Editorial Feature

Calcium Aluminate Hydrates - Phases and Structure

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Ordinary Portland cements (OPC), mainly based on lime-silica mineral phases, are often  used in the construction sector. On the contrary, calcium aluminate cements (CACs) have lime-alumina compounds as the core reactive phases.

CACs are also known as aluminous cement or high alumina cement (HAC). They were developed following the demand to produce sulfate-resistant cements.

Cement chemistry nomenclature used in a few of the notations mentioned in this article, is listed in Table 1.

Table 1. Cement Chemistry Nomenclature

Symbol Oxide species
A Al2O3 Alumina
C CaO Lime
S SiO2 Silica

Calcium Aluminate Hydrates

Three important calcium aluminate hydrates develop during HAC hydration, namely two meta-stable phases, C2AH8, and CAH10, and the stable phase C3AH6 (katoite or hydrogarnet). Besides these species, alumina tri-hydrate phases often develop during the hydration process, either as an amorphous gel or as the crystalline phase Gibbsite.

Calcium Aluminate Decahydrate (CAH10)

Metastable calcium aluminate decahydrate (CAH10) is typically produced when calcium aluminates are mixed with water at temperatures below 20 °C. CAH10 forms hexagonal platelets of weakly crystallized material, making its analysis by X-ray diffraction (XRD) extremely challenging.

When CAH10 is heated, it tends to dehydrate between 120 °C and 130 °C with an endothermic reaction, which can simply be identified using differential thermal analysis (DTA). The unit cell of CAH10 is deformed orthorhombic.

Table 2. Dehydration of calcium aluminate hydrates

Hydrate CAH10 C2AH8 C3AH6 AH3
Dehydrating temperature (°C) 100-130 170-195 300-360 210-300

Dicalcium Aluminate Octahydrate (C2AH8)

Dicalcium aluminate octahydrate (C2AH8) is another metastable calcium aluminate hydrate. It has the inclination to precipitate out of solution as a thin hexagonal platelet, as is the case with CAH10. In addition, C2AH8 develops in the temperature range of 21 °C–30 °C and dehydrates between 210 °C and 230 °C with a typical endothermic peak.

Tricalcium Aluminate Hexahydrate (C3AH6)—Katoite

Hydrogarnet (Katoite) or tricalcium aluminate hexahydrate is the most stable among all calcium aluminate hydrates. It contains a cubic unit cell and develops at higher ambient temperatures. It does not undergo any alteration into other hydrates, as is the case with CAH10 and C2AH8. It dehydrates between 300 °C and 350 °C, forming C3AH1.5 primarily, and then undergoes complete dehydration at 500 °C.

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