Tricalcium aluminate
Encyclopedia
Tricalcium aluminate Ca3Al2O6, often formulated as 3CaO·Al2O3 to highlight the proportions of the oxides from which it is made, is the most basic of the calcium aluminates
. It does not occur in nature, but is an important mineral phase in Portland cement
.
and aluminium oxide
are heated together above 1300 °C. The pure form is cubic, with unit cell dimension 1.5263 nm and has density 3064 kg·m-3. It melts with decomposition at 1542 °C. The unit cell contains 8 cyclic Al6O1818− anions, which can be considered to consist of 6 corner sharing AlO4 tetrahedra.
In Portland cement clinker
, tricalcium aluminate 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 mostly undesirable. It forms an impure solid solution phase, with 15-20% of the aluminium atoms replaced by silicon and iron, and with variable amounts of alkali metal atoms replacing calcium, depending upon the availability of alkali oxides in the melt. The impure form has at least four polymorphs:
Typical chemical compositions are:
(typically 4-8%). Sulfate ions in solution lead to the formation of an insoluble layer of ettringite
(3CaO·Al2O3·3CaSO4·32H2O) over the surface of the aluminate crystals, passivating them. The aluminate then reacts slowly to form the AFm phase
3CaO·Al2O3·CaSO4·12H2O. These hydrates contribute little to strength development.
Tricalcium aluminate is associated with three important effects that can reduce the durability of concrete:
Because they are even more basic, the alkali-loaded polymorphs are correspondingly more reactive. Appreciable amounts (>1%) in cement make set control difficult, and the cement becomes excessively hygroscopic. The cement powder flowability is reduced, and air-set lumps tend to form. They withdraw water from gypsum on storage of the cement, leading to false set. For this reason, their formation is avoided wherever possible. It is more energetically favorable for sodium and potassium to form sulfates and chlorides in the kiln
, but if insufficient sulfate ion is present, any surplus alkalis congregate in the aluminate phase. The feed and fuel in the kiln system are preferably controlled chemically to keep the sulfate and alkalis in balance. However, this stoichiometry is only maintained if there is substantial surplus oxygen in the kiln atmosphere: if "reducing conditions" set in, then sulfur is lost as SO2, and reactive aluminates start to form. This is readily monitored by tracking the clinker sulfate level on an hour-to-hour basis.
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....
. It does not occur in nature, but is an important mineral phase in 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...
.
Properties
Pure tricalcium aluminate is formed when the appropriate proportions of finely divided calcium oxideCalcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
and aluminium oxide
Aluminium oxide
Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry...
are heated together above 1300 °C. The pure form is cubic, with unit cell dimension 1.5263 nm and has density 3064 kg·m-3. It melts with decomposition at 1542 °C. The unit cell contains 8 cyclic Al6O1818− anions, which can be considered to consist of 6 corner sharing AlO4 tetrahedra.
In Portland cement 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:...
, tricalcium aluminate 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 mostly undesirable. It forms an impure solid solution phase, with 15-20% of the aluminium atoms replaced by silicon and iron, and with variable amounts of alkali metal atoms replacing calcium, depending upon the availability of alkali oxides in the melt. The impure form has at least four polymorphs:
| Alkali % m/m | Designation | Crystal |
|---|---|---|
| 0-1.0 | CI | Cubic |
| 1.0-2.4 | CII | Cubic |
| 3.7-4.6 | O | Orthorhombic |
| 4.6-5.7 | M | Monoclinic |
Typical chemical compositions are:
| Oxide | Mass % Cubic | Mass % Orthorhombic |
|---|---|---|
| SiO2 | 3.7 | 4.3 |
| Al2O3 | 31.3 | 28.9 |
| Fe2O3 | 5.1 | 6.6 |
| CaO | 56.6 | 53.9 |
| MgO | 1.4 | 1.2 |
| Na2O | 1.0 | 0.6 |
| K2O | 0.7 | 4.0 |
| TiO2 | 0.2 | 0.5 |
Effect on cement properties
In keeping with its high basicity, tricalcium aluminate reacts most strongly with water of all the calcium aluminates, and it is also the most reactive of the Portland clinker phases. Its hydration to phases of the form Ca2AlO3(OH)·nH2O leads to the phenomenon of "flash set" (instantaneous set), and a large amount of heat is generated. To avoid this, Portland-type cements include a small addition of calcium sulfateCalcium sulfate
Calcium sulfate is a common laboratory and industrial chemical. In the form of γ-anhydrite , it is used as a desiccant. It is also used as a coagulant in products like tofu. In the natural state, unrefined calcium sulfate is a translucent, crystalline white rock...
(typically 4-8%). Sulfate ions in solution lead to the formation of an insoluble layer of ettringite
Ettringite
Ettringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2312·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system...
(3CaO·Al2O3·3CaSO4·32H2O) over the surface of the aluminate crystals, passivating them. The aluminate then reacts slowly to form the 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....
3CaO·Al2O3·CaSO4·12H2O. These hydrates contribute little to strength development.
Tricalcium aluminate is associated with three important effects that can reduce the durability of concrete:
- heat release, which can cause spontaneous overheating in large masses of concrete. Where necessary, tricalcium aluminate levels are reduced to control this effect.
- sulfate attack, in which sulfate solutions to which the concrete is exposed react with the AFm phaseAFm phaseAn 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....
to form ettringiteEttringiteEttringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2312·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system...
. This reaction is expansive, and can disrupt mature concrete. Where concrete is to be placed in contact with, for example, sulfate-laden ground waters, either a "sulfate-resisting" cement (with low levels of tricalcium aluminate) is used, or slagGround granulated blast furnace slagGround-granulated blast-furnace slag is obtained by quenching molten iron slag from a blast furnace in water or steam, to produce a glassy, granular product that is then dried and ground into a fine powder.-Applications:GGBS is used to make durable concrete structures in combination with ordinary...
is added to the cement or to the concrete mix. The slag contributes sufficient aluminium to suppress formation of ettringite. - delayed ettringiteEttringiteEttringite is a hydrous calcium aluminium sulfate mineral with formula: Ca6Al2312·26H2O. It is a colorless to yellow mineral crystallizing in the trigonal system...
formation, where concrete is cured at temperatures above the decomposition temperature of ettringite (about 65 °C). On cooling, expansive ettringite formation takes place.
Because they are even more basic, the alkali-loaded polymorphs are correspondingly more reactive. Appreciable amounts (>1%) in cement make set control difficult, and the cement becomes excessively hygroscopic. The cement powder flowability is reduced, and air-set lumps tend to form. They withdraw water from gypsum on storage of the cement, leading to false set. For this reason, their formation is avoided wherever possible. It is more energetically favorable for sodium and potassium to form sulfates and chlorides in the 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...
, but if insufficient sulfate ion is present, any surplus alkalis congregate in the aluminate phase. The feed and fuel in the kiln system are preferably controlled chemically to keep the sulfate and alkalis in balance. However, this stoichiometry is only maintained if there is substantial surplus oxygen in the kiln atmosphere: if "reducing conditions" set in, then sulfur is lost as SO2, and reactive aluminates start to form. This is readily monitored by tracking the clinker sulfate level on an hour-to-hour basis.