Anhydrite is a naturally occurring anhydrous calcium sulfate mineral with the chemical formula CaSO₄. It is closely related to gypsum but lacks water of crystallization, making it harder, denser, and more thermally stable.
Gypsum (CaSO₄·2H₂O) ⇄ dehydration / hydration ⇄ Anhydrite (CaSO₄)
Anhydrite typically forms in evaporite basins beneath gypsum layers and may rehydrate to gypsum when exposed to moisture near the surface.
Chemical Composition
| Component | Typical Content |
|---|---|
| Calcium Sulfate (CaSO₄) | 95 – 99% |
| Calcium Oxide (CaO equivalent) | ~41% |
| Sulfur Trioxide (SO₃ equivalent) | ~59% |
| Silica (SiO₂) | <2% |
| Alumina (Al₂O₃) | <1% |
| Iron Oxides (Fe₂O₃) | <1% |
| Magnesium & Alkalis | Trace |
✔ Very high chemical purity
✔ No structural water
Key Physical Properties
| Property | Typical Value |
|---|---|
| Crystal System | Orthorhombic |
| Density | 2.9 – 3.0 g/cm³ |
| Color | White, grey, bluish, violet |
| Luster | Vitreous to pearly |
| Mohs Hardness | 3 – 3.5 |
| Cleavage | Perfect in three directions |
| Porosity | Very low |
| Water Solubility | Slightly soluble |
| Thermal Stability | High |
| Hydration Behavior | Slow → gypsum |
✔ Significantly harder than gypsum
✔ Stable at elevated temperatures
Key Mechanical Properties
| Property | Typical Range |
|---|---|
| Compressive Strength | 80 – 200 MPa |
| Tensile Strength | 8 – 25 MPa |
| Flexural Strength | Moderate |
| Abrasion Resistance | Moderate |
| Impact Resistance | Moderate |
| Elastic Modulus | 30 – 50 GPa |
Strengthening & Metallurgical Behavior
Strengthening Behavior:
Naturally stronger due to absence of crystal water and a dense orthorhombic lattice.
Strength can be enhanced by:
• Controlled grinding
• Binder activation in cementitious blends
Metallurgical Behavior:
⚠ Limited metallurgical use
• Sulfur content undesirable in steelmaking
• Occasionally used for slag conditioning or controlled SO₃ addition
🚫 Not used as a flux
Key Characteristics
✔ Anhydrous and dense
✔ Higher hardness than gypsum
✔ Low water absorption
✔ Excellent dimensional stability
✔ Slow-setting when hydrated
✔ Resistant to moderate heat
Refining & Processing Properties
Mining:
• Underground or open-cast mining
• Harder than gypsum — blasting often required
• Selective mining to avoid hydration zones
Processing Steps:
• Crushing
• Grinding
• Classification
• Optional activation with grinding aids
• Blending
Thermal Processing:
• No calcination required
• Stable up to ~1000 °C
• Converts to CaO + SO₃ only at very high temperatures
Available Forms
• Natural anhydrite rock
• Crushed anhydrite
• Ground anhydrite powder
• Activated anhydrite
• Synthetic anhydrite (chemical by-product)
• Anhydrite-based binders
Applications
Construction & Cement:
• Cement retarder (alternative to gypsum)
• Self-leveling floor screeds
• Anhydrite binders
• Shrinkage control agent
Chemical & Industrial:
• Sulfuric acid production (indirect)
• Desiccant (special grades)
• Chemical feedstock
• Fillers in plastics and rubber
Energy & Environment:
• FGD by-product utilization
• Waste stabilization
• Thermal storage media (experimental)
Advantages of Anhydrite
✔ Higher strength than gypsum
✔ Better moisture resistance
✔ Lower volume change
✔ Stable at higher temperatures
✔ Long shelf life
✔ Reduced risk of premature setting
Anhydrite vs Gypsum
| Feature | Anhydrite | Gypsum |
|---|---|---|
| Water Content | None | 20.9% |
| Hardness | Higher | Lower |
| Strength | Higher | Lower |
| Moisture Sensitivity | Low | High |
| Calcination | Not required | Required |