J3 



mixtures, without pulverizing and artificially mixing the mater- 

 ials prior to burning. Several plants, however, have placed on 

 the market so-called Portland cements made by grinding up to- 

 gether the underburned and overburned materials formed dur- 

 ing the burning of natural cements. Several of these brands 

 contain from 5 to 15 per cent, of magnesia; and under no cir- 

 cumstances can they be considered true Portland cements. 



In view of the conditions above noted, the writer believes that 

 the following definition will be found more satisfactory than the 

 above quoted : 



Definition of Portland cement. Portland cement is an artifi- 

 cuil product obtained by finely pulverizing the clinker produced 

 by burning to semi-fusion an intimate mixture of finely ground 

 calcareous and argillaceous material, this mixture consisting 

 approximately of one part of silica and alumina to three parts 

 of carbonate of lime (or an equivalent amount of lime oxide.) 



Composition and Constitution. Portland cement may be said 

 to tend toward a composition approximating to pure tricalcic 

 silicate (3 CaO, SiCte) which would correspond to the propor- 

 tion CaO 73-6%, SiO2 26.4%. As can be seen, however, from 

 the published analyses, actual Portland cements as at present 

 made differ in composition very markedly from this. 

 Alumina is always present in considerable quantity, forming 

 with part of the lime, the dicalcic aluminate (2 CaO, SiO2). 

 This would give, as stated by Newberry, for the general for- 

 mula of a pure Portland. 



X (3 CaO, SiO2), Y (2 CaO, AbOs). 



But the composition is still further complicated by the pres- 

 ence of accidental impurities, or intentionally added ingredi- 

 ents. These last may be simply adulterants, or they may be 

 added to serve some useful purpose, Calcium sulphate is a type 

 of the latter class. Tt serves to retard the set of the cement, and, 

 in small quantities, appears to have no injurious effect which 

 would prohibit its use for this purpose. In dome kilns, suffi- 

 cient sulphur trioxide is generally taken up by the cement from 

 the fuel gases to obviate the necessity for the later addition of 

 calcium sulphate, but in the rotary kiln its addition to the ground 

 cement, in the form of either powdered gypsum or plaster-of- 

 Paris, is a necessitv. 



