Natural gypsum

[edit] What is natural gypsum?

Natural gypsum is a soft sulphate mineral with the chemical formula CaSO4·2H2O, also known as calcium sulphate dihydrate. It consists of calcium, sulphur bound to oxygen, and water. It is an abundant mineral in sedimentary rocks that has been mined and used for construction and as a fertiliser since the time of Ancient Egypt.

[edit] Natural occurrence

The mineral gypsum was formed around 200 million years ago due to the evaporation of sea water, leaving layered deposits of calcium sulphate dihydrate, under high pressure and temperature the gypsum turns into anhydrite (CaSO4). The sedimentary rock contains varying amounts of gyspum, (around 75% to 95%), with the remaining being clay and chalk.

Gypsum and anyhdrite can occur in masses up to a few metres thick and are often mined in conjunction with one another. Gypsum has a considerably greater number of applications although finely ground and blended anhydrite and gypsum are used in large quantities as part of the mix to produce cement clinker and Portland cement.

Pure gypsum is typically white, although impurities create a wide range of different colours. It is moderately water-soluble but exhibits retrograde solubility, meaning it becomes less soluble at higher temperatures. It is fire resistant and is effective at preventing the passage of sound.

[edit] Production and use

In terms of production, gypsum rock is mined or quarried and then crushed and ground into a fine powder. When natural gypsum (CaSO4,2H2O) is ground to a powder and heated at 150° to 165° C, the majority of its chemically combined water is removed, to produce a hemi-hydrate (a crystalline hydrate containing one molecule of water for every two molecules of its compound) plaster (CaSO4,1/2H2O), better known as Plaster of Paris.

When the dry powder is then again mixed with water, it forms a paste which can set hard as the water recombines to produce Gypsum again. This process of dehydration and rehydration can be repeated almost indefinitely, meaning gypsum in its pure form can be recycled indefinitely. The prohibiting factor to this is that it is most commonly used in combination with other materials as;

• Plaster.
• Plasterboard / wallboard.
• Blockwork.
• Mortar.
• Blackboard chalk.
• In the production of cement clinker for Portland cement (blended with anhydrite).

There are a number of different products that are being developed that make use of recycled materials by mixing them with varying degrees of purity, part of this process can also involve the production of what is called synthetic gypsum.

[edit] Land fill and recycling

Gypsum itself can be recycled in a closed loop, meaning it can be recycled into the same product again, rather than down-cycled into another product. Danemark has some of the highest recycling rates in Europe with around 60% of gypsum being recycled. Whilst the European Union set a target of around 70%, countries such as Sweden, Norway, France and the UK are also gradually increasing their recycling rates. Though tonnages of recycled demolition waste are increasing, the proportion being recycled is not as high as the targets due to challenges surrounding contamination with other materials such as in plasterboard.

Once collected, plasterboards can be broken down into fine powders which can then be re-introduced, in controlled blends, to certain manufacturing processes. For closed loop recycling waste cycles, construction materials need to be pre-processed to remove nearly all other materials, as gypsum recyclers tend to accept only up to 3 per cent contamination.

Recycling gypsum waste reduces the need for virgin gypsum to be quarried and processed, which can mean significant energy and material savings for the processing of each metric ton, whilst gypsum found in landfilled plasterboard can decompose to release toxic hydrogen sulfide, and the paper can decompose to produce the potent greenhouse gas, methane.

[edit] Related articles on Designing Buildings

• Aggregate.
• Coal ash.
• Construction materials.
• Drywall construction.
• Fertilizer groundwater pollution.
• Lime plaster.
• Minerals of local and national importance.
• Mortar.
• Permission for mining or working of minerals.
• Plaster.
• Synthetic gypsum.