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.
When water is added at the time of application, the material sets and hardens, returning to its original chemical composition.
Gypsum Rock to Gypsum Plaster
By drying off some of the chemically combined water from the Gypsum rock, it is converted into gypsum plaster.
First, the gypsum rock is ground and crushed in the mill to start the process. After that gypsum is heated up on a flat metal plate above a kiln. Heating the gypsum at 120°C for one-hour results in a hemihydrate (CaSO4.1⁄2H2O) – with three-quarters of the water removed.
This Gypsum hemihydrate is known as Plaster of Paris.
Prolonged heating for over several hours results in the creation of anhydrite with practically none of the chemically combined water left.
Anhydrite sets more slowly and is a somewhat stronger plaster than hemihydrate, but it is with the downside of added production cost.
In practice, a simple production system would most probably give a mixture of the hemihydrate and anhydrite phases.
Most of the industrially produced commercial plaster today is Plaster of Paris. Gypsum sets by chemically combining with water, where it forms solid calcium sulphate dihydrate.
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Properties of Gypsum Plaster
The general properties of gypsum plaster are as follows,
- 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.
- 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.
- Gypsum plaster produces very few cracks compared to typical cement plaster.
- The plaster displays excellent adhesion to the fibrous materials.
- 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.
- It prevents rusting of metal fittings such as pipes and increases its durability of it.
- It is slightly soluble in water and can dissolve at the rate of 2gm/liter.
- As Gypsum has low thermal conductivity it ensures low consumption of energy and enables power saving.
- Practically, the gypsum is not at all affected by bacteria.
- The tensile and flexural strength of plaster is relatively high.
- 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.
- 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
Types of Gypsum Plaster
Based on the quantity of heat applied during the manufacturing process the type of gypsum plasters are as written below:
- Anhydrite plaster: this type of plaster is manufactured by heating gypsum to 1700C.
- Hemihydrate plaster: this type of plaster is produced when the gypsum is heated to more than 1700 C.
Based on the type of application of the plasters, they can be classified as written below:
- Casting Plasters
- Undercoat Plasters
- Finish Plasters
- One Coat Plasters
- Machine-applied Plasters
Procedure for Application of the Gypsum Plaster on the Internal Walls
Formerly, a 6 mm coat of gypsum plaster also called POP punning is typically applied on top of cement plaster to give a smooth finish to it before painting.
The two-stage plastering process involves various elements like sand, cement, and water that have to be mixed onsite and applied on the surface as cement plaster was done to get a smooth and even finish.
This process is now slowly being replaced by the direct application of a single coat of plaster on the surface. In this process, the plaster which is made from readymade POP powder mixed with water is applied directly to the wall surface.
the procedure for application of the plaster on the internal walls;
1. Surface Preparation and Ground Work before applying gypsum plaster
The wall surface should be prepared properly before applying the gypsum plaster, below points should be considered,
- A day before the application of Chicken mesh, the joints between the beam and brickwork should be filled with the non-shrink mortar.
- At the joint of brickwork and RCC, Chicken mesh should be fixed snugly. Also, make sure that there is an overlap of at least 100 mm on both sides.
- Concrete and brick surfaces have to be properly cured before applying the plaster over them. All the Brickwork surfaces have to be cured properly for at least 7 days before the application of plaster.
- RCC surface should be checked for any defects such as honeycombing. If required, Dressing has to be done before the application of plaster material over the surface.
- All the holes and cracks existing on the surface of the brick wall should be repaired before the application of plaster.
- The concrete surface should be raked to create better bonding between the plaster and the concrete surface. Alternatively, bonding agents (such as Bond-it by Saint Gobain) can also be applied to enhance bonding between plaster and the concrete surface.
- It should be ensured that all the electrical conduits and plumbing lines are placed inside the brickwork (If they are to be concealed).
- The RCC surfaces should be free from shuttering oil, loose materials, or any other agents before plastering starts.
- The dissimilar expansion and contraction of RCC and brick/stone masonry can cause stresses and at times separation of plaster and the surface beneath. To reduce cracks, it should be made sure that the chicken mesh (usually 20 gauge) is installed at brick and RCC interfaces, around the door frames, around the window frames, and opening for electrical & plumbing lines.
- The verticality of the wall surface should be checked using the plumb level and undulations should be removed, if any.
- 10 minutes before the plaster application, clean water should be applied on both brick and RCC surfaces to control the suction.
2. Application of Gypsum Plaster on the Walls
The Gypsum Plasters can be applied directly to the surface of brick walls, concrete blocks, or RCC. The procedure for the application of plaster on the surface is described below;
- The Plaster comes in ready-mix bags generally. Directly pour the powder into a dry vessel and mix it with water. The mixture should be stirred for at least 2-3 minutes.
- The thickness of plaster that needs to be applied on the wall should not be more than 13 mm.
- The plaster should be applied to a solid surface with firm pressure.
- Further flattening should be carried out as the plaster stiffens. When the plaster is sufficiently firm, the surface should be scoured with sponge float and water as required.
- The surface should be troweled progressively to get a smooth matt finish.
- The finished plastered surface should be protected from continuous exposure to moisture or water. The prolonged or repeated exposure to moisture or water may cause a loss of strength and/or adhesion to the plaster.
- The painting work of the surface should only be carried out once the surface has become completely dry.
3. Thickness is Recommended when applying Gypsum Plaster
The Gypsum Plasters can be applied in a thickness of 6mm to 20mm. A minimum thickness of 6 mm is recommended for later to avoid cracks and debonding.
If more than 20 mm thick of plaster built-up is to be required for a wall, then it is recommended to go first with a dash coat of cement and sand plaster for a thickness of 8-12 mm and then finish with the plaster for the remaining thickness (which is to not be less than 6mm).
4. Recommended Finish for Gypsum Plaster
Gypsum Plaster should be finished to a decent degree of smoothness before getting to paint. The surface should be usually sandpapered (and not with emery paper) to give it an additional smoother finish.
All the horizontal lines and surfaces should be measured and tested with a level. All the corners have to be checked to see if they are smoothly finished and are at the right angle.
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 of Gypsum Plaster 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.
The following are the advantages of gypsum plaster in the structure,
- 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.
- 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.
- 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.
- 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.
- High Productivity: It reduces the time of application and setting considerably when compared to conventional cement plaster.
- 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.
- 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.
- 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.
- 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.
- Fire-resistant: The plaster is highly fire-resistant which is safer.
- 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.
- Decorative application: The plaster can be easily molded into different shapes, so it is very easy to use for decorative purposes as well.
- It is a green material.
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.
Gypsum is a very soft mineral composed of calcium sulfate dihydrate (CaSO4:2H2O). Gypsum is white cementing material, which is produced by partial or complete dehydration of the mineral gypsum, generally with special retarders or hardeners added.
Gypsum is a green material that is very easy to use and gives a very smooth finish to the surface after drying. it can be used directly on any type of surface such as a brick wall, R.C.C., etc.
With fire resistance, low thermal conductivity, and very few shrinking cracks after drying gypsum is a popular choice as a plastering material. With its malleability, it can mold in any shape and gives a good aesthetic appearance to the building.
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