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Plaster of Paris – Definition, Structure, Properties, Preparation and Uses

  • Last Updated : 06 Oct, 2021

Plaster of Paris is a well-known chemical compound that is widely used in sculpting materials and gauze bandages. While we have seen numerous applications of this substance in our daily lives, Plaster of Paris is a white powdered chemical compound that is hydrated calcium sulphate that is typically produced by calcining gypsum. In other words, Plaster of Paris is often produced using heated gypsum at a high temperature.

Plaster of Paris (Calcium Sulphate Hemihydrate)

Plaster of Paris is a quick-setting gypsum plaster made of fine white powder (calcium sulphate hemihydrate) that hardens when wet and left to dry. Plaster of Paris, known since antiquity, is so named due to its production from the plentiful gypsum found in Paris. When dry, plaster of Paris does not shrink or fracture, making it an excellent material for casting moulds. It is often used to create and hold decorative plasterwork on ceilings and cornices. It’s also used in medicine to produce plaster casts to keep broken bones immobilised while they recover, however many current orthopaedic casts are composed of fibreglass or thermoplastics.

Structure of Plaster of Paris

Plaster of Paris is a chemical compound in which an atom of calcium is bonded with a combination of an atom of sulphur along with four atoms of oxygen to form sulphate. It is then bonded with two molecules of water to form calcium sulphate dihydrate.

Preparation of Plaster of Paris

Plaster of Paris is prepared from the chemical compound, calcium sulphate dihydrate, which is also known as, gypsum. Gypsum is represented by the chemical formula CaSO4.2H2O. PoP is manufactured by heating the element gypsum at a very high temperature of about 373K. When this happens at such a high-temperature value of 373K, approximate about three-fourths of its water of crystallisation is lost. forming Pop.

CaSO4 · 2H2O + Heat ⇢ CaSO4 · 0.5 H2O + 1.5 H2O (Discharged as steam)

Note: Since the presence of moisture may slow down the setting of plaster by bringing about the hydration process. Therefore, it is stored in moisture-proof containers.

Properties of Plaster of Paris

Plaster of Paris portrays the following properties:

  • It expands slowly and slightly upon setting.
  • It is highly fire-resistant.
  • It results in the formation of a thick surface to resist normal knocks after drying.
  • It is easy to spread.
  • It is easy to level.
  • It does not cause cracking of surfaces.
  • It gives a decorative interior finish.

Uses of Plaster of Paris

Plaster of Paris founds its usage in the following areas:

  1. 3D Printing: Gypsum plaster can be used for 3D Printing. The water is applied by the inkjet head.
  2. Architecture and Decorations: Plaster of Paris is used to produce fine artwork for decoration and beautification of monuments and buildings. It is also used to imitate wood or stone which is found in ancient buildings and monuments.
  3. During Burial Services: Plaster of Paris is used by executives of funeral houses in order to remake the damaged tissues and fill up the wounds.
  4. Medicines: It is used as a mould and casts. It is used to heal broken bones and cast into a supportive coating known as an orthopaedic cast.
  5. Fireproofing and fire protection systems

Sample Questions

Question 1: Why do we call calcium sulphate hemihydrate plaster of Paris?

Answer:

Plaster of Paris, or simply plaster, generally CaSO4 × 0.5 H2O is created upon heating gypsum to about a temperature of about 150 ºC. 

CaSO4 · 2H2O → CaSO4 · 0.5H2O + 1.5 H2O



Question 2: Is plaster of Paris considered to be eco friendly?

Answer:

Plaster of Paris (POP) is considered to be hazardous for the environment. It gets dissolved in the water and soil thus making it harmful. 

Question 3: State any two types of plaster and their one usage.

Answer:

  • Lime Plaster – Used for frescoes or mural paintings.
  • Clay Plaster – Used in making the interiors of the houses.

Question 4: What happens when Pop is burnt above 250 °C?

Answer:

When Plaster of Paris is burnt above 250 °C, a compound called β-anhydrite or dead burnt plaster is formed and it is a completely anhydrous product.

Question 5: What is dead burnt plaster?

Answer: 

When the chemical compound of plaster of Paris CaSO4 . 0.5 H2O is heated above the temperature of 393 K, its water of crystallisation is lost and anhydrous Calcium sulphate is left as a residue which is known as a dead burnt plaster.

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