Sep 11 2002
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Ordinary Portland cements (OPCs), often used in the construction industry, are mostly based on lime-silica mineral phases. On the other hand, calcium aluminate cements (CACs) have reactive phases that constitute lime-alumina compounds.
CACs are also known as high-alumina cements (HACs) or aluminous cements, which resulted out of a drive to create cements that are resistant to sulfate.
Some of the representations in this article use cement chemistry nomenclature, which is given in Table 1.
Table 1. Cement Chemistry Nomenclature
| Symbol |
Oxide species |
| A |
Al2O3 Alumina |
| C |
CaO Lime |
| S |
SiO2 Silica |
Phases of Calcium Aluminates
Calcium Mono-Aluminate
The main hydraulic mineral in CAC is calcium mono-aluminate (CA). The hydration of CA contributes to the high initial strength of CACs. CA is a monoclinic phase and pseudo-hexagonal, and its space group is P21/n, Z = 12, and Dx = 2.945 g/cm3. Moreover, it resembles the structure of β-tridymite with an endless 3D framework of AlO4 tetrahedra sharing corners.
Conversely, the tridymite network is altered because of the large ionic radius of Ca2+, and a part of the calcium atoms contains irregular coordination polyhedra with oxygen.
When CA is viewed under an optical microscope, it appears as asymmetrical colorless grains with inference colors that are lesser than those of CA2. Some of the Al3+ ions in the AlO4+ tetrahedra can be partly replaced by iron. A monoclinic unit cell is present in CA.
Calcium Di-Aluminate—Grossite
Calcium di-aluminate (CA2) or grossite has a monoclinic structure with the space group C2/c, Z = 4, and Dx = 2920 kg/m3. The structure of this compound is based on a framework of AlO4 tetrahedra. While some of the oxygen atoms are shared between two tetrahedra, the remaining oxygen atoms are shared among the other three. When compared to CA, the CA2 phase may be more refractory in nature, yet it is less reactive.
Self-hydrating CA2 will have lower strength than CA following 24 hours of curing. But the hydration of CA2 is believed to be faster when it is exposed to CA and also at higher temperatures.
Dodecacalcium Hepta-Aluminate—Mayenite
Mayenite or dodeca-calcium hepta-aluminate (C12A7) includes the space group I43d. The respective crystal structure of this compound is composed of Ca2+ ions with an irregular six-fold coordination with oxygen. The oxygen has an incomplete framework of corner-sharing AlO4 tetrahedra with the empirical composition Al7O1611−. One of the O2− ions in each unit formula is shared among 12 sites, which are said to increase a pair of the AlO4 groups to AlO5.
In mayenite, the calcium ions have a highly balanced shell of oxygen atoms that are irregularly spread on a hemisphere’s surface. These coordinating hemispheres exist in pairs along the axes of crystal symmetry, in which the planar faces are established by vacancies among the 12-fold positions.
Mayenite is the most reactive one among all calcium aluminate species. It occurs in HACs and hydrates rapidly, than CA. Hence, manufacturers cautiously regulate the amount of mayenite present in calcium aluminate.
Other Calcium Aluminate Phases
Sintering of CA2 results in the formation of calcium hexa-aluminate (CA6). CA6 is non-hydraulic and more refractory when compared to the other phases, and has a melting point of 1870 °C. Upon heating to high temperatures, CA6 develops in refractory castable products. The C3A phase is extremely reactive and can be found only in Portland cements.