Table of Contents
Definition
Gypsum products are materials made from a natural mineral called gypsum, which is chemically calcium sulfate dihydrate (CaSO₄·2H₂O). They are used in dentistry for models, casts, and molds because they are easy to manipulate and reproduce fine details.
"Gypsum — calcium sulfate dihydrate (CaSO₄·2H₂O) — is a mineral mined in various parts of the world, but it is also produced as a by-product of flue-gas desulfurisation in coal-fired power plants."
"The dihydrate form of calcium sulfate, called gypsum, usually appears white to milky-yellowish and is found in a compact mass in nature."
Simple Explanation:
Gypsum is a rock with two water molecules inside its crystal. Heating drives the water out to make hemihydrate powder; adding water reconverts it to a solid dihydrate cast.
Origin & Historical Note
Gypsum is naturally mined and also produced industrially. The name “Plaster of Paris” originated from deposits near Paris, France.
| Era | Application | Significance |
|---|---|---|
| Ancient Egypt | Pyramid construction with lime | First recorded use |
| Biblical Times | Alabaster in King Solomon's temple | Decorative applications |
| 18th Century | “Plaster of Paris” mining near Paris | Commercial development |
| Modern Era | Industrial byproduct utilization | Environmental sustainability |
Chemical Composition
Gypsum is known chemically as calcium sulfate dihydrate and consists of the following components:
- Calcium (Ca²⁺): provides strength and stability.
- Sulfate (SO₄²⁻): forms the mineral framework.
- Water of crystallization (2H₂O): gives reversible setting properties.
Chemical Formula: CaSO₄·2H₂O
Types of Calcium Sulfate
1. Calcium Sulfate Dihydrate (Natural Gypsum)
Natural, soft, white to yellowish rock (Mohs hardness ~2). Slightly soluble in water, essential in dental cast production.
2. Calcium Sulfate Hemihydrate (Plaster of Paris)
Formed by partial dehydration of gypsum. Two major forms — α-hemihydrate and β-hemihydrate — differ physically.
| Feature | α-Hemihydrate (Alpha) | β-Hemihydrate (Beta) |
|---|---|---|
| Formation | Calcined under steam pressure | Calcined under dry heat |
| Crystal shape | Dense, prismatic, rod-like | Porous, irregular, spongy |
| Particle size | Larger, more regular | Smaller, porous |
| Solubility | Lower | Higher |
| Water requirement | Less water needed to make workable mix | More water required |
| Strength of set product | Higher (used for dental stone) | Lower (used for model plaster) |
3. Calcium Sulfate Anhydrite
Contains no crystallization water, less soluble, and used mainly in industrial applications like cement and drywall.
| Type | Chemical Formula | Method of Production | Crystal Shape | Water Requirement | Dental Product Type |
|---|---|---|---|---|---|
| Calcium sulfate dihydrate (gypsum) | CaSO₄·2H₂O | Naturally occurring | Compact mass | N/A | Natural gypsum mineral |
| β-Hemihydrate (beta) | CaSO₄·½H₂O | Dry calcination (open kettle at 110–130°C) | Irregular, spongy | High | Plaster of Paris, Type II plaster |
| α-Hemihydrate (alpha) | CaSO₄·½H₂O | Steam pressure autoclave (125°C, 17 psi) | Dense, prismatic | Low | Dental stone, Type III |
| Modified α-Hemihydrate | CaSO₄·½H₂O | Chemical calcination (CaCl₂ or sodium succinate solution) | Cuboidal, dense | Lowest | Die stones, Type IV & V |
| Anhydrite | CaSO₄ | High temperature (>130°C) | Hexagonal or orthorhombic | None | Industrial application |