388 



CEMENTS, LIMES, AND PLASTERS. 



bon dioxide by storage on moistened filter-paper under a glass bell. 

 At the end of three weeks the increase in length of the rods was measured 

 with the following results. 



TABLE 168. 

 EFFECT OF ALUMINA. 



It will be noted that the percentage of elongation of the rods varied 

 directly with the percentage of alumina in the cements tested, proving 

 conclusively that the swelling was due to the action of the calcium sulph- 

 aluminate formed during the operation. 



Iron oxide. Iron oxide, though usually so low as to be negligible 

 in a Portland cement, occasionally is present in considerable percent- 

 ages (4 to 6 per cent). When this is the case, it can only be con- 

 sidered as equivalent to alumina in its action, allowing, of course, for 

 their difference in combining weights. This conclusion is borne out 

 by the fact that Portland cements practically free from alumina have 

 been made, containing lime, silica, and iron oxide only. 



Sulphur. Sulphur, when present in a cement mixture, may occur 

 either as a sulphide or sulphate. In the former condition it is usually 

 due to the occurrence of pyrite (iron disulphide, FeS2) either in the lime- 

 stone or in the clay. When present as a sulphate, it is usually in the 

 form of gypsum (hydrous calcium sulphate, CaSO 4 + 2H 2 0). 



In the rotary kiln, which usually has an abundantly oxidizing flame, 

 it is probable that any calcium sulphate present is dissociated (CaSO 4 

 ^CaO + S0 3 ) and the sulphur trioxide carried off, as this dissociation 

 occurs at a temperature much lower than that reached in clinkering. 

 If the flame is not sufficiently oxidizing, however, and because of imper- 

 fect draft this condition is likely to occur in vertical kilns, any lime 

 sulphate present will be reduced to the sulphide form. 



Alkalies. Small percentages of soda and potash are usually present 

 in the mixture, due mostly to their presence in the clay or shale. Alka- 

 lies have been regarded as detrimental, as inert, and as beneficial; and 

 much discussion has taken place on the subject, based mostly on purely 

 theoretical considerations. 



