Insulating materials (also known as thermal insulation materials) refer to materials or material composites with significant impedance to heat flow. The porosity of insulating refractory materials is generally above 45%, with low bulk density (not exceeding 1.5g/cm3) and low thermal conductivity (≤1.0 w·m-1k-1). There is a wide variety of insulation materials, typically classified based on material, operating temperature, form, and structure.

(1) According to their internal structure, they can be divided into three categories:

• Porous particle types, including commonly seen materials like expanded vermiculite and expanded perlite.
• Porous fiber types, including commonly seen materials like ultrafine glass wool, asbestos, and rock wool. These materials, often made of inorganic fibers, form single or composite fiber fabrics or felts with low thermal conductivity and excellent heat resistance. However, glass wool and rock wool samples tend to absorb water and produce fiber dust in the air, posing health risks and hindering their widespread use. In the 1980s, fibrous insulating materials had a significant market share, mainly applied to the insulation of building walls due to their excellent insulation and fire-resistant properties. In recent years, inorganic fibers such as aluminosilicate fibers, glass fibers, quartz fibers, zirconia fibers, carbon fibers, and silicon carbide fibers have been widely used in various advanced weaponry thermal protection projects.
• Foaming types, including inorganic, organic, and organic-inorganic hybrid types. Common inorganic types include foam glass and cement. In the 1980s, calcium silicate foam was recognized as the best-performing insulating material among hard block materials due to its small volume density, low thermal conductivity, high refractoriness, and excellent mechanical properties. However, in the 1990s, it was gradually replaced by pulp fibers because of the asbestos content in the samples. Organic types include phenolic foam, polyethylene foam, and polyurethane foam. These foam-type insulating materials have low bulk density, low thermal conductivity, and water resistance, making them widely used in the field of insulation.

(2) According to the operating temperature, they are divided into low-temperature insulation materials and high-temperature insulation materials.

• The operating temperature range of low-temperature insulation materials is 600~900℃.
• The operating temperature range of medium-temperature insulation materials is 900~1200℃.
• High-temperature insulation materials generally have an operating temperature above 1200℃.

(3) According to bulk density, they can be classified into ultra-lightweight and lightweight bricks.

• Lightweight bricks with a bulk density of 0.4~1.3g/cm3.
• Ultra-lightweight bricks with a bulk density below 0.4g/cm3.

(4) According to product shape:

• Shaped lightweight refractory bricks, including clay bricks, siliceous bricks, high-alumina bricks, and certain pure oxide lightweight bricks.
• Unshaped lightweight refractory materials, such as lightweight refractory concrete.

(5) According to raw materials, they can be divided into alumina, high alumina, silica, and magnesia insulating refractory materials. Different materials have different operating temperatures and related performance characteristics.

(6) According to material, they can be classified into organic insulating materials, inorganic insulating materials, and metallic insulating materials.

The surface heat dissipation and brick heat storage losses of industrial furnaces generally account for about 24~45% of fuel consumption. When the structural materials of the furnace body use lightweight bricks with low heat capacity and low thermal conductivity, fuel consumption can be significantly reduced. Additionally, rapid heating and cooling of the kiln can improve equipment production efficiency, reduce furnace weight, simplify furnace structure, enhance product quality, lower environmental temperatures, and improve working conditions.

The porous structure of lightweight refractory materials is loose, with a relatively large internal porosity. This structure has the disadvantage of poor resistance to slag. Molten slag can quickly infiltrate the pores of the brick, causing it to fracture. Lightweight refractory materials are not suitable for direct contact with liquid metal and slag. Due to their poor wear resistance, low mechanical strength, and poor thermal stability, they cannot be used as load-bearing structures and are not suitable for areas with contact with raw materials and high mechanical vibration.

Due to the aforementioned shortcomings of lightweight refractory materials, they are generally not used in furnace chambers where they come into direct contact with raw materials, contain slag, experience high gas flow rates, or encounter high mechanical vibrations in industrial kilns. Lightweight refractory materials are commonly used as insulation or refractory layers in kilns. Currently used lightweight refractory products include zirconia hollow spheres, alumina hollow spheres, lightweight mullite bricks, lightweight high-alumina bricks, lightweight clay bricks, and refractory fibers.