The primary chemical formula associated with gypsum plaster, as derived from its source material and set form, is CaSO4 · 2H2O. This represents the compound known as Gypsum, which is also referred to as Hydrous Calcium Sulphate.
Understanding Gypsum and Gypsum Plaster
Gypsum plaster is a building material made from gypsum rock. According to the provided information, gypsum is chemically known as Hydrous Calcium Sulphate and has the formula CaSO4 · 2H2O. This means one molecule of calcium sulfate is combined with two molecules of water.
The process of making gypsum plaster involves heating gypsum (CaSO4 · 2H2O) to remove some of its water content. This heated form is typically calcium sulfate hemihydrate (CaSO4 · ½H2O), commonly known as Plaster of Paris. When water is added to Plaster of Paris, it rehydrates and hardens, returning to its original gypsum state (CaSO4 · 2H2O).
So, while the dry powder used to make plaster is calcium sulfate hemihydrate (CaSO4 · ½H2O), the formula CaSO4 · 2H2O is crucial because it represents:
- The raw material mined from the earth (Gypsum).
- The chemical form of the plaster after it has been mixed with water and set (rehydrated Gypsum).
Chemical Formulas Involved
Let's look at the different states:
| Material State | Chemical Name | Formula | Description |
|---|---|---|---|
| Raw Material | Gypsum (Hydrous Calcium Sulphate) | CaSO4 · 2H2O | Naturally occurring mineral |
| Dry Powder | Plaster of Paris (Calcium Sulfate Hemihydrate) | CaSO4 · ½H2O | Produced by heating gypsum, used for plaster |
| Set Plaster | Rehydrated Gypsum (Hydrous Calcium Sulphate) | CaSO4 · 2H2O | Formed when Plaster of Paris mixes with water |
Based on the reference provided, which specifically states the formula for Gypsum is CaSO4 · 2H2O, this is the foundational chemical formula directly linked to gypsum plaster.
Practical Insight
When you see a bag of "gypsum plaster" in a store, it most likely contains the calcium sulfate hemihydrate (CaSO4 · ½H2O) powder. You mix this powder with water, which triggers a chemical reaction where the hemihydrate absorbs water molecules and reforms the dihydrate structure, setting into a hard, solid mass which is chemically CaSO4 · 2H2O.
Therefore, while the dry product is CaSO4 · ½H2O, the formula CaSO4 · 2H2O defines the natural mineral it comes from and the final hardened state it returns to after use.
