xv, i Witt: Portland Cement Raw Mixture H5 



not be eliminated. There is another class of errors due to the 

 composition of the substances under investigation; errors of 

 this class have been found to be largely responsible for the non- 

 concordant results of Table II. 



In the first column, the calcium is determined, and the results 

 are calculated to calcium carbonate. In the case of the calci- 

 meter or with any other method in which the carbon dioxide is 

 determined, the result in terms of calcium carbonate must be 

 calculated from the total carbon dioxide found. This, of course, 

 assumes that all calcium is present as carbonate and that all 

 carbon dioxide is combined with calcium, which in this case is 

 not true. Carbonates other than calcium are present in small 

 amount in the limestone, causing the results to be slightly too 

 high. However, the calcium in the two siliceous materials is 

 not present as carbonate and consequently is not included when 

 the determination by calcimeter is made. The net result is that 

 the values obtained are Ipw. 



In the acid-alkali titration method, the alkalinity or acid- 

 consuming power is calculated to calcium carbonate. In this 

 case the errors due to apparatus and to manipulation may be 

 made very small. However, there are much greater errors due 

 to the composition of the mixture. On being treated with 

 standard acid, all carbonates go into solution and are calculated 

 to calcium carbonate. Of the calcium in the siliceous materials, 

 a portion goes into solution without neutralizing acid equivalent 

 to that which would be required for calcium carbonate, and a 

 portion is undissolved by the acid. Some of the iron and the 

 aluminium in these materials go into solution and consequently 

 neutralize the acid, thereby tending to produce a higher result. 37 



The results obtained by determining the acid-insoluble matter 

 were so irregular that no further analyses by that method were 

 made. 



Slight errors are caused by the presence of moisture in the 

 raw mixture as it comes from the tube mill. It has not been 

 found practicable to eliminate these in the routine work. The 

 moisture content of the material as it enters the tube mill is 

 determined at regular intervals and is kept as low as possible 



37 S. B. Newberry, Cem. and Eng. News, March (1903) 35, describes a 

 double titration method by which correction for magnesium may be made. 

 With the raw mixture under investigation, however, the error due to the 

 effect of iron and aluminium cannot be corrected. The error due to mag- 

 nesium is not corrected because it tends to compensate the other one. 



