What Is Xonotlite Calcium Silicate?
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.
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.