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gypsum
Gypsum has a long history as a cementitious material. It is mostly used for painting and architectural decoration. It may be said that everyone often says gypsum, so we must really understand it in order to better apply it.

Gypsum has a wide range of uses in the chemical, pharmaceutical, arts and crafts, architectural sculpture, and building materials industries. For example, in the cement industry, gypsum can be used as a retarder for silicate cement, which can be used to configure silicate and aluminate self-stressing cement, and can also be used as raw material for producing calcium sulfate early strength cement.

The raw materials for the gypsum material for producing gypsum are: natural dihydrate gypsum (CaSO4·2H2O), anhydrite (CaSO4) and industrial by-product gypsum.


Dihydrate gypsum can generally be divided into five categories according to physical properties:

Transparent gypsum: flaky crystal, colorless and transparent like glass

Fiber gypsum: fibrous crystal, silky luster

Alabaster: fine grain block, white transparent

Ordinary gypsum: dense granular, impure

Earth plaster: earthy, not coalesced or slightly knotted, not pure

gypsum powder grinding production line

1. Dihydrate gypsum:

It belongs to the monoclinic system and generally crystallizes in the form of columns and plates, often forming a double crystal like a dovetail. Its crystal lattice is composed of an ion-bonding layer composed of calcium ions (Ca2+) and sulfate ions (SO42) alternating with water molecules to form a layered structure. The close combination of calcium and sulfate ions is much stronger than binding to water molecules. Therefore, when heating dihydrate gypsum, the first step is to crack the water molecules where the binding force between the calcium ions and the sulfate ions is relatively weak, and then lose water from the crystal lattice to become another gypsum product. I will mention it below.

The natural dihydrate gypsum is divided into 5 grades, and the calcium value is calculated according to the calcium oxide (CaO) value (the calculated calcium value of the gray calcium powder is almost the same).

Level 1: ≥95, Level 2: 94~85, Level 3: 84~75, Level 4: 74~65, Level 5: 64~55

It is generally believed that the production of high-strength construction gypsum and dihydrate gypsum should be better than Grade II. It is good to produce ordinary building plaster, and the grade should be above Grade 4.

Therefore, finding good gypsum raw materials is not an easy task. In addition to the appearance, the test data is critical!


2. Natural anhydrite:

It is mainly composed of anhydrous calcium sulfate (CaSO4). The ore layer of anhydrite is generally located under the dihydrate gypsum. The anhydrite usually turns into dihydrate gypsum under the action of mineral water, and occasionally contains 5~10% or more in natural anhydrite. Dihydrate gypsum. Natural anhydrites are pure, colorless or white, often containing impurities and appearing dark gray, sometimes reddish or blue.

The crystal lattice of anhydrite is a unit structure composed of four fractions in each grid, and the crystal lattice is compact, and it has higher stability than other kinds of calcium sulfate crystal lattice. Nowadays, anhydrite is used for self-leveling mortar to adjust the setting time, and micro-expansion can also reduce the occurrence of cracks, which also contributes to fluidity. (The expansion value in the water is said to be unstable, I have not studied it in depth)


3. Industrial by-product gypsum:

In some chemical industries, the production of by-products containing calcium sulfate as a main component can also be used as a raw material for gypsum cementitious materials. E.g:

Fluorine gypsum: wastes removed during the preparation of hydrofluoric acid. Fluorite powder (CaF2) and sulfuric acid (H2SO4) are mixed in proportion to produce a reaction:

CaF2+H2SO4=CaSO4+2HF↑

The HF gas is collected by cooling to form hydrofluoric acid, and the CaSO4 residue is fluorogypsum. Since a certain amount of sulfuric acid and hydrogen fluoride remain in the fluorogypsum, it is acidic, so it is neutralized with lime.

Phosphogypsum: Obtained in the manufacture of phosphoric acid, which reacts with phosphoric acid and phosphoric acid to form phosphoric acid and gypsum. Phosphogypsum crystals are very close to natural dihydrate gypsum.



4. Various variants of plaster

Gypsum cementitious materials are generally obtained by using dihydrate gypsum as a raw material and heat-treating under certain conditions. The process of dehydration of dihydrate gypsum will give various hemihydrate gypsum and anhydrite gypsum variants according to different conditions, and their structure and properties are different.

When dihydrate gypsum is heated to 110 ° C ~ 170 ° C in dry air, it is dehydrated into β-hemihydrate gypsum, heating is continued to 200-360 ° C, and β-hemihydrate gypsum is converted into β-anhydrous gypsum. The dihydrate gypsum is heat-treated under the condition of a temperature of 120 to 140 ° C and a saturated steam pressure in the presence of liquid water, and then dehydrated into α-hemihydrate gypsum, and further heated to 200-230 ° C to be converted into α-anhydrous gypsum.

--hemihydrate gypsum is the main component of ordinary building gypsum, and α-hemihydrate gypsum is the main component of high-strength building gypsum. At the same time, as a cementitious material, its macroscopic properties vary greatly. For example, the water requirement of standard consistency, α-hemihydrate gypsum is about 0.4~0.45, and β-hemihydrate gypsum is about 0.7~0.85. The compressive strength of the test piece is only 7.0~10.0 MPa for β-hemihydrate gypsum, and 24~40 MPa for α-hemihydrate gypsum! !

Observed by scanning electron microscopy: α-hemihydrate gypsum is a compact, intact, coarse primary particle

The β-hemihydrate gypsum tablet is a sheet-like, irregular, secondary particle composed of fine individual grains.

Particle size particles: α-hemihydrate gypsum is about three times that of β-hemihydrate gypsum.

Internal specific surface area: β-hemihydrate gypsum is more than twice as much as α-hemihydrate gypsum.

The following chemical reactions occur after the hemihydrate gypsum is added with water:

CaSO4· 0.5H2O+1.5H2O=CaSO4·2H2O+17.7~19.26KJ (heat)

(There are more complicated gypsum variants that are not mentioned)


5.Gypsum Powder Processing Machine: