Gypsum Plasterboard: Manufacture & Additives

 

Plasterboard is one of the great versatile materials in modern construction. Made from the simple materials of gypsum and paper it is used in most building types in a wide range of applications including wall lining, partitions, sound control and fire protection

The main environmental impacts associated with plasterboard result from the production process, transportation and disposal. Efforts directed by government currently concentrate on reducing the quantity of plasterboard being diverted from landfill to be recycled.

Around 270 million m2 of plasterboard is manufactured annually using some 3 million tonnes of gypsum. Representing around 60% of the total annual output, gypsum used in plasterboard is generated by from natural and synthetic sources:

 

Naturally occurring gyspum


Gypsum (CaSO4.2H2O) is the hydrated form of Calcium Sulphate (CaSO4). Calcium Sulphate is a by-product of the evaporation of lake and seawater and occurs in beds up to several metres thick. Calcium Sulphate Dihydrate (Gypsum) is commonly hydrated by groundwater.

The UK is largely self-sufficient in gypsum. The material can be found in seams in the Pennines and the Midlands, but the biggest source of extraction is in East Sussex where mining extracts some 100,000 tonnes of gypsum a year from 100 m below the High Weald. The mined rock is reduced to powder through a process of crushing and grinding.

 

Synthetic or Glue Gas Desulphurisation (FGD) gypsum


This gypsum is a by-product of the desulphurisation of the flue gases of coal-fired powerstations. Sulphur dioxide emission control systems remove sulphur from combustion gases using 'scrubbers.' One particular type of scrubber that uses lime or limestone reagent system produces 'FGD gypsum' which is chemically nearly identical to mined natural gypsum.

Though currently contributing towards a significant proportion of overall gypsum production in the UK, this is likely to have peaked as the country moves towards reducing dependence on coal-fired power stations (assuming no substantial breakthrough in carbon capture and storage technology / change in energy policy).

 

Plasterboard production


'Stucco' is produced by removing around 75% of the water from the gypsum through heating using natural gas to a temperature of over 150'C. This process of 'calcination' changes calcium sulphate from its dihydrate state (gypsum) to its hemihydrate state.
The hemihydrate calcium sulphate (CaSO4.½H2O) is now ready to form plasterboard.

Water, as well as other additives to control production and performance, is added to the stucco. The slurry is poured onto a moving belt of facing paper where a further paper is applied as a top cover and spreads the slurry to set the width and thickness of the plasterboard.

The resulting wet sandwich of paper and plaster is cut to board lengths as the plaster sets quickly before being transferred to a drying line which carries the boards through a gas-heated kiln. When the slurry has dried and hardened it has effectively reverted back to being gypsum rock.

Once dry, the boards are trimmed and stacked ready for despatch and use.

 

Common additives


• Starch - protects the physical bond between gypsum crystals and facing paper during drying.
• Lignosulphates - Improves the flow of the slurry so less water is required, resulting in denser plaster.
• Potassium sulphate - Causes the gypsum to precipitate out more quickly due to a common ion effect.
• Foaming agent (detergent) - Forms a foam in the mix, resulting in a less denser plaster.
• Silicone - plasterboard is inherently vulnerable to moisture, silicone is added for use in damp conditions
• Wax - also added to provide resistance to moisture
• Vermiculite - added to specifically designated fire resistant boarding
• Glass fibre - also added to provide increased fire protection.

 

Impacts

Pro

Can enable quicker and cheaper construction

Pro

Non-toxic in use

Pro

Gypsum can regulate humidity through moisture absorption

Pro

Lining paper is typically 90% recycled

Pro

Recyclable - content of new plasterboard can be up to 25% recycled

Con

Production and calcination use significant amounts of natural gas and fossil-derived electricity

Con

Global warming effect of burning of gas used to dry feedstock

Con

Plasterboard and facing paper production contribute significant levels of toxicity and eutrophication to water, as well as toxicity to land

Con

Transport of plasterboard to site and to disposal adds to embodied energy

Con

Though non-hazardous, it requires unique monocell landfill disposal

Con

Handling during construction risks injury

 

Metrics

Thermal conductivity: 0.16 W/m.K
Density: 800 Kg/m3
Specific heat capacity: 840 J/kg.K
Embodied energy: 6.75 MJ/kg
(ICE V2.0)

 

 

Waste


Though not the greatest source of environmental impact, the waste disposal of plasterboard is the most notorious. It would seem that from every archetypal skip, there juts a slab of plasterboard headed for landfill. Current waste figures are staggering:

• New construction waste arisings are estimated to be 300,000 tonnes per year as a result of :
- over-ordering by the contractor
- incorrect specification
- damaged plasterboard
- off-cuts arising during construction
• Manufacturing waste is currently around 500 tonnes annually
• The quantity of waste from demolition and refurbishment has yet to be determined.

 

Government and the plasterboard industry are engaged in ways of reducing waste:


• The Ashdown Agreement between WRAP and the Gypsum Products Association sets annual targets in order to reduce waste and increase recycling.
• The Plasterboard Sustainability Partnership set up by DEFRA to promote awareness of sustainability throughout the plasterboard supply chain.
• Site Waste Management Plans
• The Environment Agency's Waste Protocols Project which establishes the quality required for recycling gypsum from waste plasterboard.
• Industry take-back schemes

The recovery and recycling of plasterboard is still in its early days, but the ambition will continue to be that of eliminating plasterboard waste going to landfill.

 

Designing- out waste


• Waste from plasterboard off-cuts is usually assumed to be between 5 - 20%. According to BRE and CRWP surveys the true figure is nearer 30%.

• Waste streams are multiplied by the type of plasterboard specified: eg moisture, fire and acoustic each require a unique waste stream. This can be reduced by using fewer different grades and sizes.

• At the outset of the design its useful to acknowledge the size of any product in the application of it. Plasterboard comes as 600 (usually laid horizontally), 900 and 1200 wide and in may lengths including 2400, 2700, 3000 etc. If the job is big enough, the manufacturer will fabricate custom lengths.

• When designing drylining, it is useful to draw each board and work out the most efficient combinations to reduce off-cutting.

• Design for reconstruction by using battens on dry joints. Some partition systems are designed for disassembly, but rarely used.

• Its always wise to avoid using drylining around complex / curved forms. In these situations it is better to go for simple plaster and paint.

• Flood construction: The Environment Agency advises use of sacrificial materials in areas prone to flooding. A better approach is to use solid wall construction with moisture, water and frost resistant materials, non-eroding, non-absorbent materials, all with no cavities and finished with cement render or tiling.

 

 

 


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