All FAQ

What Is Calcium Silicate? — Definition, Properties & Industrial Applications

Calcium silicate is a rigid, asbestos-free thermal insulation material produced from lime (CaO) and silica (SiO₂) reacted under saturated steam in an autoclave. The resulting board is composed of interlocking needle-like xonotlite crystals with over 90% air porosity by volume. It is one of the most widely specified high-temperature industrial insulation materials, used in applications from cement kilns to fire-rated doors.

What Is Calcium Silicate?

Calcium silicate insulation is a pre-formed rigid board or pipe section manufactured through a hydrothermal reaction between lime and silica raw materials. The process takes place in a high-pressure autoclave at approximately 190-220°C and 12-18 bar, where raw materials dissolve and recrystallize as interlocking xonotlite crystals (6CaO·6SiO&sub2;·H&sub2;O).

The key distinction of calcium silicate is its combination of rigidity, compressive strength, and high-temperature stability. Unlike fibrous insulation such as mineral wool or ceramic fiber, calcium silicate boards are load-bearing — they support their own weight and structural loads without deformation, suitable as backup insulation behind refractory linings and as core material in fire-rated doors.

During autoclave curing, xonotlite crystals grow as fine needles that interlock to form a rigid, porous matrix. With over 90% of the board volume being air trapped within sub-micron pores, the material insulates through the low thermal conductivity of still air. The calcium silicate crystal framework provides mechanical strength and thermal stability.

Calcium silicate is classified under ASTM C533, which defines Type I (general purpose, to 650°C) and Type II (high-temperature, to 1000°C). For applications above 650°C, only xonotlite-based products should be specified — tobermorite-based calcium silicate degrades and shrinks at higher temperatures. Learn more about crystal phases in the article on what is xonotlite.

Chemical Composition & How It's Made

The fundamental chemistry of calcium silicate insulation is the reaction between calcium oxide and silicon dioxide in the presence of water under elevated temperature and pressure:

The industrial manufacturing process proceeds through four stages:

1. Raw Material Preparation — Quicklime (CaO) is slaked with water to form calcium hydroxide slurry. This is mixed with finely ground silica (SiO&sub2;, typically quartz flour) at a precisely controlled CaO/SiO&sub2; molar ratio between 0.85 and 1.00. Small amounts of reinforcing fibers (cellulose or alkali-resistant glass fiber) are added to improve green strength during forming.
2. Slurry Mixing and Pre-Reaction — The slurry is mixed under high shear to ensure complete dispersion. A partial pre-reaction occurs at 80-95°C, forming a tobermorite gel precursor (C-S-H gel) that gives the slurry sufficient body for casting or filter-pressing.
3. Autoclave Curing — The formed boards or pipe sections are loaded into a high-pressure autoclave. Under saturated steam at 190-220°C and 12-18 bar for 8-16 hours (depending on product thickness), the tobermorite gel dissolves and recrystallizes as interlocking xonotlite needles. This is the critical phase transformation that determines the product's ultimate temperature rating.
4. Drying, Sizing, and Quality Control — After autoclave curing, boards are dried in multi-zone ovens at 105-150°C to remove free water. Boards are then precision-cut to final dimensions (tolerances typically ±2 mm on thickness) and undergo QC testing including density measurement, compressive strength testing, thermal conductivity verification, and XRD analysis to confirm the xonotlite crystal phase.

The entire process is asbestos-free — no asbestos is used at any stage of production. Mingfa's manufacturing facility in Laizhou, Shandong operates a 108,000 m² production plant with dedicated autoclave capacity, serving customers across 70+ countries. For details of the manufacturing facility, see the About Mingfa page.

Key Properties

Calcium silicate insulation is characterized by a specific set of physical and thermal properties that define its suitability for industrial applications. The table below summarizes the key parameters across standard and high-temperature grades:

Several properties merit further explanation:

Thermal Conductivity

Thermal conductivity of calcium silicate increases with both density and mean temperature. At ambient, values are typically 0.050-0.055 W/m·K for standard-density boards. At 400°C mean temperature, values reach approximately 0.10-0.13 W/m·K. This temperature dependence is approximately linear. For engineering calculations, thermal conductivity should be evaluated at the mean temperature between hot face and cold face, not at ambient.

Density and Mechanical Strength

Calcium silicate is available from approximately 170 kg/m³ (ultra-light insulating grade) to 900 kg/m³ (structural grade). Compressive strength scales with density: a 200 kg/m³ board achieves 2-3 MPa, a 450 kg/m³ board achieves 8-12 MPa, and a 900 kg/m³ board can exceed 20 MPa. This makes calcium silicate suitable for everything from simple pipe insulation to refractory backup linings in industrial furnaces.

Fire Performance

Calcium silicate is classified as A1 non-combustible under EN 13501-1, the highest fire classification available. It does not contribute to fire in any stage, produces no smoke, and generates no flaming droplets. This suits fire-rated construction including fire door cores, structural steel fire protection, and compartmentation barriers.

Common Industrial Applications

Calcium silicate insulation serves a broad range of high-temperature industrial applications. The material's combination of thermal performance, compressive strength, and non-combustibility makes it the preferred choice in the following sectors:

• Cement Kilns — Calcium silicate boards serve as backup insulation behind refractory brick linings in rotary cement kilns, reducing shell temperature by 40-80°C and lowering fuel consumption. Pre-heater cyclones, tertiary air ducts, and cooler sections also use calcium silicate. Typical specification: 50-100 mm thick, 200-250 kg/m³ density, Type II grade for hot zones.
• Steel Ladles and Tundishes — Calcium silicate boards provide permanent backup insulation behind working refractory linings, maintaining steel temperature during ladle transfer and casting. Specified at 40-75 mm thickness depending on ladle capacity and hold time requirements.
• Aluminum Reduction Cells — Sidewalls and covers of Hall-Heroult reduction cells are insulated with calcium silicate boards. Operating at 200-350°C on the cold face, the insulation must resist fluoride attack and maintain dimensional stability. Densities of 350-450 kg/m³ are commonly specified.
• Glass Furnaces — Container and float glass furnaces use calcium silicate as crown insulation, sidewall backup, and bottom paving, reducing heat loss through the furnace shell. Typical thicknesses range from 50 mm (crown) to 100 mm (bottom paving). See more at calcium silicate insulation engineering guide.
• Fire Doors — Calcium silicate board is the core material in fire-rated door assemblies achieving 60-, 90-, and 120-minute fire resistance ratings. The A1 classification, dimensional stability, and low thermal conductivity make it an ideal fire door core. Typical densities: 350-450 kg/m³.
• Pipe and Equipment Insulation — Pre-formed calcium silicate pipe sections are used for steam lines, hot oil pipes, process piping, and flue gas ducts in power plants, refineries, and chemical facilities. Sizes range from 21 mm to 610 mm nominal pipe diameter with insulation thickness from 25 mm to 150 mm.

For a full overview of Mingfa's product range across these application sectors, visit the Products page.

Calcium Silicate vs Other Insulation Materials

Engineers specifying industrial insulation must evaluate calcium silicate against several alternative materials. The comparison below covers the three most common alternatives for high-temperature applications.

The choice between calcium silicate and alternative materials depends on the specific application requirements:

• Choose calcium silicate when compressive strength, rigidity, or structural support is required — backup insulation behind refractory linings, fire door cores.
• Choose ceramic fiber when the application exceeds 1100°C or requires very low thermal mass and rapid thermal cycling.
• Choose mineral wool when lower cost is the priority, the application is below 650°C, and structural load-bearing is not needed.
• Choose fiberglass for applications below 450°C where lowest installed cost per square meter is the selection criterion.

In many industrial furnace designs, calcium silicate is used as the backup or cold-face layer behind a ceramic fiber hot-face layer, combining the structural rigidity of calcium silicate with the higher temperature tolerance of ceramic fiber.

Where to Buy Calcium Silicate Insulation

Calcium silicate insulation is manufactured globally, with the largest concentration of production capacity in Shandong Province, China. Mingfa Insulation has been manufacturing calcium silicate insulation in Laizhou, Shandong since 1991 — over three decades of specialized production experience.

When evaluating suppliers, procurement engineers should confirm:

• ISO 9001 certification — Mingfa holds ISO 9001 certification with a documented quality management system.
• Product certifications — ASTM C533 compliance, EN 13501-1 A1 fire classification, and regional standards relevant to the destination market.
• Crystal phase verification — For applications above 650°C, confirm the product is xonotlite-based through XRD analysis. Request test certificates.
• Manufacturing scale — Mingfa's 108,000 m² facility in Shandong has dedicated autoclave production lines ensuring reliable supply.
• Export experience — Products exported to 70+ countries, with familiarity with international shipping documentation and destination-country regulations.

Mingfa supplies calcium silicate boards, pipe sections, and custom-cut shapes in densities from 170 to 900 kg/m³ and thicknesses from 20 mm to 150 mm. For technical specifications, pricing inquiries, or to request samples, contact our team through the Contact page. For background on the company and manufacturing capabilities, see About Mingfa.

Frequently Asked Questions