Dodecacalcium hepta-aluminate
 Crystal structure of C12A7
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| Names | |
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| Other names
C12A7
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| Identifiers | |
| Properties | |
| Ca12Al14O33 | |
| Molar mass | 1,386.66 g·mol−1 |
| Appearance | Clear to black solid, depending on synthesis and doping |
| Density | 2.68 g·cm−3 |
| Melting point | 1,400 °C (2,550 °F; 1,670 K) |
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Refractive index (nD)
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1.614–1.643 |
| Structure | |
| Cubic | |
| I43d | |
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a = 1.1989 nm
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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 | |
Dodecacalcium hepta-aluminate (12CaO·7Al2O3, Ca12Al14O33 or C12A7) is an inorganic solid that occurs rarely in nature as the mineral mayenite. It is an important phase in calcium aluminate cements and is an intermediate in the manufacture of Portland cement. Its composition and properties have been the subject of much debate, because of variations in composition that can arise during its high-temperature formation.
Polycrystalline C12A7 can be prepared via a conventional solid-state reaction, i.e., heating a mixture of calcium carbonate and aluminium oxide or aluminium hydroxide powders, in air. It is not formed in oxygen or in moisture-free atmosphere. It can be regrown into single crystals using the Czochralski or zone melting techniques.
In Portland cement kilns, C12A7 is an early reaction product of aluminium and calcium oxides in the temperature range 900–1200 °C. With the onset of melt-phases at higher temperatures, it reacts with further calcium oxide to form tricalcium aluminate. It thus can appear in under-burned kiln products. It also occurs in some natural cements.
The mineral as normally encountered is a solid solution series with end-members Ca12Al14O33 and Ca6Al7O16(OH). The latter composition loses water only at high temperature, and has lost most of it by the melting point (around 1400 °C). If material heated to this temperature is rapidly cooled to room temperature, the anhydrous composition is obtained. The rate of re-absorption of water to form the hydrous composition is negligible below 930 °C.
C12A7 has a cubic crystal symmetry; Ca12Al14O33 has a lattice constant of 1.1989 nm and a density of 2.680 g·cm−3 while Ca6Al7O16(OH) has 1.1976 nm and 2.716 g·cm−3. The unit cell consists of 12 cages with the inner diameter of 0.44 nm and a formal charge of +1/3, two of them host free O2− ions (not shown in the infobox structure). These ions can easily move through the material and can be replaced by F−, Cl− (as in the mineral chlormayenite) or OH− ions.
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