Dodecacalcium hepta-aluminate

Dodecacalcium hepta-aluminate (12CaO.7Al₂O₃) occurs rarely in nature as the mineral mayenite. It is an important mineral 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,^[1] because of variations in composition that can arise during its high-temperature formation.

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Composition and Structure

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. The mineral is cubic. The crystal of Ca₁₂Al₁₄O₃₃ has cell dimension 1.1983 nm and density 2680 kg.m^-3 while that of Ca₆Al₇O₁₆(OH) has 1.1976 nm and 2716 kg.m^-3.

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

The mineral reacts rapidly with water, with considerable heat evolution, to form 3CaO.Al₂O₃.6H₂O and Al(OH)₃ 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.

In Portland cement kilns, it 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.

References

1. ^ H F W Taylor, Cement Chemistry, Academic Press, 1990, ISBN 0-12-683900-X, pp 36-38