Siliceous Materials: The primary siliceous material is diatomite, which contains amorphous SiO₂ and reacts easily with lime in water below 100°C to form a colloid. Other alternatives include bentonite, fly ash, quartz sand, silica fume, and siliceous shale powder.
Calcareous Materials: Lime is the main calcareous material, preferably with an effective CaO content of at least 85%. Fresh lime is hydrated to form lime slurry, with CaO particles smaller than 1μm, ensuring good suspension and easy adhesion to SiO₂ particles.
Reinforcing Fibers: While asbestos was once a common reinforcing fiber, its use is now banned internationally due to health risks. Alternatives include alkali-free glass fibers, alkali-resistant mineral wool fibers, and brucite fibers, which are unique to China and harmless to humans. Organic fibers such as pulp, hemp, and polypropylene are also used as substitutes.
Additives: To promote setting, enhance strength, and reduce density, additives like sodium silicate, calcium silicate powder, SiO₂ ultrafine powder, and polyacrylamide are used. Sodium silicate acts as a setting accelerator, while calcium silicate powder and SiO₂ ultrafine powder serve as nucleating agents. Polyacrylamide not only accelerates setting but also forms a network structure by linking dispersed phases, enhancing overall strength.
The production process involves weighing the raw materials according to the formulation and mixing them with water in a mixer. The amorphous SiO₂ in diatomite reacts with hydrated lime below 100°C to form a gel, creating particle clusters with CaO adsorbed on the SiO₂ surface.
These clusters undergo hydrothermal synthesis in an autoclave under saturated steam pressure and temperature. When the CaO/SiO₂ ratio is around 0.8, calcium-rich CaO·SiO₂·H₂O forms initially, followed by low-crystallinity Tobermorite (5CaO·6SiO₂·5H₂O), and finally, a highly crystalline Tobermorite framework with over 90% porosity. After drying to remove moisture, the material becomes calcium silicate insulation, which can be shaped into various products. When the CaO/SiO₂ ratio is around 1.0, Xonotlite (6CaO·6SiO₂·H₂O) is formed. Calcium silicate boards can be cut, nailed, and shaped into plates, blocks, or pipes.
In China, calcium silicate boards are available in three bulk densities: 240, 220, and 170 kg/m³, with international standards typically not exceeding 220 kg/m³. The compressive strength is above 0.4 MPa, and the flexural strength exceeds 0.2 MPa. The thermal conductivity at 70±5°C ranges from 0.049 to 0.064 W/(m·K).
If you’re interested in learning more about the applications of calcium silicate boards, feel free to check out this article ” What is a Calcium Silicate Board Used For? “.
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