Learning the Difference Between Plaster of Paris and Gypsum

Gypsum and Plaster of Paris are two materials that are sometimes used interchangeably in different industries, especially construction, art, and medicine. Though they resemble each other, they differ when it comes to composition, properties, and usage. It is important to know the difference between gypsum and Plaster of Paris in order to choose the proper material for a particular project. This article discusses the characteristics, applications, and main differences between these two products.

What is Gypsum?

Gypsum is a naturally occurring mineral that is made up of calcium sulfate dihydrate (CaSO₄·2H₂O). It occurs in sedimentary rock and is mined in many parts of the globe. Gypsum is soft, light, and can be scratched with a fingernail. Its natural color is usually white or colorless, but impurities may impart to it gray, brown, or pink colors.

Gypsum has been utilized for thousands of years, tracing its history back to ancient times. Gypsum was used by the Egyptians in the pyramids, for example. Nowadays, it is heavily utilized within the construction sector for the production of drywall, plaster, and cement. Gypsum is also utilized within the agricultural sector as a soil conditioner and within the food sector as a coagulant.

One of the main characteristics of gypsum is that it can hold water. When heated, gypsum releases its water and becomes a powder called Plaster of Paris.

What is Plaster of Paris?

Plaster of Paris is a white powder that is produced by heating gypsum to a certain temperature (about 150°C) to drive off half of its water content. The chemical composition of Plaster of Paris is calcium sulfate hemihydrate (CaSO₄·½H₂O). When it is mixed with water, it rehydrates and solidifies into a solid mass, which makes it a great material for molding and casting.

The origin of the name "Plaster of Paris" is from the extensive use in Paris in the 17th century to construct decorative pieces in buildings. Currently, it is largely applied in art for sculpture making, in medicine for casting, and in construction for wall and ceiling patching.

Key Differences Between Gypsum and Plaster of Paris

Although gypsum and Plaster of Paris are closely related, they have some very important differences:

Chemical Composition:

  • Gypsum is calcium sulfate dihydrate (CaSO₄·2H₂O), which includes two water molecules.
  • Plaster of Paris is calcium sulfate hemihydrate (CaSO₄·½H₂O), which includes a half molecule of water.

Formation:

  • Gypsum is a naturally occurring mineral within sedimentary rock formations.
  • Plaster of Paris is a product made from gypsum by heating it to evaporate half of its water content.

Water Content:

  • Gypsum has approximately 20% water content by weight.
  • Plaster of Paris has less water, and because of this, it can rehydrate and harden when water is added.

Hardening Process:

  • Gypsum does not harden when water is added; it stays in its natural form.
  • Plaster of Paris sets rapidly when dissolved in water and hence is best suited for casting and molding.

Uses:

  • Gypsum is mainly applied in building for the manufacture of drywall, plaster, and cement. It is also utilized in farming and food processing.
  • Plaster of Paris is utilized in art, medical science, and building for molds, casts, and patching compounds.

Setting Time:

  • Gypsum has no setting time as it does not set when dissolved in water.
  • Plaster of Paris has a comparatively short setting time, ranging from 5 to 15 minutes, depending on the composition.

Strength and Durability:

  • Gypsum is softer and less durable than Plaster of Paris.
  • Plaster of Paris, after hardening, is more durable and stronger and can be used in structural applications.

Applications of Gypsum and Plaster of Paris

Gypsum Applications:

  • Uses in Construction: Gypsum is predominantly used in construction for the manufacturing of drywall, plaster, and cement. It is applied as a fire-resistant material too.

  • Agriculture: It is applied in agriculture as a conditioner for soils to enhance its structure and yield basic nutrients such as calcium and sulfur.

  • Food Industry: It is utilized in the food industry as a coagulant to prepare tofu and other foods. 

Applications of Plaster of Paris

  • Art and Sculpture: Artiste and sculptors use Plaster of Paris to make molds and casts.
  • Medicine: Orthopedics use Plaster of Paris to prepare casts to hold broken bones in position in order to heal.
  • Construction: Plaster of Paris is utilized to repair walls and ceilings as well as to make ornamental work.

Conclusion

Overall, although difference between gypsum and Plaster of Paris have close similarities, they are utilized for different uses with different characteristics. Gypsum is a naturally found mineral that is utilized mainly in construction and agriculture, whereas Plaster of Paris is a processed product obtained from gypsum, utilized for molding, casting, and medical purposes. It is important to know the differences between these two materials in order to choose the appropriate material for particular projects, achieving the best results and longevity. Whether you're on a building site, painting, or splinting a broken bone, being aware of the special characteristics of gypsum and Plaster of Paris will assist you in making good choices.

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