Gypsum
On this page 4 sections
Classification
| Type | Name | Main application |
|---|---|---|
| Type I | Impression plaster | Impressions |
| Type II | Model plaster | Study models, mounting |
| Type III | Dental stone | Working casts |
| Type IV | High-strength, low-expansion stone | Dies |
| Type V | High-strength, high-expansion stone | Dies for high-shrinkage alloys |
Setting reaction
Calcium sulphate hemihydrate reacts with water to form calcium sulphate dihydrate.
Important points:
- The reaction is exothermic.
- Dihydrate crystals form and interlock.
- Setting expansion occurs due to outward thrust of growing crystals.
Factors affecting setting time
| Factor | Effect |
|---|---|
| Increased water/powder ratio | Increases setting time |
| Increased spatulation | Decreases setting time |
| Warm water (within normal working range) | Generally accelerates setting |
| Potassium sulphate | Accelerator |
| Borax | Retarder |
| Sodium chloride | Concentration-dependent effect |
Borax is a retarder of gypsum setting reaction.
Increased powder-to-liquid ratio
What happens when increased powder is added to liquid followed by rapid mixing.
- More nuclei of crystallisation are available.
- Setting generally becomes faster.
- Increased mixing also accelerates setting.
Remember: The water/powder ratio and manipulation affect the final strength and setting expansion.