Gypsum Composition, Hydration, and Metal Interaction in Construction

Commercial Gypsum Composition and Purity

The commercial gypsum composition is formed by a mixture of Hemihydrate (HH) and some anhydrous Dihydrate (DH) nuclei, besides the impurities brought from the quarry.

Chemical Composition and Proportions

The typical proportions are:

• Hemihydrate (HH): 20% to 70%
• Anhydrite: 80% to 90%

Sources of Gypsum Impurities

Once obtained, it is necessary to distinguish two types of impurities:

Raw Material Impurities

These come from the source materials. If the gypsum stone contains impurities, these impurities will transfer to the final product.

Manufacturing Process Defects

These are impurities or defects produced during the manufacturing process, such as under-calcined (incocidos) and over-calcined (overcooked) material.

Gypsum Hydration Mechanisms

Two primary forms of hydration can occur:

Supersaturated Solution Hydration

This process involves creating a supersaturated solution, culminating in the precipitation of Dihydrate (DH). Since DH has low solubility, it will solubilize other components, leading to a supersaturated DH solution.

Topochemical Hydrate Crystallization

DH hydrate crystallization is produced topochemically without prior dissolution of anhydrous phases. The material absorbs water without dissolving and moisturizes the DH, crystallizing on the grain.

Effects of Plaster on Common Metals

The contact of plaster (cast) with various metals affects corrosion resistance:

Interaction with Ferrous Metals (Iron/Steel)

Contact with cast iron or steel hinders the formation of insoluble products that typically work as a barrier against subsequent corrosion reactions. Moreover, the plaster prevents corrosion reactions when the material is in contact with the cast.

Interaction with Zinc

The effect is similar to that observed with ferrous metals.

Interaction with Lead

The plaster forms a very insoluble lead sulfate, creating a thin protective layer on the material.

Interaction with Aluminum

Aluminum, commonly used in metallic carpentry, is protected against corrosion by a layer of aluminum oxide. The plaster contact is generally stable.

Interaction with Copper

Copper has little tendency to corrode unless there are special circumstances. It has been proven that contact with plaster is quite stable. Note that water must be present for corrosion to occur.

Classification and Applications of Plaster

Standard Gypsum Types

The main types of construction gypsum are:

• Gypsum YG (Thick Construction Plaster): Used in construction, based on Calcium Sulfate Anhydrite II.
• Gypsum YF (Fine Construction Plaster): Similar to YG, but with a finer grain size.
• Gypsum YP (Prefabricated Plaster): Characterized by greater purity and strength for construction.
• Plaster E-30 (Regulatory Setting Additives): Incorporates regulatory setting additives.
• Plaster E-35 (Special Plaster): Higher purity than the other types.

Specific Applications