Use : As a bounding compound.
Raw Materials : limestone, Sulphuric acid
CaCO3+ H2SO4 = CaSO4+H2O
Different form of synthetic or byproduct gypsum are now available for use and are being produced at economical rates which help the gypsum producers. In the business of making cement, gypsum costs usually very economical. Synthetic gypsum reduce costs as well as provide other manufacturing, environmental, and political benefits.
The few sources of synthetic gypsum are power utilities and acid neutralization. Many electric power utilities use high-sulfur bituminous coal as their source of fuel and in case of acid neutralization the chemical gypsum produced is of high quality, but it also is fine and wet, making it difficult handle.
Natural gypsum: CaSO4. 2H2O
Taking the example of synthetic gypsum manufacturing by power plant
As the power plant has the excess SO2 produced at the plant, these gases are called as Flue gases which are exposed to slurry of hydrated lime.
This lead s to the formation of calcium sulphite (CaSO3. ½ H2O)
This calcium sulphite is then oxidised to form the required gypsum. Of the formula of CaSo4.2H2O
The most popular source used is lime or limestone. In this process, when the slurry is precipitated, it results in a byproduct known as Flue Gas Desulfurization (FGD) material, or synthetic gypsum.
In many cases, synthetic gypsum is available to cement producers at a lower cost than natural gypsum and is very fine material and can have moisture ranging from 10% to 15% after a dewatering process.
Synthetic gypsum forms include FGD (Flue Gas Desulfogypsum ) or DSG or ( DeSulfoGypsum) from the scrubbing of SO2 gases from coal-fired power plants, Titanogypsum from the production of TiO2 pigments, Phosphogypsum from the production of phosphate fertilizers and Fluorogypsum from the production of hydrofluoric acid.
FGD forms of synthetic gypsum are extremely hard to handle due to the sticky nature of this material in wet form In wet form it can benefit the cement grinding process by inducing inherent moisture into the mill, thus providing cooling that may otherwise be accomplished by use of a water spray system. As a result of this potential moisture introduction into the grinding process, gypsum dehydration is reduced by using FGD gypsum.
FGD gypsum produced from coal-fired boilers do not contain hazardous materials and in wet form can be stored in open stockpiles without environmental impact resulting from fugitive emissions. Coal-fired power plants with installed SO2 scrubbers can produce a high purity, high quality gypsum raw material by reacting combustion gases with a lime or limestone slurry and further allowing the intermediate product to become oxidized to calcium sulfate dihydrate or gypsum. This sustainable material is interesting as an averted form of SO2 pollution (the primary cause of acid rain). Comparisons have been made of the quality parameters for initial set, false set, early and late strength gain of Type I and Type II cements produced with 100% FGD gypsum versus cements produced with 100% natural gypsum. The results show no variation in the cements produced and reveal no impact on cement performance.