The molecular formula of dihydrate gypsum is CaSO4,2H2O, and its chemical structure is a calcium sulfate crystal with two crystal waters. Under different heating conditions, its structural water is easily removed, becoming various crystals of hemihydrate gypsum and anhydrous gypsum.
When heated at 65 ℃, dihydrate gypsum begins to release structural water, but the dehydration rate is relatively slow. At around 107 ℃ and a water vapor pressure of 971mmHg, the dehydration rate rapidly increases. As the temperature continues to rise, dehydration accelerates. At 70-190 ℃, dihydrate gypsum rapidly dehydrates into alpha hemihydrate gypsum or beta hemihydrate gypsum. When the temperature continues to rise to 220 ℃ and 320-360 ℃, semi hydrated gypsum continues to dehydrate into alpha soluble anhydrous gypsum. But the anhydrous gypsum generated at 220 ℃ is relatively easy to absorb water in the air and become semi hydrated gypsum. The anhydrous gypsum formed between 450-750 ℃ is insoluble anhydrous gypsum. This type of anhydrous gypsum, commonly known as "dead burning" gypsum, is difficult to dissolve in water, almost non condensable, and lacks strength. At 800 ℃, anhydrous gypsum begins to decompose into CaO, SO2, and O2, and its coagulation ability mainly relies on the coagulation effect of CaO rather than gypsum. This decomposition becomes more intense after 1050 ℃ and only ends at 1350 ℃. Under a reducing atmosphere, it is beneficial for the decomposition of CaSO4.
character:
Usually white and colorless, colorless and transparent crystals are called transparent gypsum, sometimes turning gray, light yellow, light brown and other colors due to impurities. The stripes are white. Transparent. Glass luster, cleavage surface pearl luster, fibrous aggregate silk luster. The cleavage is extremely complete, and the angle between the cleavage planes is 66 and 114, forming a diamond shaped body. Sexual fragility. Hardness of 1.5~2. There may be slight changes in different directions. Relative density 2.3.
Under polarized light: colorless. Two axis crystal (+). 2V=58。 Ng=1.530，Nm=1.523，Np=1.521。 As the temperature increases, 2V decreases and reaches zero at approximately 90 ℃.
Crystalline water stage:
There are three stages of discharging crystal water during heating: 105~180 ℃. Firstly, one water molecule is discharged, and then half of the water molecule is immediately discharged, transforming into calcined gypsum Ca [SO4]? 0.5H2O， Also known as gypsum or semi hydrated gypsum. At 200~220 ℃, the remaining half of the water molecule is discharged and transformed into type III hard gypsum Ca [SO4]? εH2O(0.06<ε<0.11)。 At about 350 ℃, it transforms into type II gypsum Ca [SO4]. At 1120 ℃, it further transforms into type I hard gypsum. Melting temperature is 1450 ℃.
Structure:
The microporous structure and heat dehydration properties of gypsum and its products endow them with excellent sound insulation, heat insulation, and fire resistance properties.
Gypsum belongs to the monoclinic crystal system with high cleavage degree and is prone to cracking into thin flakes. Heating gypsum to 100-200 ° C and losing some crystalline water can yield semi hydrated gypsum. It is an air hardening cementitious material with two forms, α and β, both of which are diamond shaped crystals, but with different physical properties. Alpha type semi hydrated gypsum has good and solid crystallization; β - type hemihydrate gypsum is a crystal with flakes and cracks, with very fine crystals and a much larger specific surface area than α - type hemihydrate gypsum.
When producing gypsum products, alpha type hemihydrate gypsum requires less water than beta type gypsum, and the products have higher density and strength. Alpha type hemihydrate gypsum, also known as high-strength gypsum, is usually distilled in a saturated steam medium using a steam pressure vessel; The beta type hemihydrate gypsum, also known as building gypsum, is calcined using a frying pan or rotary kiln with an open device. Chemical gypsum, an industrial by-product, has the same properties as natural gypsum and does not require excessive processing. The slurry of semi hydrated gypsum mixed with water forms dihydrate gypsum again, which quickly solidifies and hardens during the drying process to obtain strength, but softens when it comes into contact with water.
Formation of dihydrate gypsum:
Gypsum used in the production of building materials, including natural gypsum and chemical gypsum. Natural gypsum includes natural dihydrate gypsum (CaSO4? 2H2O) and natural anhydrous gypsum (CaSO4). Natural dihydrate gypsum has a softer texture, while natural anhydrous gypsum has a harder texture, hence it is also known as hard gypsum. Natural dihydrate gypsum is commonly used in industrial production, and when referring to "gypsum", it mostly refers to natural dihydrate gypsum.
Pure dihydrate gypsum is transparent or colorless, with crystal forms such as fibrous, needle like, and flake like. Natural dihydrate gypsum ore often contains a lot of impurities. From the occurrence, there are transparent gypsum, fiber gypsum, snowflake gypsum, flake gypsum, muddy gypsum or soil gypsum. The content of dihydrate gypsum in gypsum is often referred to as grade, which is used to grade gypsum. Grade I gypsum contains over 95% dihydrate gypsum, Grade II contains over 85% dihydrate gypsum, and Grade III contains over 75%. The production of building gypsum boards mostly requires the use of grade three or higher gypsum.
Chemical gypsum generally refers to various by-products in industrial production, which are industrial waste residues containing a certain amount of dihydrate gypsum and many impurities. When referring to these gypsum, it is customary to add the name of the original main product type or gypsum source type before it, such as phosphogypsum, fluorogypsum, smoke exhaust desulfurization gypsum, saltpeter gypsum, etc.
Gypsum exists in various forms, and after heating, it gradually transforms from dihydrate gypsum to another form. After absorbing moisture, the opposite change occurs. The production of gypsum building products is based on the properties of gypsum.
Gypsum is heated and dehydrated to become hemihydrate gypsum (or other forms of dehydrated gypsum), which is mixed with water to form gypsum slurry. The hemihydrate gypsum and other dehydrated gypsum in the gypsum slurry hydrate and harden to form dihydrate gypsum, which is then molded into hardened gypsum products.
Half water gypsum has a short setting time, with initial setting time of 4-8 minutes and final setting time of 10-13 minutes for building gypsum; High strength gypsum sets initially in 3-8 minutes and finally in 15-30 minutes. The short production cycle of gypsum building materials fully utilizes the fast setting and hardening characteristics of gypsum materials.
