Background

Portland cements (OPC), which are commonly used in the construction industry, are based mainly upon lime-silica mineral phases, whereas in calcium aluminate cements the main reactive phases are lime-alumina compounds. Calcium aluminates (CAC) may go under the other names of aluminous cement or high alumina cement (HAC). Calcium aluminate cements evolved from a drive to develop sulphate resistant cements.

Some of the notation used later in this article uses cement chemistry nomenclature, which is documented 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 (CA)

CA (calcium mono-aluminate) is the principal hydraulic mineral present in calcium alumina cement. Its hydration contributes to the high early strength of CACs. Calcium mono-aluminate is a monoclinic phase, pseudo-hexagonal its space group is P21/n, Z=12 and Dx= 2.945 g/cm-3. It resembles the structure of b-tridymite with an infinite three-dimensional framework of AlO4 tetrahedra sharing corners. However the large ionic radius of the Ca2+ distorts the tridymite network and a section of the calcium atoms has irregular co-ordination polyhedra with oxygen. Under the optical microscope CA appears as irregular colourless grains, with inference colours less than that of CA2. Some of the Al3+ ions in the AlO4+ tetrahedra may be partially replaced by iron. CA has a monoclinic unit cell. Figure 1 shows a model of its crystal structure.

AZoM - Metals, Ceramics, Polymer and Composites : Calcium Aluminate Cements (CAC) – Phases and Structure of Calcium mono-Aluminate

Figure 1. Crystal structure of calcium mono-aluminate.
Calcium Di-Aluminate (CA2) – Grossite

Grossite or Calcium di-aluminate (CA2) has a monoclinic structure with the space group C2/c, Z=4 and D x=2920 kg m-3. Its structure is based upon a framework of AlO4 tetrahedra. Some of the oxygen atoms are shared between two tetrahedra and the rest distributed between the other three. The CA2 phase tends to be more refractory in nature than CA, however it is less reactive. CA2 hydrating on its own will impart a lower strength than CA after 24 hours of curing. However the hydration of CA2 is reported to be accelerated at higher temperature and also in the presence of CA. Grossite’s crystal structure is shown in figure 2.

AZoM - Metals, Ceramics, Polymer and Composites : Calcium Aluminate Cements (CAC) – Phases and Structure of Calcium di-Aluminate

Figure 2. Crystal structure of Calcium di-aluminate (Grossite).
Dodecacalcium Hepta-Aluminate (C12A7) - Mayenite

Dodeca-calcium hepta-aluminate (C12A7) or mayenite has the space group I43d. Its respective crystal structure is composed of Ca2+ ions with irregular six-fold co-ordination with oxygen, which has an incomplete framework of corner-sharing AlO4 tetrahedra that has the empirical composition Al7O1611-. In each unit formula one of the O2- ions is distributed between twelve sites, which is thought to increase two of the AlO4 groups to AlO5.

The calcium ions in mayenite have a highly balanced shell of oxygen atoms, which are roughly distributed on the surface of a hemisphere. These co-ordinating hemispheres occur in pairs along the axes of crystal symmetry, in which the planar faces are defined by vacancies amongst the twelve-fold positions.

Mayenite is the most reactive of all calcium aluminate species occurring in HACs, and will hydrate very rapidly in contrast to CA. Due to this fact the amount of mayenite contained in calcium aluminate is very carefully regulated by manufacturers.

AZoM - Metals, Ceramics, Polymer and Composites : Calcium Aluminate Cements (CAC) – Phases and Structure of dodecA-Calcium hepta-Aluminate

Figure 3. Crystal structure of Mayenite
Other Calcium Aluminate Phases

Calcium hexa-aluminate (CA6) forms as a result of the sintering of CA2. It is non-hydraulic and more refractory than the other phases, having a melting point of 1870°C. CA6 is formed in refractory castable products when they are heated to high temperatures. The C3A phase is very reactive and is only present in Portland cements.