investigators, generally some method of the second class 
is employed in control work because of simplicity and rapidity. 
While all methods are subject to errors arising from the appa- 
ratus and from the procedure employed, errors due to the nature 
of the constituents of the raw mixture may greatly influence 
the results. The causes of errors that may arise have been 
studied. 
" An investigation of certain phases of the subject is now in progress. 
