What is plaster? Gypsum is a kind of mineral, which belongs to the monoclinic series of natural minerals, and its main component is calcium sulfate. In industrial production, gypsum is often used as an important raw material with various functions and uses. First, gypsum is widely used in the construction industry, such as manufacturing plasterboard, gypsum blocks, and other building materials. In addition, gypsum also has important applications in agriculture, medicine, food, cosmetics, and other fields. For example, in agriculture, gypsum can be used as fertilizer to increase soil pH and promote crop growth; in medicine, gypsum can be used as a laxative and topical agent; and in food and cosmetics, gypsum can be used as an additive and coagulant. It is worth noting that although the chemical composition of gypsum and lime are similar, they are not the same substance. Lime is mainly used to neutralize acidic substances in soil, while gypsum is mainly used in a variety of applications in industrial production. Gypsum plays the following roles in concrete: 1. As the setting agent of concrete, it can delay the cement's rapid setting and adjust the cement's setting time. 2. Improve the strength of concrete and improve the performance of concrete. 3. Adjust the water demand of concrete and control the amount of water secreted by concrete. 4. Adjust the slump of concrete and improve the workability of concrete. 5. Control the shrinkage of concrete and improve the crack resistance of concrete. 6. Improve the durability of concrete and prolong the service life of the concrete.
Principle of gypsum in concrete The main mechanism of gypsum in concrete is that when heated, gypsum will be dehydrated to form hemihydrate gypsum, also known as burning gypsum. Complete dehydration of gypsum produces anhydrous gypsum, also known as anhydrite. Mineral anhydrite has very low activity, also known as dry cement. When there is no gypsum in cement, tricalcium aluminate in cement will react with water quickly, and tricalcium silicate will also have significant hydration. The rapid setting may occur if the cement content is higher and the specific surface area is larger. The hydration process is accompanied by significant heat release, followed by the hardening of cement paste, called rapid setting. The rapid setting makes fresh concrete lose its plasticity, which is extremely disadvantageous to producing concrete. Therefore, a suitable amount of gypsum should be added when cement grinding. When the content is high, more gypsum should be added, but too much gypsum will have an adverse effect and make the cement produce a pseudo-setting phenomenon.
The additional amount of gypsum in concrete There is a certain range of gypsum addition in concrete. The national standard stipulates that the SO content of general Portland cement does not exceed 3.5% (the SO content of slag silicate mud does not exceed 4.0%). The additional amount of gypsum varies with different types of cement. For example, special cement, such as Portland cement, high alumina cement, and white cement, has a high gypsum content of 8% to 12%. In comparison, the gypsum content of ordinary Portland cement is generally about 5%. In short, the additional amount of gypsum in concrete must be determined according to specific conditions, generally the type of cement, concrete performance, and other conditions.
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