Calcium aluminoferrite (Ca
2(Al,Fe)
2O
5) is a dark brown crystalline phase commonly found in
cements. In the cement industry it is termed tetra-calcium aluminoferrite or ferrite. In
cement chemist notation (CCN), it is abbreviated as C
4AF meaning 4CaO·Al
2O
3·Fe
2O
3 in the
oxide notation. It also exists in nature as the rare mineral
brownmillerite.
In the absence of elements other than
calcium,
aluminium,
iron and
oxygen, calcium aluminoferrite forms a
solid solution series of formula Ca
2(Al
xFe
1–x)
2O
5 for all values of x in the range 0–0.7.
[1]: 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
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.
In
Portland cement
clinker, calcium aluminoferrite (C
4AF) occurs as an "interstitial phase",
crystallizing from the melt as the reaction end-product of the
flux components (Al
2O
3 and Fe
2O
3) added in the raw materials to lower the high melting point of the oxide mix CaO-SiO
2. 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
transition metals 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, Al
2O
3 and Fe
2O
3 often make up 15% of the mass of the calcium aluminoferrite. This substitution reduces its
melting point to around 1350 °C.
The typical chemical composition of calcium aluminoferrite (C
4AF) for three different clinker bulk Fe
2O
3 contents is given in the next table.
[1]: 10 ,
[2]: 160 The calcium aluminoferrite mean composition is clearly dominated by CaO (43.7 wt. %), Al
2O
3 (19.3 wt %), and Fe
2O
3 (25.4 wt. %). The mean contents in SiO
2 (4.2 wt. %) and other oxides are marginal.
Oxide | Mass % | Mass % | Mass % |
---|---|---|---|
Bulk Fe 2O 3 in clinker |
0.29 | 2.88 | 4.87 |
SiO 2 |
4.0 | 2.6 | 6.1 |
Al 2O 3 |
20.2 | 20.8 | 17.0 |
Fe 2O 3 |
24.5 | 23.9 | 27.7 |
CaO | 44.6 | 46.4 | 40.2 |
MgO | 3.7 | 3.1 | 4.9 |
Na 2O |
0.1 | 0.1 | 0.1 |
K 2O |
0.1 | 0.1 | 0.1 |
TiO 2 |
1.9 | 2.7 | 1.7 |
Mn 2O 3 |
0.1 | 0.3 | 1.5 |
ZnO | 1.1 | 0.1 | 0.9 |
Note that the sum of Al
2O
3 and Fe
2O
3 contents in calcium aluminoferrite (C
4AF) in the table here above remains constant at 44.7 mass % as illustrated in the table here below.
Oxide | Mass % | Mass % | Mass % |
---|---|---|---|
Bulk Fe 2O 3 in clinker |
0.29 | 2.88 | 4.87 |
Al 2O 3 |
20.2 | 20.8 | 17.0 |
Fe 2O 3 |
24.5 | 23.9 | 27.7 |
Sum | 44.7 | 44.7 | 44.7 |
Calcium aluminoferrite (C
4AF) has little effect upon the physical properties of cement. On
hydration it forms
[1]: 175
hydrogarnet (4 CaO·Al
2O
3· n H
2O) and
hydrated iron oxide
gel. In principle, this is a fast and energetic
exothermic 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
AFm phases, which have negligible
strength-giving properties. In the case of
calcium aluminate cements (CAC),
[2]: 726 the situation is less clear-cut, but there is little contribution to early strength. Calcium aluminoferrite is also present in calcium sulfoaluminate (CSA) cements, and again contributes no strength.
The three other main mineral phases of Portland cement clinker are: