By heating of gypsum found in nature (CaSO4+2H2O) to 150 to 160°C , 1,5 mol water evaporates of 2 moles in the chemical structure and it is changed as CaSO4+1/2H2O called calcination process. Gypsum loses mass of around 15% during calcination, i.e. if 1000 kg of gypsum plaster is calcined, approximately 850 kg plaster remains, and 150 kg of water vapor is emitted from the chimney. Gypsum found steady work in nature, becomes an unstable structure after calcination. When mixed with water again it becomes stable structure in nature.
CaSO4 + 2H2O CaSO4 + ½H2O + 3/2H2O Dehidrasyon (Kalsinasyon)
CaSO4 + 2H2O + 3/2H2O CaSO4 + 2H2O Rehidrasyon
The plaster is breathable construction material is able to get moisture in the atmosphere and give the moisture in the body when the environment is dry. It absorbs the heat during the fire and left the water in the body so it is fire resistant. In the heat and sound insulation compared to other building materials Gypsum is irreplaceable.
According to the production and place of use type gypsum is divided into two classes, α-alpha and β-beta gypsum plaster, gypsum as alpha and beta chemical structures (CaSO4 + ½H2O) are the same. The differences between them occur in the crystal structure. The reason of differences is calcination process method. We can understand the difference between types of two gypsum plasters by analyzing water/gypsum ratio and strength values.
Calcined gypsum is mixed with chemical additives, it is converted into a variety of building materials. Gypsum-based building materials produced basically as follows.
- Satin Plaster
- Plaster of Paris
- Hand Plaster
- Spray Plaster
- Construction Plaster
- Joint Filler
- Seramic Mould Plaster
In addition, the manufacturer has developed its own special products.
GYPSUM PHASE ANALYSIS AND CRYSTAL WATER
Phase analysis of gypsum purity of 100% is as in the following table. Dehydrate gypsum is the plaster in nature. After calcination process gypsum dehydrate transform to hemihydrate, AII anhydride (dry plaster) and AIII soluble (unstable gypsum)..
Hemihidrat; in other words named as stucco is the plaster calcined by losing 1,5 moles of the 2 moles of water./p>
AII Anhidrit; (anhydrite) lost all 2 moles of water in the body (burned).
AIII unstable plaster; (Soluble anhydrite) unstable gypsum in the later stages of the process can turn into hemihydrate or dehydrate.
|CaSO4 + 2H2O Phases and Characteristics|
|Solubility in 20°C
|Dihydrate||CaSO4 + 2H2O||172,17||20,92||2,31||2,05|
|Hemihydrate||CaSO4 + ½H2O||145,15||6,21||2,619 β - > 2,757 α||8,8 β - 6,7 α|
|Soluble Anhydrite||CaSO4 III||136,14||0||2,58||8,8 β - 6,7 α|
|Insoluble Anhydrite||CaSO4 II||136,14||0||2,93 - 2,97||2,7|
GYPSUM PRODUCT CHARACTERISTICS DEPENDS ON PURITY RATE
Depending on the gypsum purity characteristics the product obtained after the calcination vary. Crystal water, setting time and flow diameter such as product information, features will vary depending on the gypsum purity changes are shown in the following table. These values are generated based rotary furnace calcination system. The values can be subject to wide variations depending on the calcination method.
- Rotary Kiln ( Rotary Kiln )
- Vertical Kiln ( Vertical Kiln )
- Flash Kalsinasyon ( Flash Calcination)
- Otoklav ( Autoclave )
- Indirect Ovens (Indirect Kettle or indirect Rotary Kiln)
- Anhydrite Ovens
The method of calcinations
Product properties and some technical values obtained depending on gypsum calcination methods are given in the table below. These values may vary depending on the production standard and equipment quality.