Gypsum Plaster: Types & Specifications

In construction, various types of gypsum plaster serve distinct purposes. Casting plaster, with its fine texture, is ideal for crafting molds and decorative elements. Gypsum veneer plaster, known for its smooth finish, is often applied for decorative coatings. Basecoat plaster acts as an initial layer on walls before painting or finishing. Finish plaster, providing a polished surface, is commonly used for the final decorative touch.

One-coat plaster streamlines the process by combining base and finish coats in a pre-mixed form. Textured plaster, infused with aggregates, introduces diverse finishes on walls. This range of gypsum plasters caters to specific needs, allowing builders to choose the most suitable option for their construction or renovation projects.

What Is Gypsum Plaster?

Gypsum plaster is white cementing material, which is produced by partial or complete dehydration of the mineral gypsum, generally with special retarders or hardeners added.

It is applied in a plastic state (with water) to the surface, where it sets and hardens by chemical recombination of the gypsum with water. It can be applied over the brick, block, or concrete surface to form a smooth surface.

The core ingredient of all gypsum plasters is gypsum rock–hydrous calcium sulfate–which has a water content of about 20% in chemical combination. During processing, about 3/4 of this chemically combined water is removed from the gypsum rock employing a controlled heating process called calcination.

Types of Gypsum Plaster

Based on the quantity of heat applied during the manufacturing process the types of gypsum plasters are as written below:

1. Anhydrite plaster: this type of plaster is manufactured by heating gypsum to 1700C.
2. Hemihydrate plaster: this type of plaster is produced when the gypsum is heated to more than 1700 C.

Based on the types of application of the plasters, they can be classified as written below:

1. Casting Plasters
2. Undercoat Plasters
3. Finish Plasters
4. One Coat Plasters
5. Machine-applied Plasters

Read More: What Is ACP Sheet | Types of ACP Sheet | Applications of ACP Sheet | Why Choose ACP Sheet

Properties of Gypsum Plaster

The general properties of gypsum plaster are as follows,

1. It is very lightweight compared to conventional cement plaster and therefore the usage of gypsum for plastering the wall and ceiling surface does not increase the structural load on the building.
2. As Gypsum produces much less heat than compare to cement plaster it does not shrink during drying and hardening processes which cement plasters are prone to.
3. Gypsum plaster produces very few cracks compared to typical cement plaster.
4. The plaster displays excellent adhesion to the fibrous materials.
5. Gypsum is a non-combustible mineral and contains high content (nearly 50%) of crystal water. In the incident of fire, it behaves like a barrier and protects the blockwork, concrete, and steel under the plaster.
6. It prevents rusting of metal fittings such as pipes and increases its durability of it.
7. It is slightly soluble in water and can dissolve at the rate of 2gm/liter.
8. As Gypsum has low thermal conductivity it ensures low consumption of energy and enables power saving.
9. Practically, the gypsum is not at all affected by bacteria.
10. The tensile and flexural strength of plaster is relatively high.
11. It is very malleable when mixed with water and can be molded in whichever shape is desired. It is most suitable for aesthetic work in the building.
12. After the plaster is properly set and with the plaster backing and backgrounds dry, the plaster can be painted with any type of paint except cement paint. It is mentioned in ‘SP 62: S & T 1997’, (Handbook on Building Construction Practices Excluding Electrical Work).

Gypsum Plaster Specifications

The following are the specifications of gypsum plaster,

• Color of the surface after drying of plaster: White
• Setting Time of plaster: 25-30 Minutes
• Area covered by plaster (considering 12mm thickness): 21 sq. per 25 kg Bag
• Compressive Strength of plaster: 60-70 kg/cm²
• Shelf Life of plaster: 4 Months
• Package size of one bag: 25 kg per bag

Top Gypsum Plaster Brands Available in India

• Gyproc One Coat Elite from Saint Gobain
• MagicPlast from Magicrete
• Diamond Plaster
• Stucko from Shubham Plaster
• JK Gypgold from JK Lakshmi Cement

Storage At Site

The following care should be taken when storing the gypsum plaster at the site;

• As the exposure to moisture reduces the setting time and strength of plaster, the plaster bags should be stored properly where there is no possibility of it coming in contact with water or moisture.
• At the site, the plaster bags have to be stored on an elevated platform made of either brick and timber or concrete.
• The plaster bags should be kept at least some certain distance away from the walls so that there is no chance of water or moisture reaching the bags.
• The minimum shelf life of Plaster is between 3 to 4 months from the date of manufacture. But if properly stored it can be extended.
• The shelf life of plaster can be increased by another 6 months. If it is stored properly under suitable conditions of temperature and humidity.

Advantages

The following are the advantages of gypsum plaster in the structure,

1. Ease of Application (Workability): Gypsum is very easy to apply and level on the surface. It can be directly applied over the brick wall or ceiling without any separate finish, which makes it very workable and easy to apply.
2. No Shrinkage Cracks: The heat produced in the gypsum-water reaction is less as compared to the reaction of cement with water, as such the shrinkage cracks in the plaster are fewer as compared to traditional cement plaster.
3. Quick Setting Time: The plaster sets within 25-30 minutes which is very quick compared to other plasters. Before painting the surface, it has to be dried up. In the case of plaster, the painting could be started 72 hours after the application.
4. No curing time required: The plaster does not require any curing like traditional cement plaster, which in turn saves water and time during the construction work.
5. High Productivity: It reduces the time of application and setting considerably when compared to conventional cement plaster.
6. High Performance: The plaster is very durable and lightweight which reduces the dead load on the structure and departs excellent high strength after drying. The plaster exhibits impact-resistant behavior.
7. Smooth Finish: Due to the ease of application and leveling of the plaster, the walls come out perfectly lined, leveled, and smooth. Perfect right angles can be achieved in the corners.
8. Reduced Supervision: For traditional cement plasters, where cement and sand have to be properly proportioned careful quality checking is needed. So skillful supervision is required. While the application of plaster does not require the same amount of quality checks thus supervision efforts are reduced.
9. Readily available raw materials: Gypsum is a readily available raw material, while for traditional cement plaster natural sand, (which is one of the raw materials needed) may not be available or hard to obtain on-site.
10. Fire-resistant: The plaster is highly fire-resistant which is safer.
11. Low thermal conductivity: Gypsum has low thermal conductivity. It saves electrical costs for heating and cooling rooms in a building in winter and summer respectively. Thus gypsum has good insulation properties.
12. Decorative application: The plaster can be easily molded into different shapes, so it is very easy to use for decorative purposes as well.
13. It is a green material.

Disadvantages

The following are gypsum plastering disadvantages

• Since the plaster retains the dampness from the outside, it cannot be used for the outside walls of the building.
• The gypsum also cannot be done in areas that are always damp such as bathrooms, W/C, and choked or wash areas, etc.
• For the same thickness of plaster, gypsum cost more than traditional cement mortar plaster (cement and sand). Though, in areas where natural/river sand is hard to come by for construction, cement mortar plastering would require a 6 mm gypsum layer on top to finish it making cement plaster more costly.
• The plaster cannot be stored for more than 3 months even in a dry place and conditions, otherwise, it deteriorates.

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