Calcium aluminates
Calcium aluminates are a class of inorganic compounds formed by the combination of calcium oxide (CaO) and aluminum oxide (Al₂O₃) in varying stoichiometric ratios, with key phases including monocalcium aluminate (CA, CaO·Al₂O₃), tricalcium aluminate (C₃A, 3CaO·Al₂O₃), dodecacalcium hepta-aluminate (C₁₂A₇, 12CaO·7Al₂O₃), and others such as 5CaO·3Al₂O₃ and 3CaO·5Al₂O₃.[1][2] These materials serve as the primary active components in calcium aluminate cements (CACs), which are non-Portland hydraulic cements distinguished by their high alumina content (typically 30-80% Al₂O₃) and low silica compared to ordinary Portland cement.[3][4] CACs are valued for their rapid hardening and early strength development, achieving significant compressive strength within hours of setting, which contrasts with the slower hydration of Portland cements.[3] They also demonstrate superior resistance to high temperatures (up to 1800°C), chemical attack from acids and sulfates, abrasion, and impact, though long-term strength can diminish due to the conversion of hydration products like CAH₁₀ and C₂AH₈ into more porous phases such as C₃AH₆.[3][5] These properties arise from the hydration mechanism, where aluminates react with water to form calcium aluminate hydrates and gibbsite (Al(OH)₃), enabling quick binding without requiring sulfate activators in pure forms.[1] The primary applications of calcium aluminates and CACs span construction, refractories, and specialized engineering contexts. In building chemistry, they are used in rapid-hardening mortars, self-leveling floor screeds, tile adhesives, and repair concretes for infrastructure like roads and bridges.[3] Refractory castables for steel and glass industries rely on their thermal stability, while corrosion-resistant linings for sewers and chemical plants leverage their sulfate and acid resistance.[3][6] Emerging uses include biomedical scaffolds for bone grafts due to biocompatibility and bioactivity, as well as oil well cements where reinforcement addresses inherent brittleness.[7][8] Overall, calcium aluminates represent a versatile class of materials tailored for demanding environments where standard cements fall short.[9]Composition and Phases
Principal Phases
Calcium aluminates comprise a series of binary compounds formed between calcium oxide (CaO) and aluminum oxide (Al₂O₃) in varying stoichiometric ratios within the CaO-Al₂O₃ system.[10] The principal phases are dodecacalcium hepta-aluminate (C₁₂A₇, 12CaO · 7Al₂O₃ or Ca₁₂Al₁₄O₃₃), tricalcium aluminate (C₃A, 3CaO · Al₂O₃ or Ca₃Al₂O₆), monocalcium aluminate (CA, CaO · Al₂O₃ or CaAl₂O₄), dicalcium aluminate (CA₂, CaO · 2Al₂O₃ or CaAl₄O₇), and monocalcium hexa-aluminate (CA₆, CaO · 6Al₂O₃ or CaAl₁₂O₁₉).[10][11] These phases exhibit distinct stability ranges in the phase diagram of the system. Monocalcium aluminate (CA) is stable at elevated temperatures, with congruent melting above 1600°C.[12] Dodecacalcium hepta-aluminate (C₁₂A₇) represents a low-temperature phase, forming and remaining stable in the range of approximately 1300–1400°C.[13] Tricalcium aluminate (C₃A) undergoes incongruent melting at around 1540°C.[10] Among these, minor phases include mayenite (C₁₂A₇), grossite (CA₂, CaAl₄O₇), hibonite (CA₆, CaAl₁₂O₁₉), and metastable compounds such as pentacalcium trialuminate (C₅A₃, 5CaO · 3Al₂O₃ or Ca₅Al₆O₁₄).[14][15][16]Crystal Structures
Calcium aluminates exhibit diverse crystal structures that underpin their reactivity in cementitious materials, primarily composed of aluminum in tetrahedral coordination within oxygen polyhedra. The principal phases, such as monocalcium aluminate (CA), display a monoclinic crystal system with space group P2₁/n, featuring a three-dimensional framework of corner-sharing AlO₄ tetrahedra that form ditrigonal rings, with calcium cations occupying interstitial sites to balance the charge.[17] This stuffed tridymite-like arrangement results in layered conformations of the tetrahedral rings, contributing to the phase's structural stability.[18] Dicalcium aluminate (CA₂) adopts a monoclinic crystal system with space group C2/c, characterized by a framework structure consisting of isolated AlO₄ tetrahedra, AlO₆ octahedra, and oxygen triclusters (O atoms bridged to three Al atoms).[15][19] Calcium cations are coordinated by oxygen atoms in irregular polyhedra that link the structural units.[15] This configuration, featuring mixed tetrahedral and octahedral aluminum coordinations along with triclusters, distinguishes CA₂ from other phases and contributes to its refractory properties.[19] Tricalcium aluminate (C₃A) primarily crystallizes in a cubic system with space group Pa-3, where the structure comprises isolated AlO₄ tetrahedra surrounded by calcium polyhedra, forming a highly symmetric framework with a lattice parameter of approximately 1.526 nm.[20] Polymorphism in C₃A includes a cubic form stable at high temperatures and an orthorhombic variant induced by substitutions such as sodium, involving a transition that distorts the tetrahedral coordination and alters hydration behavior.[21][22] The cubic phase features 72 calcium atoms and 48 aluminum atoms per unit cell, with all aluminum in tetrahedral sites.[23] Dodecacalcium hepta-aluminate (C₁₂A₇), also known as mayenite, has a cubic crystal system with space group I-43d and a lattice parameter of about 1.199 nm, consisting of a cage-like framework built from AlO₄ tetrahedra that enclose 12 sub-nanometer cages, six of which contain extra-framework O²⁻ ions for charge compensation.[24] Recent studies have explored defects and substitutions in C₁₂A₇, such as the replacement of O²⁻ with electrons to form an electride phase, which introduces localized electronic states and enhances conductivity, as demonstrated in applications for catalysis.[25] This electride modification, achieved through reduction, shifts the material from an insulator to a conductor while preserving the overall framework integrity.[25]Physical and Chemical Properties
Physical Properties
Calcium aluminates typically appear as white to gray powders, depending on the purity and processing conditions of the specific phase.[26][27] The densities of the principal calcium aluminate phases vary based on their composition, with values ranging from approximately 2.7 to 3.0 g/cm³ for the more common hydraulic phases. These densities contribute to the lightweight nature of materials derived from calcium aluminates, such as certain refractories and cements. Representative densities for key phases are summarized below:| Phase | Chemical Formula | Density (g/cm³) |
|---|---|---|
| Tricalcium aluminate (C₃A) | Ca₃Al₂O₆ | 3.04 |
| Monocalcium aluminate (CA) | CaAl₂O₄ | 2.98 |
| Dodecacalcium hepta-aluminate (C₁₂A₇) | Ca₁₂Al₁₄O₃₃ | 2.7 |