574 CEMENTS, LIMES, AND PLASTERS. 



Newberrys carry the experiments further, making a silica-, iron oxide, 

 lime mixture entirely free from alumina. This was made to correspond 

 to the formula 



(3CaO.Si0 2 ) + X (2CaO.Fe 2 3 ), 



and contained about 7 per cent of iron oxide. On burning this gave 

 a black, fusible clinker. When powdered this was dark-gray, and 

 gave a slow-setting hard and sound cement. 



Their final conclusions were, that though "iron oxide evidently 

 combines with lime in the same manner as alumina," the amount of 

 iron oxide present in ordinary clays is so small that "it is quite unneces- 

 sary, in working with ordinary clays, to take the iron oxide into con- 

 sideration in calculating the amount of lime required." 



In view of the manufacture of cements containing appreciable per- 

 centages of iron oxide, it seems advisable to take this constituent into 

 consideration in proportioning mixes, and this has accordingly been 

 done in the formula given earlier in this volume. 



Replacement of lime by magnesia. The possibility of this replace- 

 ment has been flatly denied by some of our leading authorities on cement 

 chemistry, while it has been maintained, but less confidently, by others. 

 To the present writer it seems certain that magnesia is absolutely in- 

 terchangeable with lime, due regard being paid to their differences 

 in atomic weight. It is only necessary to adduce the example of the 

 high-burned natural cements, such as the Akron, to make it clear that 

 a cement containing 15 to 20 per cent of magnesia can be made at almost 

 clinkering temperature. Recent experiments by Newberry seem to con- 

 firm this conclusion. It is to be noted, however, that a Portland cement 

 carrying high percentages of magnesia will necessarily differ considera- 

 bly from our present-day lime Portlands. It is even probable that 

 the differences in physical and technical properties will be so great that 

 it will be necessary to market such magnesia Portlands under some dis- 

 tinct trade-name. 



Replacement of lime by other bases. Magnesia is not the only 

 base that can replace, either partly or entirely, the lime of a normal 

 Portland-cement clinker. Other alkaline earths can be so substituted, 

 as was proven in the course of Richardson's recent experiments. He 

 describes * this phase of his work as follows: 



"Clinkers have been made in which baryta (BaO) and strontia 

 (SrO) are the bases. They must be burned at a very much higher 

 temperature than similar clinkers containing lime. In powder these 



* Engineering News, vol. 53, p. 85. Jan. 26, 1905. 





