Calcium Sulfate Anhydrite (CaSO4)
Key Features
- Anhydrous form — no setting reaction, safe in moisture-containing environments
- Neutral pH — compatible with epoxy-amine systems without cure interference
- Good whiteness (≥90%) for light-colored composite applications
- Stable at elevated temperatures — contributes to fire resistance
- Cost-effective white bulk filler for UPR and epoxy casting compounds
Calcium sulfate anhydrite (CaSO₄) is the anhydrous form of calcium sulfate, distinct from gypsum (CaSO₄·2H₂O) and semi-hydrate hemihydrate (CaSO₄·0.5H₂O). Anhydrite is a naturally occurring mineral produced by the thermal dehydration of gypsum at temperatures above 400°C, yielding a stable, dimensionally consistent mineral filler with low moisture sensitivity.
As a filler in composite resin systems, anhydrite offers moderate density (2.96 g/cc), good whiteness, and a neutral pH that is compatible with most resin chemistry. Its particle shape is irregular but equidimensional, providing moderate mechanical reinforcement and good optical opacity in light-colored composite systems. The sulfate chemistry of anhydrite makes it particularly compatible with epoxy-amine systems, where it does not cause the pH-related cure issues sometimes seen with carbonate or hydroxide fillers.
Calcium sulfate anhydrite finds use in filled UPR and epoxy casting compounds, grout and anchoring materials, and as a cost-effective white extender in composite primers and filler compounds. One key advantage over hemihydrate (plaster of Paris) is the absence of setting reactions — anhydrite does not hydrate and expand when contacted with moisture, making it safe for use in moisture-containing environments. It also provides better fire resistance than organic fillers due to its inorganic nature and stability at elevated temperatures.
Specifications
| Parameter | Value |
|---|---|
| Appearance | White powder |
| True density | 2.96 g/cm³ |
| CaSO₄ content | ≥ 95% |
| pH (10% slurry) | 7.0–8.5 |
| Chemical formula | CaSO₄ (anhydrous) |
| Moisture content | ≤ 0.3% |
| Whiteness (R457) | ≥ 90% |
| Bulk density (tapped) | 0.8–1.2 g/cm³ |
| Median particle size (d50) | 5–15 µm |
Applications
FAQ
Gypsum (CaSO₄·2H₂O) contains ~21% water of crystallization which is released at 120–180°C, making it unsuitable for hot-process composites or high-temperature applications. Anhydrite (CaSO₄) is the dehydrated form with no bound water, stable to >1000°C. In thermoset composites, anhydrite is preferred as it avoids the risk of water release causing bubbles or voids during cure exotherm. Gypsum's setting reaction in the presence of moisture is also a processing concern that anhydrite avoids.
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