Dodecacalcium hepta-aluminate (12CaO.7Al₂O₃) occurs rarely in nature as the mineral mayenite

Mayenite

Mayenite , Ca12Al14O33, is a rare cubic mineral of cubic symmetry, originally reported from Eifel volcanic complex . It is also found at pyrometamorphic sites: e.g., in the Hatrurim Formation and in some burned coal dumps. The formula can be written as [Ca12Al14O32]O, which refers to the unique...

. It is an important mineral phase in Calcium aluminate cements

Calcium aluminate cements

Calcium aluminate cements are cements consisting predominantly of hydraulic calcium aluminates. Alternative names are "aluminous cement", "high-alumina cement" and "Ciment fondu" in French...

 and is an intermediate in the manufacture of 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...

. Its composition and properties have been the subject of much debate, because of variations in composition that can arise during its high-temperature formation.

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 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...

s, 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

Tricalcium aluminate

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 thus can appear in under-burned kiln products. It also occurs in some Natural cements

Cement

In the most general sense of the word, a cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The word "cement" traces to the Romans, who used the term opus caementicium to describe masonry resembling modern concrete that was made from crushed...

.