Dodecacalcium hepta-aluminate 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.
Synthesis
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.
Composition and structure
The mineral as normally encountered is a solid solution series with end-members Ca12Al14O33 and Ca6Al7O16. The latter composition loses water only at high temperature, and has lost most of it by the melting point. 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 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. These ions can easily move through the material and can be replaced by F−, Cl− or OH− ions. The confusion regarding composition contributed to the mistaken assignment of the composition Ca5Al3O33. Studies of the system have shown that the solid solution series extends also to the accommodation of other species in place of the hydroxyl group, including halides, sulfide and oxide ions.
Properties and applications
C12A7 is an important mineral phase in calcium aluminate cements and is an intermediate in the manufacture of Portland cement. It reacts rapidly with water, with considerable heat evolution, to form 3CaO·Al2O3·6H2O and Al3 gel. The formation of the hydrate from this mineral and from monocalcium aluminate represents the first stage of strength development in aluminous cements. Because of its higher reactivity, leading to excessively rapid hydration, aluminous cements contain relatively low amounts of dodecacalcium hepta-aluminate, or none at all. C12A7 has potential applications in optical, bio and structural ceramics. Some amorphous calcium aluminates are photosensitive and hence are candidates for optical information storage devices. They also have desirable infrared transmission properties for optical fibers. While undoped C12A7 is a wide-bandgap insulator, electron-doped electride C12A7:e− is a metallic conductor with a conductivity reaching 1500 S/cm at room temperature; it may even exhibit superconductivity upon cooling to 0.2–0.4 K. C12A7:e− is also a catalyst that has potential applications in the ambient-pressure synthesis of ammonia. Electron doping is achieved by extracting O2− ions from the C12A7 structure via chemical reduction. The injected electrons occupy a unique conduction band called 'the cage conduction band', and migrate through the C12A7:e− crystal by tunneling. They can be readily and reversibly replaced with hydride ions by heating C12A7:e− in a hydrogen atmosphere. Owing to this reversibility, C12A7:e− does not suffer from hydrogen poisoning – irreversible deterioration of properties upon exposure to hydrogen which is common to traditional catalysts used in the ammonia synthesis.
