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What Is Dihydrate Gypsum?

From : clirik    Date : 2019-01-11 13:23

 What is Dihydrate gypsum?
Introduction of Dihydrate gypsum:
The molecular formula of dihydrate gypsum is CaSO4, 2H2O, and its chemical structure is calcium sulfate crystal with two crystal waters. The structure water is easily released during heat treatment under different conditions, and it becomes hemihydrate gypsum and anhydrite of various crystals.
When the temperature is heated at 65 ° C, the dihydrate gypsum begins to release the structural water, but the dehydration rate is relatively slow. At a temperature of about 107 ° C and a water vapor pressure of 971 mmHg, the dehydration speed rapidly increases. As the temperature continues to rise, the dehydration is accelerated. At l 70-l90 ° C, the dihydrate gypsum is dehydrated at a rapid rate to become α-hemihydrate gypsum or β-hemihydrate gypsum. When the temperature continues to rise to 220 ° C and 320 ~ 360 ° C, the hemihydrate gypsum continues to dehydrate into α-soluble anhydrite. However, the anhydrous 100 cream produced at 220 ° C is more likely to absorb water into the hemihydrate gypsum in the air. The anhydrite formed during the period of 450 to 750 ° C is insoluble anhydrite. This anhydrite is what we usually call "dead burning" gypsum; it is very hard to dissolve in water, hardly condenses, and does not have strength. At 800 ° C, the anhydrite began to decompose into CaO and SO2 plus O2, etc., the coagulation ability at this time mainly depends on the coagulation of CaO rather than gypsum. This decomposition is more intense after 1050 ° C and ends at 1350 ° C. In the reducing atmosphere, it is beneficial to the decomposition of CaSO4.
After curing, the dihydrate gypsum will not decrease its solubility in water after long-term storage? No, the cured dihydrate gypsum will be dehydrated into gypsum after long-term placement. The data shows: dihydrate gypsum is 2.08g/L, α-hemihydrate gypsum is 6.20g/L, β-hemihydrate gypsum It was 8.15 g/L, the soluble anhydrite was 6.30 g/L, and the natural anhydrite was 2.70 g/L. So its solubility will not decrease but rise!
application of Dihydrate gypsum:
(1) Used as a retarder in the cement industry after rinsing and drying
(2) After rinsing and drying, it is used to produce ordinary β-type gypsum powder, and to produce gypsum products such as gypsum blocks and large plates;
(3) directly used in agriculture to make land improvement agents;
(4) Used directly for road construction materials.
Gypsum and dihydrate gypsum:
Theoretically, the amount of water required for chemical reaction between gypsum and water is 18.6%; in the process of model making, the actual amount of water added is much larger than this value, and the purpose is to obtain a certain flow of gypsum slurry for pouring, and at the same time obtain A smooth surface model; excess moisture leaves a lot of capillary pores after drying, making the plaster model water absorbent.
Water absorption is an important parameter of the plaster model, which directly affects the speed of the blank during grouting. The water absorption rate of the ceramic plaster mold is generally between 38 and 48%.
Gypsum powder should be placed in a dry place. Do not splash water or cast gypsum when using it. The gypsum bag should be clean and prevent the used gypsum residue or other debris from being mixed into the bag.
Usually white, colorless, colorless transparent crystal called transparent gypsum, sometimes due to impurities such as gray, light yellow, light brown and other colors. The streak is white. Transparent. Glass luster, cleavage surface pearl luster, fibrous aggregate silky luster. The cleavage is extremely complete, and medium, and the cleavage sheet is split into a rhomboid with an angle of 66 and 114. Crisp. Hardness 1.5~2. The direction is slightly different. The relative density is 2.3.
Under the polarizer: colorless. Two-axis crystal (+). 2V = 58. Ng=1.530, Nm=1.523, Np=1.521. 2V decreases with increasing temperature, and 2V is zero at approximately 90 °C.
Crystal water stage:
When heating, there are 3 stages of discharging crystallization water: 105~180°C, first discharging one water molecule, then immediately discharging half of the water molecules, and converting into calcined gypsum Ca[SO4]?0.5H2O, also known as plaster or hemihydrate gypsum. . 200~220 °C, the remaining half of the water molecules are discharged and converted into type III anhydrite Ca[SO4]?εH2O (0.06<ε<0.11). At about 350 ° C, it is converted to type II gypsum Ca [SO4]. Further converted to type I anhydrite at 1120 °C. The melting temperature was 1450 °C.
The microporous structure and heat dehydration of gypsum and its products make it excellent in sound insulation, heat insulation and fire resistance.
Gypsum is a monoclinic system with high degree of cleavage and easy to split into thin slices. The gypsum is heated to 100 to 200 ° C, and part of the crystal water is lost to obtain hemihydrate gypsum. It is a gas-hardening gelling material with both alpha and beta forms, all of which are rhombohedral but have different physical properties. The α-type hemihydrate gypsum crystallizes well and is solid; the β-type hemihydrate gypsum is a plate-like and cracked crystal, the crystal is very fine, and the specific surface area is much larger than that of the α-type hemihydrate gypsum.
When producing gypsum products, α-type hemihydrate gypsum requires less water than β-type, and the product has higher density and strength. Usually, it is made by steaming autoclave in a saturated steam medium, which is α-type hemihydrate gypsum, also called high-strength gypsum. It is made of β-type hemihydrate gypsum by using a wok or rotary kiln open device. plaster. Industrial by-product chemical gypsum has the same properties as natural gypsum and does not require excessive processing. The slurry of semi-hydrated gypsum and water is re-formed into dihydrate gypsum, which is rapidly coagulated and hardened during the drying process to obtain strength, but softens when exposed to water.
Dihydrate gypsum formation:
Gypsum for the production of building materials, including natural gypsum and chemical gypsum. Natural gypsum is natural dihydrate gypsum (CaSO4? 2H2O) and natural anhydrite (CaSO4). The natural dihydrate gypsum texture is soft, and the natural anhydrous gypsum texture is hard, so it is also called anhydrite. Commonly used in industrial production is natural dihydrate gypsum. Generally speaking, "gypsum" refers to natural dihydrate gypsum in most cases.
Pure dihydrate gypsum is transparent or colorless, and has a crystal form such as fiber, needle or sheet. Natural dihydrate gypsum mines often contain more impurities. From the perspective of production, there are transparent gypsum, fiber gypsum, alabaster, flake gypsum, argillaceous gypsum or earth gypsum. The content of dihydrate gypsum in gypsum, often referred to as grade, is used to classify gypsum. The first grade gypsum contains more than 95% of dihydrate gypsum, the second grade contains more than 85% of dihydrate gypsum, and the third grade contains more than 75%. More than three grades of gypsum are used to produce building gypsum boards.
Chemical gypsum, generally refers to the by-products of various industrial productions, is industrial waste residue, which contains a certain amount of dihydrate gypsum, and also contains more impurities. When referring to these gypsums, it is customary to add the original main product type or gypsum source in front of them. The name of the type, such as phosphogypsum, fluorogypsum, desulfurization gypsum, and mirabilite gypsum.
Gypsum has many forms of existence. After the gypsum is heated, it will be gradually converted from dihydrate gypsum to another form. After moisture absorption, the opposite change will occur. The production of gypsum building products is based on this property of gypsum.
The gypsum is heated and dehydrated to become hemihydrate gypsum (or other type of dehydrated gypsum), the hemihydrate gypsum is mixed with water to synthesize gypsum pulp, the dehydrated gypsum such as hemihydrate gypsum in the gypsum slurry is hydrated and hardened to form dihydrate gypsum, and the gypsum slurry is molded into the mold. Made into a hardened gypsum product.
The semi-water gypsum has a short setting time, the building gypsum is initially coagulated in 4-8 minutes, and the final setting is 10-13 minutes; the high-strength gypsum is initially coagulated in 3-8 minutes, and the final setting is 15 to 30 minutes. The production cycle of gypsum building materials is short, which makes full use of the characteristics of quick-setting and hardening of gypsum materials.

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