CONSTITUTION, ETC., OF PORTLAND CEMENT. 573 



He further notes that of the two constituents of the clinker the celit 

 is almost inert, being usually unattacked by the water, while the alit 

 furnishes most of the lime needed for the setting effect. As the celit 

 is a solution of dicalcic salts (2CaO.Si0 2 4-2CaO.Al 2 3 ), while the alit is a 

 solution of tricalcic compounds (3CaO.Si02 + 3CaO.Al 2 3 ), the lower- 

 limed cements are, therefore, the less hydraulic. This agrees with 

 experience. 



This theory differs from Le Chatelier's in that it considers setting 

 as due only indirectly to the presence of silicates and aluminates. 



"The strength of the Portland cement after setting is due entirely to- 

 the crystallization of calcium hydrate under certain favorable conditions, 

 and not at all to the hydration of the silicates or the aluminates, since 

 in this act of hydration nothing can take place which would tend to 

 bind these silicates and aluminates together." 



The formation of lime silicates and aluminates during clinkering 

 is on this theory only a convenient way of securing indirectly a very 

 active lime hydrate, which is itself the real cementing material. 



Replacement of silica by other acids. Various oxides of the silica 

 group have been substituted by Richardson in his series of synthetic 

 cements. Titanic oxide (Ti02), stannic oxide (Sn02), and plumbic 

 oxide (PbO) have been so used. "The ground clinker in each case 

 has been found to set rapidly, although the resulting test pieces were 

 not volume constant, the temperature obtainable in our furnace being 

 evidently insufficient to bring about a thorough combination between 

 these oxides and lime. Cements have been made in which phosphoric 

 acid (P20 5 ) has been substituted for silica." 



Replacement of alumina by iron oxide. Some difference of opinion 

 appears concerning the extent to which the alumina of a Portland ce- 

 ment may be replaced by iron oxide. 



This problem was taken up by the Newberrys in the classic researches 

 before cited. They prepared mixtures of pure iron oxide and calcium 

 carbonate in such proportions as to correspond to the formula 2CaO.- 

 Fe 2 3 , which in percentages is equivalent to CaO 41.3 per cent, Fe 2 3 

 58.7 per cent. "On burning, the material fused to a black slag, which 

 yielded a brown color on grinding. Mixed with water to a paste, this 

 powder showed no heating, and did not set or harden in air or cold water. 

 A part placed in steam however, after setting one day in air, hardened 

 rapidly, and after several hours in boiling water showed no cracking 

 and appeared very hard. From this experiment it appears that lime 

 and iron oxide readily combine, yielding a product which is constant 

 in volume, though it shows no hardening properties in the cold." The 



