- 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.
Properties of the pure phase
In the absence of elements other than
calcium, aluminium, ironand oxygen, calcium aluminoferrite forms a solid solutionseries of formula Ca2(AlxFe1-x)2O5 for all values of x in the range 0-0.7 [H F W Taylor, "Cement Chemistry", Academic Press, 1990, ISBN 0-12-683900-X, pp 28-32] .Compositions with x > 0.7 do not exist at ordinary pressures (see dicalcium aluminate). The crystal is orthorhombic, and is normally lath-like. Its densityvaries 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
Portland cementclinker, 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, manganeseand zinc. There is also a substantial amount of magnesiumand 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 [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] :
Behavior in cements
Calcium aluminoferrite has little effect upon the physical properties of cement. On hydration it forms [Taylor, ibid, p 175] 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[Hewlett, ibid, p 726] , 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.
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