Gypsum is a type of mineral that is composed of hydrated calcium sulfate in its chemical form. One of its key functions is to serve as a retarding agent of cement. This is due to its ability to regulate the rate at which the cement hardens, making it an essential component in the manufacturing of cement. Without gypsum, the setting time of cement may be too fast, making it difficult to work with and decreasing its overall quality. Therefore, gypsum is considered an indispensable ingredient in the production of cement.
What is Gypsum ?
Gypsum is a mineral that is naturally found in the earth’s crust. It is obtained by mining deposits that were formed by ancient seabeds. This mineral is composed of calcium sulfate and water, and is available in various colours including white, grey, yellow, red, and brown. Gypsum has numerous applications in manufacturing, industry, and agriculture due to its versatile nature.
One of the significant advantages of using gypsum is its natural fire-resistant property. This makes it a valuable resource in construction and building materials as it can prevent the spread of flames. Additionally, gypsum is used in the manufacturing of drywall and plasterboard due to its ability to provide insulation and soundproofing. It is also a popular choice for agricultural purposes as it can improve soil quality and promote plant growth.
Overall, the abundance and versatility of gypsum make it an essential mineral for various industries. Its natural fire-resistant property, in particular, is a valuable asset that can be utilized in numerous applications to enhance safety and prevent damage from fire.
Fig 1: Gypsum
When is Gypsum added in the cement ?
Cement production involves the formation of clinkers, which are cooled and combined with a small amount of gypsum before undergoing the final grinding process. The percentage of gypsum added to the mixture differs depending on the type of cement being produced. For ordinary Portland cement, the percentage typically ranges between 3 to 4%, whereas for quick-setting cement, the amount can be reduced to as low as 2.5%.
Role of Gypsum in Cement
Gypsum is commonly added to cement to slow down its hydration process when mixed with water. When water is added to cement, it quickly reacts with the C3A particles, leading to rapid hardening. This short time frame doesn’t allow sufficient time for transportation, mixing, and placement of the cement. To prevent this, gypsum is introduced, and upon contact with water, it reacts with the C3A particles to form ettringite. These fine-grained crystals coat the surface of the C3A particles but are too small to bridge the gaps between cement particles. As a result, the cement mix remains plastic and workable, allowing for more time for mixing, transporting, and placing. The duration of these processes significantly influences the strength, composition, and workability of the final concrete product. Thus, gypsum is considered a retarding agent for cement, as it delays the process of hydration.
Effects of Gypsum on Cement
Gypsum plays an important role in the manufacturing of cement by preventing the occurrence of Flash Setting. In addition, it also serves to slow down the setting time of cement, which allows for a longer period of time for mixing, transporting and placing the cement.
When water is added to cement, it triggers a chemical reaction between the aluminates and sulfates present in the cement, leading to the release of heat. However, gypsum acts as a coolant in this process and helps to reduce the temperature by regulating the heat of hydration.
Furthermore, the use of gypsum in cement results in a significant increase in strength and hardness compared to non-gypsum cement. Additionally, the amount of water required for the hydration process in gypsum-based cement is relatively lower.