Compositions of the ferrite phase in clinkers

In many clinkers, the ferrite phase is closely mixed with aluminate due to a similarity in cell parameters, oriented intergrowth can occur (MIS). The close admixture often renders X-ray microanalysis difficult or unreliable. For ordinary Portland cement clinkers, the compositions found in dilferent laboratories are nevertheless remarkably consistent. Table 1.2 includes an average value based on the results of investigations using X-ray microanalysis (H8,K1,B2,U1,H3,B4) or chemical analysis of separated material (Yl). Table 1.3 includes suggested site occupancies corresponding to these data. 

As with the aluminate phase, the average compositions take into account the requirement that these site occupancies should be reasonable from the standpoint of crystal chemistry. There is no basis for allocating cations to octahedral and tetrahedral sites separately as the preferences of some of the eations, especially Mg, in this structure are unknown, as is the temperature at which equilibrium is attained. This temperature probably varies between clinkers, and may be expected to affect the distribution. 

The typical composition differs markedly from that of C4AF (46.1% CaO, 21.0% AljOj, 32.9% Fc20,). It contains about 10% of impurity oxides and is much lower in Fc20,. It approximates to CajAl-Fco Mgo 2Sio,i.sTio.o.s05 which is derived from C AF by replacing some of the Fe by Mg and an equal amount by Si and Ti . Harrisson el oL (H3) observed that, for data from several laboratories, the number of magnesium atoms per formula unit agreed with that of (Si + Ti) this supports the above interpretation. 

Sulphate-resisting Portland cements have relatively high ratios of iron to aluminium, and the ferrite phase cannot have the composition given above if it contains most of the iron. Tables 1.2 and 1.3 include a tentative composition and atomic ratios corresponding to it, based on scanty data for the interstitial material as a whole (G3,G4) and the requirement of reasonable site occupancies. 

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