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.
Photomicrograph (0.11 mm) of clinker polished section showing tetra-calcium aluminoferrite (C 4AF) (white) and tricalcium aluminate (C 3A) (grey) occupying interstitial space between alite (C 3S) (blue) and belite (C 2S) (orange) crystals. These are false (interference) colors.
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 meltincongruently 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 clinkeredit
In Portland cementclinker, 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.
Chemical composition of calcium aluminoferrite (C 4AF) as a function of the bulk concentration (mass %, or wt. %) of Fe 2O 3 in the clinker[1][2]
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.
Sum of Al 2O 3 and Fe 2O 3 in C 4AF in the above table
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
Behavior in cementsedit
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 oxidegel. 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.
See alsoedit
The three other main mineral phases of Portland cement clinker are:
^ abcHewlett P.C. (Ed.) (1998). Lea's Chemistry of Cement and Concrete: 4th Ed, Arnold, ISBN0-340-56589-6.
Further readingedit
Taylor, Harry F. W. (1997). Cement Chemistry. Thomas Telford. ISBN978-0-7277-2592-9.
Peter Hewlett; Martin Liska (2019). Lea's Chemistry of Cement and Concrete. Butterworth-Heinemann. ISBN978-0-08-100795-2.
May 05, 2024
calcium, aluminoferrite, dark, brown, crystalline, phase, commonly, found, cements, cement, industry, termed, tetra, calcium, aluminoferrite, ferrite, cement, chemist, notation, abbreviated, meaning, 4cao, oxide, notation, also, exists, nature, rare, mineral, . Calcium aluminoferrite Ca2 Al Fe 2 O5 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 C4 AF meaning 4CaO Al2 O3 Fe2 O3 in the oxide notation It also exists in nature as the rare mineral brownmillerite Photomicrograph 0 11 mm of clinker polished section showing tetra calcium aluminoferrite C4 AF white and tricalcium aluminate C3 A grey occupying interstitial space between alite C3 S blue and belite C2 S orange crystals These are false interference colors Contents 1 Properties of the pure phase 2 Phases in Portland cement clinker 3 Behavior in cements 4 See also 5 References 6 Further readingProperties of the pure phase editIn the absence of elements other than calcium aluminium iron and oxygen calcium aluminoferrite forms a solid solution series of formula Ca2 Alx Fe1 x 2 O5 for all values of x in the range 0 0 7 1 28 32 Compositions with x gt 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 editIn Portland cement clinker calcium aluminoferrite C4 AF occurs as an interstitial phase crystallizing from the melt as the reaction end product of the flux components Al2 O3 and Fe2 O3 added in the raw materials to lower the high melting point of the oxide mix CaO SiO2 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 Al2 O3 and Fe2 O3 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 C4 AF for three different clinker bulk Fe2 O3 contents is given in the next table 1 10 2 160 The calcium aluminoferrite mean composition is clearly dominated by CaO 43 7 wt Al2 O3 19 3 wt and Fe2 O3 25 4 wt The mean contents in SiO2 4 2 wt and other oxides are marginal Chemical composition of calcium aluminoferrite C4 AF as a function of the bulk concentration mass or wt of Fe2 O3 in the clinker 1 2 Oxide Mass Mass Mass Bulk Fe2 O3 in clinker 0 29 2 88 4 87 SiO2 4 0 2 6 6 1 Al2 O3 20 2 20 8 17 0 Fe2 O3 24 5 23 9 27 7 CaO 44 6 46 4 40 2 MgO 3 7 3 1 4 9 Na2 O 0 1 0 1 0 1 K2 O 0 1 0 1 0 1 TiO2 1 9 2 7 1 7 Mn2 O3 0 1 0 3 1 5 ZnO 1 1 0 1 0 9 Note that the sum of Al2 O3 and Fe2 O3 contents in calcium aluminoferrite C4 AF in the table here above remains constant at 44 7 mass as illustrated in the table here below Sum of Al2 O3 and Fe2 O3 in C4 AF in the above table Oxide Mass Mass Mass Bulk Fe2 O3 in clinker 0 29 2 88 4 87 Al2 O3 20 2 20 8 17 0 Fe2 O3 24 5 23 9 27 7 Sum 44 7 44 7 44 7Behavior in cements editCalcium aluminoferrite C4 AF has little effect upon the physical properties of cement On hydration it forms 1 175 hydrogarnet 4 CaO Al2 O3 n H2 O 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 See also editThe three other main mineral phases of Portland cement clinker are Alite C3 S or 3CaO SiO2 Belite C2 S or 2CaO SiO2 Tricalcium aluminate C3 A or 3CaO Al2 O3References edit a b c d Taylor H F W 1990 Cement Chemistry Academic Press ISBN 0 12 683900 X a b c Hewlett P C Ed 1998 Lea s Chemistry of Cement and Concrete 4th Ed Arnold ISBN 0 340 56589 6 Further reading editTaylor Harry F W 1997 Cement Chemistry Thomas Telford ISBN 978 0 7277 2592 9 Peter Hewlett Martin Liska 2019 Lea s Chemistry of Cement and Concrete Butterworth Heinemann ISBN 978 0 08 100795 2 Retrieved from https en wikipedia org w index php title Calcium 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