Calcium aluminoferrite

  Calcium Aluminoferrite (Ca₂(Al,Fe)₂O₅) is a dark brown crystalline phase commonly found in cements. It also exists rarely in nature as brownmillerite.

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Properties of the pure phase

In the absence of elements other than calcium, aluminium, iron and oxygen, calcium aluminoferrite forms a solid solution series of formula Ca₂(Al_xFe_1-x)₂O₅ for all values of x in the range 0-0.7^[1]. Compositions with x > 0.7 do not exist at ordinary pressures (see dicalcium aluminate). The crystal is orthorhombic, and is normally lath-like. Its density varies from 4026 kg.m^-3 (x = 0) to 3614 kg.m^-3 (x = 0.7). All compositions melt incongruently in the range 1400-1450°C. They are ferromagnetic, progressively more so as iron content increases. These phases are easily prepared from the oxides.

Phases in Portland cement clinker

In Portland cement clinker, calcium aluminoferrite occurs as an "interstitial phase", crystallizing from the melt. Its presence in clinker is solely due to the need to obtain liquid at the peak kiln processing temperature (1400-1450°C), facilitating the formation of the desired silicate phases. Apart from this benefit, its effects on cement properties are little more than those of a diluent. Its forms an impure solid solution that deviates markedly in composition from the simple chemical formula. The calcium aluminoferrite phase acts as a repository for many of the minor elements in the clinker. Most of the transitional elements in the cement are found in the ferrite phase, notably titanium, manganese and zinc. There is also a substantial amount of magnesium and silicon, and because of this, oxides other than CaO, Al₂O₃ and Fe₂O₃ often make up 15% of the mass of the calcium aluminoferrite. This substitution reduces the melting point to around 1350°C.

Typical chemical compositions for various clinker bulk Fe₂O₃ contents are^[2]:

Behavior in cements

Calcium aluminoferrite has little effect upon the physical properties of cement. On hydration it forms^[3] 4CaO.Al₂O₃.nH₂O and hydrated iron oxide gel. In principle, this is a fast and energetic reaction, but precipitation of an insoluble layer of hydrated iron oxide upon the calcium aluminoferrite crystal surface forms a barrier to further reaction. In the case of Portland cement, subsequent slow reaction with dissolved sulfate forms an AFm phase, which has negligible strength-giving properties. In the case of calcium aluminate cements^[4], the situation is less clear-cut, but there is little contribution to early strength. Calcium aluminoferrite is also present in sulfoaluminate cements, and again contributes no strength.

References

1. ^ H F W Taylor, Cement Chemistry, Academic Press, 1990, ISBN 0-12-683900-X, pp 28-32
2. ^ P. C. Hewlett (Ed)Lea's Chemistry of Cement and Concrete: 4th Ed, Arnold, 1998, ISBN 0-340-56589-6, p 160 and Taylor, ibid, p 10
3. ^ Taylor, ibid, p 175
4. ^ Hewlett, ibid, p 726