74 BULLETIN 572, U. S. DEPARTMENT OF AGRICULTURE, 
feldspar was completely volatilized and the clinker which remained 
was of such composition that the percentage of silica amounted to 
3.2 times that of the alumina and the sum of these constituents 
amounted to exactly one-half the percentage of the lime present. 
The clinker obtained in this way, therefore, has a composition with 
respect to these constituents which falls within the limits required 
for Portland cement. It was observed also that when a quantity 
of calcium chloride equivalent to the alkalies in the feldspar was 
substituted for a part of the lime, volatilization took place in about 
half the time required when the ignition was made with lime alone: 
and that with the same mixture, on extending the time of ignition, 
almost complete decomposition of the feldspar could be effected at a 
temperature as low as 1,050°. Substitution of sodium chloride for 
calcium chloride gave approximately the same results. 
In a later publication ! it was shown that'when feldspar and lime 
in the proper proportion to make cement are digested with water 
at a steam pressure of 10 to 15 atmospheres, about 90 per cent of the 
potash in the feldspar passes into solution in the form of the hydroxide 
and the residue simply requires ignition for the manufacture of 
Portland-cement clinker. When the proportion of lime taken is 
reduced much below the limit stated, only partial decomposition 
of the feldspar results even at increased pressures, but by adding 
to such a mixture a quantity of a soluble calcium salt equivalent to 
the alkalies in the feldspar the amount of potash recovered may then 
be increased considerably. | 
The conclusion was drawn from these experiments that potash 
could be set free from feldspar by substituting the latter for clay in © 
the manufacture of cement; that the potash would be volatilized 
to a greater or less extent and could be recovered in the flue dust; 
and that it should be possible to obtain raw materials of the proper 
composition to yield Portland-cement clinker on ignition. 
In a series of experiments recently described by Anderson and 
Nestell,? cement mixtures from 10 different plants in this country 
and Japan were ignited in a way similar to that used in the ignition 
of the feldspar-lime mixtures. In every respect the results corre- 
sponded to those which were obtained in this laboratory by the ignition 
of feldspar with ime. Thus it was found that with different cement 
mixtures volatilization of the potash began at about 1,100°; at 
— 1,200° the percentage of potash volatilized in a given time was dif- 
ferent for the different mixtures, but when the temperature was 
increased to 1,300°, and maintained for 1 hour, almost complete 
volatilization resulted in every case. When heating an hour at 1,200° 
the addition of 5 per cent of sodium chloride to the raw material gave 
an increase in volatilization of the potash from 19 per cent to 66 per 
cent, while the use of calcium chloride was reported to be even more 
1 Ross, W. H., J. Ind. Eng. Chem., 9, 467 (1917). 2 J. Ind. Eng. Chem., 9, 253 (1917). 
