2 BULLETIN" 572, TJ. 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 1 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, 2 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 lime. 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 



i Ross, \V. H . J. Ind. Eng. Chem., 9, 467 (1917). * J. Ind. Eng. Chem., 9, 253 (1917). 



