Gypsum powder works by undergoing a chemical reaction with water, transforming from a dry powder into a solid, rigid material.
Understanding Gypsum and Its Transformation
Natural gypsum is a mineral composed of calcium sulfate dihydrate (CaSO₄·2H₂O). This means each calcium sulfate molecule is bound to two molecules of water. In this natural state, gypsum is relatively soft.
To create the gypsum powder commonly used in construction and other applications (often referred to as plaster of Paris when it's the hemihydrate form), natural gypsum rock is processed, primarily through heating.
The Heating Process
As mentioned in the reference, when natural gypsum is heated in air, it loses its water molecules:
- Partial Dehydration: When heated to temperatures around 150–170°C (300–340°F), gypsum loses about three-quarters of its water content. It converts into calcium sulfate hemihydrate (CaSO₄·½H₂O). This material is often called bassanite or simply plaster. This is the most common form of gypsum powder used for molding, casting, and plastering.
- Complete Dehydration: If heated further to higher temperatures (above 170°C), it loses almost all remaining water, converting into anhydrous calcium sulfate (CaSO₄). This form is called anhydrite. Anhydrite can also be used as a desiccant or binder, depending on its form and processing, but the hemihydrate is the primary "plaster" form that sets quickly with water.
Here's a simple overview of the forms:
| Form | Chemical Formula | Water Content | Common Name(s) | Properties |
|---|---|---|---|---|
| Natural Gypsum | CaSO₄·2H₂O | Dihydrate | Gypsum Rock | Soft mineral |
| Partially Dehydrated Gypsum | CaSO₄·½H₂O | Hemihydrate | Plaster (of Paris), Bassanite | Powder; Reacts with water to set |
| Completely Dehydrated Gypsum | CaSO₄ | Anhydrous | Anhydrite | Can be a binder or desiccant |
The Setting Mechanism: How the Powder Works
The magic of gypsum powder (specifically the calcium sulfate hemihydrate form, plaster) lies in its ability to reverse the dehydration process when mixed with water.
- Rehydration: When calcium sulfate hemihydrate powder is mixed with water, it readily absorbs the water molecules. The hemihydrate structure is unstable in the presence of sufficient water.
- Recrystallization: The absorbed water molecules allow the calcium sulfate to reform its original dihydrate structure (CaSO₄·2H₂O). This process is not just simple absorption; the dissolved calcium sulfate and water molecules rearrange and crystallize, growing tiny interlocking crystals of calcium sulfate dihydrate.
- Hardening: As these new gypsum crystals grow and interlock, they form a dense, rigid matrix. This crystallization process is what causes the mixture to solidify and harden. The excess water that doesn't become part of the crystal structure eventually evaporates, leaving behind the solid gypsum mass.
Think of it like assembling a 3D puzzle where the water acts as the medium allowing the puzzle pieces (calcium sulfate and water molecules) to find each other and lock into place, forming a solid structure.
Practical Applications
This rehydration and setting property makes gypsum powder incredibly useful:
- Plastering: Used to create smooth finishes on walls and ceilings.
- Drywall (Gypsum Board): The core of drywall panels is made from set gypsum plaster sandwiched between paper or fiberglass mats.
- Casting and Molding: Used to create casts for medical purposes (like broken bones) or decorative items due to its ability to capture fine detail.
- Art and Sculpture: Plaster of Paris is a common medium for sculpting and creating models.
In essence, gypsum powder works by harnessing the reversible chemical process of hydration and dehydration, allowing it to be transformed from a pliable mixture back into a hard, rock-like state by simply adding water.
