Anorthite (CaO·Al₂O₃·2SiO₂), belonging to the triclinic crystal system, is characterized by its low density, minimal thermal expansion, and low thermal conductivity. Its raw materials are readily available and cost-effective. However, with a melting point of 1550℃ and relatively low strength, Anorthite has rarely been used as the primary crystalline phase in refractory materials. On the other hand, mullite is renowned for its exceptional refractory properties, including high heat resistance, excellent thermal shock resistance, chemical erosion resistance, creep resistance, and high load-bearing softening temperature.
Combining these advantages, researchers have developed Anorthite/mullite composite lightweight refractory materials, which feature low thermal conductivity and high operational temperatures. These materials integrate the low density and thermal conductivity of Anorthite with the high strength and superior high-temperature performance of mullite. Applications of such materials are exemplified by products like Anorthite Insulating Fire Brick, which leverages these properties for industrial efficiency.
Anorthite has a chemical composition of CaO·Al₂O₃·2SiO₂, with a theoretical density of 2.74–2.76 g/cm³ and a hardness of 6–6.5. Its dielectric constant is 6.2, with a thermal expansion coefficient of 4.82×10⁻⁶/℃ and a room-temperature thermal conductivity of 1.503 W/m·K. Naturally occurring in basic igneous rocks, pure Anorthite is rare due to its complete solid solubility with albite at any temperature, necessitating synthetic production. Consequently, research on Anorthite remains limited.
Theoretically, Anorthite melts at 1550℃, significantly higher than potassium feldspar (1290℃) and albite (1190℃). This property makes it a potential candidate for medium-temperature refractory applications, particularly in products like Anorthite Insulating Fire Brick, designed to meet specific industrial needs.
Mullite is classified into three types: α-mullite, which corresponds to the pure form of 3Al₂O₃·2SiO₂ and is referred to as the 3:2 type; β-mullite, which contains excess Al₂O₃ in solid solution, resulting in a slightly expanded lattice and is known as the 2:1 type; and γ-mullite, which incorporates small amounts of TiO₂ and Fe₂O₃. Mullite crystallizes in the orthorhombic system, and its average structure consists of chains of [AlO₆] octahedrons linked along their edges, aligned parallel to the c-axis. These chains are positioned at the four corners and the center of the (001) projection plane of the unit cell. At the midpoint of the unit cell (Z=1/2), the octahedral chains connect with [SiO₄] and [AlO₄] tetrahedrons, forming double chains that also run parallel to the c-axis. Within this structure, the bridging oxygen atoms and other oxygen sites lose their occupancy, while the remaining oxygen atoms and the Al and Si ions in the T-sites undergo displacement, accompanied by the substitution of Si by Al in the tetrahedrons.
The chain-like arrangement of mullite’s structure results in the formation of long columnar or needle-shaped crystals extending along the c-axis. In high-alumina products, these needle-like mullite crystals interlock to create a robust skeletal network, endowing mullite with a series of exceptional properties. It has a melting point of 1870℃, a specific gravity of 3.03, and a hardness of 6.7. Its linear expansion coefficient is small, measuring 5.3×10⁻⁶/℃ from 20 to 1000℃, with a thermal conductivity of 13.8 KJ/m·h·℃. Mullite also possesses a relatively low elastic modulus, ranging from 160 to 200 GPa, which is approximately half that of Al₂O₃ or SiC. In addition to the columnar and needle-like crystals, plate-like mullite crystals can also form. XRD analysis has identified these plate-like crystals as β-mullite, corresponding to the 2Al₂O₃·SiO₂ (2:1) type.
Mullite is chemically stable, even insoluble in HF. However, its primary weakness lies in the charge imbalance within its structural oxygen, making it susceptible to decomposition by oxides such as Na₂O and K₂O. Sintered and fused mullite can be reduced by CO at temperatures above 1000℃, leading to the formation of corundum and gaseous SiO₂.
Mullite Insulation Brick exhibits high refractoriness, excellent thermal shock resistance, chemical erosion resistance, creep resistance, high load-bearing softening temperature, good volume stability, and strong electrical insulation properties. These qualities make it an ideal advanced refractory material, widely utilized in industries such as metallurgy, glass, ceramics, chemistry, power generation, defense, gas, and cement.
