CaSO 4 o2H 2 O= H 2 O %×4.778 The calculation formula of anhydrite: CaSO 4 =(SO 3 -H 2 O %×2.22)×1.70 However, this analysis method can only analyze the grade of gypsum ore. The content of dihydrate gypsum, hemihydrate gypsum and anhydrite in the gypsum can not be analyzed. How do we determine the content of gypsum in the gypsum? In all walks of life, the content of each component in the plaster of Paris is different, and its use is also different. In our construction industry, what is basically needed is hemihydrate gypsum. The more it is, the better the quality. The above is a little experience we have gained through experiments.
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In the daily analysis of gypsum, we generally first chemically analyze the composition of crystal water and sulfur trioxide contained in the gypsum, and use them to convert the content of CaSO 4 o2H 2 O and CaSO4 in the gypsum.
The calculation formula of dihydrate gypsum:
We know that the main mineral component of raw gypsum is dihydrate gypsum. According to the interatomic bond in the gypsum structure, it can be written as [Ca 2 o(H 2 O) 3 ][(SO 4 )2oH 2 O], among which 3 H 2 O and The two Ca 2+ van der Waals forces have weak mutual attraction force, and it is easy to lose three crystal waters. The two SO42- and one H2O have stronger hydrogen bonding interactions than the intermolecular van der Waals force between the two molecules. This crystal water is more difficult to lose than the first three, and the structure of [(SO 4 ) 2oH 2 O] 4+ is as follows:
However, in the 95% ethanol solution, there is a hydrogen bond in addition to the intermolecular force between the ethanol and water molecules. The above several forces are arranged as follows:
2 SO 4 2- and 1 H 2 O>ethanol and H 2 O >2Ca 2+ with 3 H 2 O
Therefore, after the 95% ethanol solution is added to the gypsum, the water in the ethanol solution can only be combined with the anhydrous gypsum to form the hemihydrate gypsum, and cannot continue to react with the hemihydrate gypsum to form the dihydrate gypsum.
After consulting with foreign materials, we have done a lot of related experiments and learned that raw gypsum loses attached water at around 50 °C, and when it burns to above 107 °C and not more than 190 °C, about 3/4 of the crystal water is lost. Semi-hydrated gypsum, the temperature rises above 400 ° C, can lose all of the crystal water to become anhydrite. According to the above characteristics of gypsum, after experiment, it is considered that the following methods can be used to determine each phase in the gypsum.
First, the adhesion of water, the determination of residual anhydrite:
In about 1 gram of calcined gypsum, about 1 ml of a 90%-95% ethanol solution is added, so that the anhydrous gypsum can absorb the water in the ethanol solution to become hemihydrate gypsum. However, under this condition, due to the action of the ethanol solution, the hemihydrate gypsum and the newly formed hemihydrate gypsum will not continue to be hydrated into dihydrate gypsum, and after drying at 45 ° C, the alcohol solution and excess water are removed by constant weight.
Attached water content:
Residual anhydrite water absorption:
Residual anhydrite content: CaSO 4 = 15.116
In the above formula: W1--ethanol solution original weight before hydration sample W2--ethanol solution hydration sample weight above analysis sample, attached water and anhydrite can only have one, because there is no water Gypsum is very easy to absorb water, so there is no anhydrite in the presence of water.
Second, the determination of hemihydrate gypsum:
In the sample after the determination of the anhydrite, there were only two components, hemihydrate gypsum and dihydrate gypsum, and 1 ml of distilled water was added to the sample. Converting all of the hemihydrate gypsum, including both intrinsic and newly formed, to dihydrate gypsum;
At this time, the added weight of the sample is the amount of crystal water added by the hydration of the hemihydrate gypsum into dihydrate gypsum, and dried at 45oC to remove excess water.
The amount of water absorbed by the sample after hydration with distilled water:
Content of hemihydrate gypsum: CaSO 4 o1/2 H 2 O=5.37 (c-3b)
In the above formula: W3--The weight of the sample after hydration of distilled water III. Determination of gypsum:
In addition, a sample is weighed and dried at 400oC until constant weight. At this time, all kinds of gypsum containing different crystal water lose crystal water, and the weight of the sample is reduced by hemihydrate gypsum and dihydrate gypsum. The sum of the weight of crystal water and attached water.
Water content of all samples:
The hemihydrate gypsum contains crystallization water weight: CaSO 4 o1/2 H 2 O%×0.062
The content of dihydrate gypsum: CaSO 4 o2H 2 O=4.78 (Aa- CaSO 4 o1/2 H 2 O%×0.062)
In the above formula: W4--Based 400oC Original weight of sample before drying W5--Based on 400oC drying to constant weight of sample weight gypsum phase analysis After the general laboratory can be carried out, the calcination temperature and calcination time of gypsum can be Take control. The following is the phase analysis test of the gypsum mine in Quwo County, Shanxi Province, after calcination at different temperatures, and the phase analysis of the gypsum in the gypsum, the hemihydrate gypsum and the dihydrate gypsum. The examples are as follows: 
Of course, with the development of science, the chemical identification technology for gypsum is getting deeper and deeper, and more and more methods are being used.
Gypsum is a kind of natural mineral product formed by calcium sulfate with different water content. It is very rich in reserves throughout our province. The building gypsum is a semi-hydrated gypsum which is formed by calcination of natural dihydrate gypsum at a low temperature of 107-170 ° C, also called hot gypsum. With the calcination temperature, the properties of the product, the content of dihydrate gypsum, hemihydrate gypsum and anhydrite gypsum are also different, and the application is also different. This requires accurate analysis of each phase in the building gypsum to control the calcined gypsum. quality.