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What Is Xonotlite Calcium Silicate?

Author's Note: This article draws on Mingfa's 34+ years of calcium silicate R&D and manufacturing experience in Laizhou, Shandong. Technical claims are validated against our in-house laboratory data and published industry standards. About our engineering team

What Is Calcium Silicate?

Calcium silicate is a rigid, lightweight thermal insulation material produced from lime (CaO) and silica (SiO&sub2;) as its primary raw materials. The manufacturing process combines these raw materials with water, cellulose or glass fibre reinforcement, and reactive siliceous additives into a slurry, which is then moulded and cured in high-pressure steam autoclaves at 190-220°C. Under these hydrothermal conditions, the lime and silica react to form xonotlite crystals (6CaO·6SiO&sub2;·H&sub2;O) -- a stable calcium silicate hydrate phase that gives the material its exceptional thermal and mechanical properties. The cured product is approximately 90% air by volume, trapped within an interlocking crystalline matrix, giving it a density range of 170 to 900 kg/m³ depending on the grade. Calcium silicate insulation is rated for continuous service from sub-zero temperatures to 650°C for standard grades and up to 1100°C for high-temperature (xonotlite-based) grades. It is classified as Euroclass A1 non-combustible under EN 13501-1, contains no organic binders after curing, and is fully recyclable. First developed in the 1940s and commercialised in the 1950s, calcium silicate has become one of the most widely specified industrial insulation materials worldwide for applications in power generation, petrochemical processing, marine, and building services.

Xonotlite is a calcium silicate hydrate mineral with the chemical formula 6CaO·6SiO&sub2;·H&sub2;O. It is the preferred crystal phase for high-temperature calcium silicate thermal insulation. All Mingfa insulation products are xonotlite-based.

Why Xonotlite Matters for Insulation

Not all calcium silicate hydrate phases are suitable for high-temperature use. The key difference between xonotlite and the more common tobermorite phase (5CaO·6SiO&sub2;·5H&sub2;O) is the amount of chemically bound water:

• Tobermorite: Contains approximately 11% bound water by weight. Above 650°C, this water is driven off, causing the crystal structure to collapse. The board shrinks significantly (2% or more) and loses mechanical integrity.
• Xonotlite: Contains only approximately 2.1% bound water by weight. This makes it thermally stable to approximately 1050°C. At 750-800°C, xonotlite undergoes a topotactic transformation to wollastonite (CaSiO&sub3;) with near-zero dimensional change.
• About Mingfa — manufacturing process — xonotlite crystal formation

The Topotactic Transformation

The term "topotactic" means the crystal transformation occurs without major structural rearrangement. When xonotlite transforms to wollastonite at 750-800°C, the calcium silicate framework remains intact. The board does not shrink, crack, or lose its insulating properties. This is the fundamental reason xonotlite-based insulation can serve reliably at 1000-1100°C. A tobermorite board, by contrast, would be structurally destroyed at these temperatures.

How Xonotlite Is Produced

Xonotlite occurs naturally as a rare hydrothermal mineral, found in metamorphic limestone contact zones. For industrial production, xonotlite is synthesized in autoclaves at 190-220°C and 12-18 bar saturated steam pressure. Under these conditions, a tobermorite gel precursor (formed at lower temperature) dissolves and recrystallizes as interlocking needle-like xonotlite crystals. The CaO/SiO&sub2; molar ratio must be precisely controlled between 0.85 and 1.00 -- deviations from this range produce tobermorite or unreacted silica instead of pure xonotlite.

Identifying Xonotlite Products

Not all manufacturers achieve pure xonotlite. Some lower-cost products are tobermorite-based and are only suitable to 650°C. Mingfa uses XRD (X-ray diffraction) and SEM (scanning electron microscopy) analysis to verify the crystal phase of every formulation. When specifying calcium silicate insulation for applications above 650°C, request XRD verification of the xonotlite phase from your supplier. Learn more about our manufacturing process on the About Mingfa page.

Why Xonotlite Matters for Industrial Insulation

The topotactic transformation from xonotlite to wollastonite at 750 to 800 degrees C is what makes calcium silicate insulation viable at high temperatures. Xonotlite-based insulation maintains its dimensions and insulating properties at service temperatures where tobermorite-based products would undergo significant shrinkage and structural degradation. This is why xonotlite is the preferred crystal phase for calcium silicate insulation rated above 650 degrees C, including Mingfa's LG-High Temperature series rated for continuous service at 1100 degrees C.

For Mingfa's refractory-grade calcium silicate products using xonotlite technology, see the high-temperature insulation panel specifications.