Calcium aluminoferrite
Encyclopedia
Cement
In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...
s. In the cement industry it is termed ferrite. It also exists in nature as the rare mineral brownmillerite
Brownmillerite
Brownmillerite is a mineral with an interesting structure. Its chemical formula is Ca22O5. It is named for Lorrin Thomas Brownmiller , chief chemist of the Alpha Portland Cement Company, Easton, Pennsylvania....
.
Properties of the pure phase
In the absence of elements other than calciumCalcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
, aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, calcium aluminoferrite forms a solid solution
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
series of formula Ca2(AlxFe1-x)2O5 for all values of x in the range 0-0.7.
Compositions with x > 0.7 do not exist at ordinary pressures (see dicalcium aluminate
Calcium aluminates
Calcium aluminates are a range of minerals obtained by heating calcium oxide and aluminium oxide together at high temperatures. They are encountered in the manufacture of refractories and cements....
). The crystal is orthorhombic, and is normally lath-like. Its density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
varies from 4026 kg⋅m−3 (x = 0) to 3614 kg⋅m−3 (x = 0.7). All compositions melt
Melting
Melting, or fusion, is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid...
incongruently in the range 1400-1450°C. They are ferromagnetic, progressively more so as iron content increases. These phases are easily prepared from the oxide
Oxide
An oxide is a chemical compound that contains at least one oxygen atom in its chemical formula. Metal oxides typically contain an anion of oxygen in the oxidation state of −2....
s.
Phases in Portland cement clinker
In Portland cementPortland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
clinker
Clinker (cement)
thumb|200px|right|Typical clinker nodulesthumb|200px|right|Hot clinkerIn the manufacture of Portland cement, clinker is lumps or nodules, usually 3-25 mm in diameter, produced by sintering limestone and alumino-silicate during the cement kiln stage.-Uses:...
, calcium aluminoferrite occurs as an "interstitial phase", crystallizing
Crystallization
Crystallization is the process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid–liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid...
from the melt. Its presence in clinker is solely due to the need to obtain liquid at the peak kiln
Cement kiln
Cement kilns are used for the pyroprocessing stage of manufacture of Portland and other types of hydraulic cement, in which calcium carbonate reacts with silica-bearing minerals to form a mixture of calcium silicates...
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
Diluent
A diluent is a diluting agent.Certain fluids are too viscous to be pumped easily or too dense to flow from one particular point to the other. This can be problematic, because it might not be economically feasible to transport such fluids in this state.To ease this restricted movement, diluents...
. Its forms an impure solid solution that deviates markedly in composition from the simple chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
. The calcium aluminoferrite phase acts as a repository for many of the minor elements in the clinker. Most of the transitional elements
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
in the cement are found in the ferrite phase, notably titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
, manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...
and zinc
Zinc
Zinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
. There is also a substantial amount of magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
and silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, 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:
| Oxide | Mass % | Mass % | Mass % |
|---|---|---|---|
| Fe2O3 in Clinker | 0.29 | 2.88 | 4.87 |
| SiO2 | 4.0 | 2.6 | 6.1 |
| Al2O3 | 20.2 | 20.8 | 17.0 |
| Fe2O3 | 24.5 | 23.9 | 27.7 |
| CaO | 44.6 | 46.4 | 40.2 |
| MgO | 3.7 | 3.1 | 4.9 |
| Na2O | 0.1 | 0.1 | 0.1 |
| K2O | 0.1 | 0.1 | 0.1 |
| TiO2 | 1.9 | 2.7 | 1.7 |
| Mn2O3 | 0.1 | 0.3 | 1.5 |
| ZnO | 1.1 | 0.1 | 0.9 |
Behavior in cements
Calcium aluminoferrite has little effect upon the physical properties of cement. On hydrationMineral hydration
Mineral hydration is an inorganic chemical reaction where water is added to the crystal structure of a mineral, usually creating a new mineral, usually called a hydrate....
it forms 4CaO⋅Al2O3⋅nH2O and hydrated iron oxide
Iron(III) oxide-hydroxide
A number of species are dubbed iron oxide-hydroxide. These chemicals are oxide-hydroxides of iron, and may occur in anhydrous or hydrated forms...
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
Portland cement
Portland cement is the most common type of cement in general use around the world because it is a basic ingredient of concrete, mortar, stucco and most non-specialty grout...
, subsequent slow reaction with dissolved sulfate forms an AFm phase
AFm phase
An AFm phase is an "alumina, ferric oxide, monosulfate" phase. AFm phases are important in the hydration of hydraulic cements.They are crystalline hydrates with general, simplified formula 3 CaO·2O3·CaSO4·nH2O....
, which has negligible strength-giving properties. In the case of calcium aluminate cements
Calcium aluminate cements
Calcium aluminate cements are cements consisting predominantly of hydraulic calcium aluminates. Alternative names are "aluminous cement", "high-alumina cement" and "Ciment fondu" in French...
, 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.