Heat transfer in insulating bricks and other refractory materials involves two parts: heat transfer through the solid phase and heat transfer through the gas phase. At low temperatures, heat transfer is primarily through the solid phase; at high temperatures, heat transfer through the gas phase becomes crucial.

To reduce the thermal conductivity of the solid phase, it is necessary to select a solid phase with low thermal conductivity and to minimize the contact area between solid particles. For example, the more complex the crystal structure, the more phonons are scattered, and the lower the thermal conductivity. Polycrystalline structures have relatively poor structural integrity and regularity, and coupled with the effects of grain boundary impurities and distortion, their thermal conductivity is lower than that of single crystals. Furthermore, pores can scatter phonons. Generally, increasing porosity can reduce thermal conductivity. Additionally, using spherical microparticles, so that heat is transferred only through the tiny contact area between particles, can also significantly reduce the thermal conductivity of the solid.

Therefore, the ways to improve the thermal insulation capacity of insulating bricks are: ① increasing porosity; ② reducing pore size; ③ increasing the complexity of the microstructure at each level; ④ reducing solid-phase contact; ⑤ adding light-blocking substances to hinder radiative heat transfer.


Methods for Manufacturing Lightweight Insulating Bricks

1. Add lightweight aggregate, coat the lightweight aggregate with a matrix, and then use cement bonding, chemical bonding, or ceramic bonding through sintering to create a porous material;

2. Add combustible material, burn it off at high temperature, leaving pores in situ to form a porous material;

3. Create a foam slurry, solidify it with cement, and then sinter it to obtain a porous material.

Below are detailed examples of using these processes to manufacture insulating refractory materials, so that readers can understand the application and construction techniques of insulating refractory materials from a performance perspective.

Thermal Insulation Bricks Made from Hollow Alumina Spheres

Alumina hollow sphere bricks with anti-spasting properties were prepared using hollow alumina spheres, fused alumina powder, α-Al₂O₃ micro powder, SiO₂ micro powder, sillimanite, and clay powder as raw materials. The raw material composition is shown in Table 1.

1.png As shown in Table 1, hollow alumina spheres are lightweight aggregate, ensuring high-temperature resistance and reducing density; alumina powder is an inert filler; sillimanite is an expanding agent, reducing firing shrinkage and improving thermal shock resistance; alumina micro powder, silica micro powder, and clay powder are binders, lowering the sintering temperature, forming a mullite bonding phase, and improving the high-temperature resistance and thermal shock resistance of the thermal insulation bricks.

The brick-making materials were prepared according to the set batching scheme. After mixing, molding, and natural drying for 24 hours, the bricks were dried at 110℃ for 24 hours and then fired at 1600℃ for 4 hours to obtain the thermal insulation bricks. When the sillimanite content is 12%, the thermal insulation brick produced has the following properties: bulk density 1.48 g/cm³, compressive strength 26 MPa, load softening temperature 1710℃, linear change after reheating at 1600℃ for 2 hours +0.3%, and thermal shock resistance (1100℃ - water cooling) 17 cycles. The produced thermal insulation brick is lightweight, high-strength, high-temperature resistant, and thermally shock resistant.


Insulating Bricks Made with Lightweight Aggregates and Combustible Materials

Lightweight thermal insulation bricks were prepared by extrusion molding using polystyrene foam balls, expanded perlite powder, mullite fiber, clay, and silica sol as raw materials. The raw material composition is shown in Table 2.


Table 2 shows that expanded perlite is a lightweight raw material, further reducing bulk density and improving thermal insulation performance; mullite fiber acts as a reinforcing agent, preventing shrinkage and cracking during drying and sintering; clay is a binder, forming the matrix.


2.png The formula used is as follows: 15%–30% binding clay, 3%–10% mullite fiber, 15%–28% expanded perlite, 1.5%–3.5% polystyrene waste foam balls, and 15%–25% silica sol. The manufacturing process involves uniformly mixing the binding clay, expanded perlite powder, and polystyrene waste foam balls. Simultaneously, the mullite fiber is thoroughly and evenly dispersed in the binder using a mixer at a certain speed, and water is added to form a slurry. Then, the powder and slurry are mixed and stirred until the mixture is dispersed into a uniform viscous substance. After standing for a certain period, it is extruded and molded. After drying at room temperature for two days or at 70℃ for several hours, it is sintered to obtain the finished product.

III. Production of Anorthite Insulating Bricks Using Traditional Foaming Method

Anorthite-bonded mullite lightweight insulating refractory bricks were developed using kyanite as the main raw material and white cement as the binder. The composition of the raw materials is shown in Table 3. Anorthite-mullite composite insulation material is a composite material with mullite as the main crystalline phase and anorthite as the bonding phase. Anorthite has a melting point of only 1550℃, but it possesses characteristics such as a low coefficient of thermal expansion and low thermal conductivity. Therefore, the heat-insulating and refractory material made from the composite of anorthite and mullite can combine the advantages of anorthite's low thermal conductivity with the high service temperature of mullite products, while also exhibiting a lower sintering temperature, higher strength, and better thermal shock stability.

3.png Anorthite-mullite composite insulation material is formed using a foaming method. First, a foaming agent is prepared by synthesizing rosin soap from rosin and sodium hydroxide, and then adding bone glue to create a stable foam. Second, water is added to kyanite and white cement powder to form a slurry. Next, the foam is added, stirred, and then poured into molds. Then, it is cured, dried, and fired to obtain a green body. Finally, after cutting, fine grinding, and inspection, the finished product is obtained. In the foaming process, the foaming agent is the most important factor.

As shown in Table 4, mullite-calcium feldspar insulating bricks exhibit strong high-temperature resistance and excellent thermal insulation performance, making them a high-quality insulating and refractory material.

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