The composition of the tricalcium aluminate and ferrite phases in Portland cement determined by the use of an electron-probe microanalyser

Abstract

Synopsis Each of the four main phases of Portland cement contains various elements in solid solution, the extent and nature of which may affect the hydraulic properties. A direct determination of the composition of these phases has only become possible by the use of the electron-probe microanalyser. The paper describes the analysis of the tricalcium aluminate and calcium alumino-ferrite phases. It was difficult to obtain reliable and accurate analyses of these in Portland cement clinkers and results were confirmed by the investigation of laboratory preparations where the tricalcium aluminate and calcium alumino-ferrite phases were predominant. The tricalcium aluminate phase was found to be approximately: (Ca_2·78Na_0·16K_0·02) (Al_1·75Fe_0·16Mg_0·07Si_0·l0)O₆ The calcium alumino-ferrite phase in a normal Portland cement clinker was found to be: Ca_5·97Al_3·23Fe_2·13Si_0·14Mg_0·15Ti_0·23Mn_0·09O₁₅ and that in a sulphate-resisting cement clinker to be: Ca_6·16Al_2·24Fe_3·22Si_0·14Mg_0·07Ti_0·12Mn_0·03O₁₅ The properties of a Portland cement are largely governed by the relative amounts of the four major phases present. The normal ‘Bogue’ method of compound calculation from the chemical analysis assumes that no solid solution occurs in these phases and that the cement clinker is at equilibrium. This method of compound calculation is compared, for a series of cement clinkers, with a calculation where solid solution is taken into account, with a calculation where the ferrite composition as given by X-ray diffraction is used, and also with the direct determination of the phases by X-ray diffraction and optical microscopy. It seems likely that the normal method of compound calculation underestimates the C₃A content of sulphate-resisting cement clinkers.