Phase Equilibria and Crystal Phases in the CaO-Al2O3 System
The binary phase diagram of CaO-Al2O3 shows two refractory end-members CaO and Al2O3 with melting points of 2,570°C and 2,050°C, respectively [25, 26]. There is a deep eutectic with a minimum at 1,390°C and five intermediate crystalline phases, of which three hydrates are important as cements .
Monocalcium aluminate (CaAl2O4) is the most important phase in CAC. Addition of water to CaAl2O4 (CA) eventually leads to the formation of the crystalline hydrates 3CaOAl2O36H2O and Al2O33H2O, which dominate the initial hydration of CAC . Monocalcium aluminate CaAl2O4 does not have a spinel structure, even though it is stochiometrically equivalent to Mg-aluminate spinel. The crystal structure of this phase is monoclinic, pseudo hexagonal with a p2/n space group. The structure of the CA phase resembles that of tridymite and is formed from a framework of corner – linked AlO4 tetrahedra. Large Ca2+ ions distort the aluminate framework, reducing symmetry. As a consequence, the coordination environment of Ca2+ is irregular.
The CA2 phase (CaO2Al2O3) occurs as the natural mineral grossite . This phase is a monoclinic C2/2 phase and is also formed from a framework of corner – linked AlO4 tetrahedra. Some of the oxygens in the framework are shared between two tetrahedral and some are shared between three. The CA2 phase does not react well with H2O and is not necessarily useful in refractory CAC. The CA6 phase also occurs naturally, as the mineral hibenite. This phase has a similar structure to P-Al2O3 and is nonreactive and its presence is not desired in CAC.
The C12A7 phase reacts very rapidly with water and becomes modified to produce the hydrated phase 11CaO7Al2O3Ca(OH)2. The C12A7 phase is cubic with a space group of 143 d. The basic structure is one of a corner shared AlO4 framework. The Ca2+ ions are coordinated by six oxygen atoms but the coordination polyhedron is irregular. It has been suggested, by infrared spectroscopy, that some of the aluminum ions are coordinated by five oxygen atoms. This hydrated phase (Ca11Al7Ca(OH)2) is closely related to the naturally occurring mineral, Mayenite, a cubic mineral with M2M symmetry and a large (11.97 A) unit cell, closely related to the garnet structure.