Rock gypsum is a naturally occurring evaporite mineral composed primarily of calcium sulfate dihydrate (CaSO₄·2H₂O). Unlike decorative gypsum varieties, rock gypsum occurs as massive, bedded, granular, or nodular deposits and is the primary industrial raw material for plaster, cement retarders, gypsum boards, and soil conditioners.
Chemical Composition
| Component | Typical Range |
|---|---|
| Calcium Sulfate Dihydrate (CaSO₄·2H₂O) | 70 – 95% |
| Water of Crystallization | ~20.9% |
| Anhydrite (CaSO₄) | 0 – 15% |
| Silica (SiO₂) | 1 – 10% |
| Alumina (Al₂O₃) | 0.5 – 5% |
| Iron Oxides (Fe₂O₃) | 0.2 – 3% |
| Carbonates & Clay | Variable |
✔ Purity varies widely by deposit
✔ Industrial grading depends on CaSO₄·2H₂O content
Key Physical Properties
| Property | Typical Value |
|---|---|
| Crystal System | Monoclinic |
| Appearance | White, grey, buff, brown |
| Density | 2.2 – 2.4 g/cm³ |
| Mohs Hardness | ~2 |
| Texture | Massive, granular, nodular |
| Porosity | Low – Moderate |
| Solubility | Slightly soluble in water |
| Dehydration Temperature | 100 – 150 °C |
| Thermal Stability | Low – Moderate |
📌 Easily scratched
📌 Moisture-sensitive
Key Mechanical Properties
| Property | Typical Range |
|---|---|
| Compressive Strength | 15 – 50 MPa |
| Tensile Strength | 2 – 7 MPa |
| Flexural Strength | Low |
| Abrasion Resistance | Poor |
| Impact Resistance | Very poor |
| Elastic Modulus | 5 – 15 GPa |
⚠ Not load-bearing
⚠ Strength improves after processing into products
Strengthening & Metallurgical Behavior
Strengthening Behavior:
No metallurgical strengthening mechanisms.
Strength is enhanced through:
• Calcination (plaster formation)
• Controlled hydration
• Fiber or polymer reinforcement in gypsum boards
Metallurgical Behavior:
🚫 Not suitable as a flux or metal additive
• Sulfur content is undesirable in metallurgy
• Decomposes before metallurgical temperatures
Key Characteristics
✔ Soft and workable
✔ Consistent chemical behavior
✔ Low calcination temperature
✔ Rapid setting after hydration
✔ Fire resistance in boards
✔ Good acoustic and thermal insulation
Refining & Processing Properties
Mining:
• Open-cast or underground mining
• Ripping or light blasting
• Selective mining for purity control
Processing Steps:
• Crushing
• Grinding
• Drying
• Calcination (100 – 180 °C)
• Classification and blending
Thermal Processing:
Produces:
• Plaster of Paris (CaSO₄·½H₂O)
• Anhydrite (CaSO₄)
Available Forms
• Run-of-mine rock
• Crushed gypsum
• Gypsum powder
• Calcined gypsum (plaster)
• Gypsum boards
• Gypsum granules (agriculture)
Applications
Construction & Building:
• Cement set retarder
• Gypsum plaster
• Drywall / gypsum boards
• Decorative mouldings
• False ceilings
Industrial & Chemical:
• Mold making
• Ceramic casting
• Paint filler
• Industrial plasters
Agriculture:
• Soil conditioner
• Calcium and sulfur source
• Salinity management
Advantages of Rock Gypsum
✔ Abundant and cost-effective
✔ Low energy processing
✔ Fire resistance
✔ Improves cement quality
✔ Enhances soil structure
✔ Environmentally safe
Rock Gypsum vs Anhydrite
| Feature | Rock Gypsum | Anhydrite |
|---|---|---|
| Water Content | Present | Absent |
| Processing Temperature | Low | Higher |
| Reactivity | High | Lower |
| Main Use | Plaster, cement | Chemical, cement |