Tricalcium aluminate
| Names | |
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| Other names
aluminate, C3A
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| Identifiers | |
| ECHA InfoCard | 100.031.744 |
| Properties | |
| Ca3Al2O6, or 3CaO·Al2O3 | |
| Molar mass | 270.193 g/mol |
| Density | 3.064 g/cm3 |
| Melting point | 1,542 °C (2,808 °F; 1,815 K) (decomposes) |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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| Infobox references | |
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.
Pure tricalcium aluminate is formed when the appropriate proportions of finely divided calcium oxide 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:
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 sulfate (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.
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