Calcium Sulfate vs. Plaster of Paris

What's the Difference?

Calcium sulfate and Plaster of Paris are both forms of calcium sulfate hemihydrate, but they differ in their physical properties and uses. Calcium sulfate is a naturally occurring mineral that is commonly found in the form of gypsum. It is used in various industries, including construction, agriculture, and food production. On the other hand, Plaster of Paris is a specific type of calcium sulfate hemihydrate that has been heated and then cooled to form a white, powdery substance. It is widely used in the medical field for casting broken bones and creating molds, as well as in arts and crafts for sculpting and creating decorative objects. Overall, while both substances are derived from calcium sulfate, Plaster of Paris has been processed to have specific properties that make it suitable for various applications.


AttributeCalcium SulfatePlaster of Paris
Chemical FormulaCaSO4CaSO4 · 0.5H2O
Common NameCalcium SulfatePlaster of Paris
AppearanceWhite powder or crystalsWhite powder
UsesUsed in construction, as a soil conditioner, in food and pharmaceutical industriesUsed in construction, for making molds and casts, in art and crafts
Setting TimeSlower setting timeRapid setting time
HardnessLess hardHarder
Water SolubilityPartially soluble in waterHighly soluble in water
Chemical ReactivityReacts with acidsReacts with acids

Further Detail


Calcium sulfate and Plaster of Paris are two commonly used materials in various industries and applications. While they share some similarities, they also have distinct attributes that make them suitable for specific purposes. In this article, we will explore the characteristics of both Calcium Sulfate and Plaster of Paris, highlighting their composition, properties, uses, and differences.


Calcium sulfate, with the chemical formula CaSO4, is a naturally occurring mineral compound. It is composed of calcium, sulfur, and oxygen atoms. On the other hand, Plaster of Paris is a specific form of calcium sulfate hemihydrate (CaSO4 · 0.5H2O). It is created by heating gypsum, a hydrated calcium sulfate mineral, to remove the water content and form a fine white powder.


Both Calcium Sulfate and Plaster of Paris exhibit unique properties that make them valuable in various applications. Calcium sulfate is a white, odorless powder that is insoluble in water. It has a relatively low hardness and can be easily crushed into a fine powder. Plaster of Paris, on the other hand, is also a white powder but has a higher solubility in water compared to calcium sulfate. When mixed with water, it forms a paste that can be molded into different shapes before hardening into a solid mass.


Calcium sulfate finds extensive use in industries such as construction, agriculture, and food. In construction, it is commonly used as a binder in cement, contributing to the strength and durability of concrete structures. It is also utilized as a soil amendment in agriculture to improve soil structure and provide essential nutrients to plants. Additionally, calcium sulfate is an approved food additive and is used as a firming agent, coagulant, or stabilizer in various food products.

Plaster of Paris, on the other hand, is widely recognized for its applications in the field of art, medicine, and construction. In the art industry, it is used for creating sculptures, molds, and casts due to its ability to set quickly and retain fine details. In medicine, Plaster of Paris is commonly used for immobilizing broken bones by creating casts that provide support and protection during the healing process. Furthermore, it is utilized in construction for creating decorative elements, such as ornamental moldings and ceiling tiles.


While Calcium Sulfate and Plaster of Paris share similarities in terms of their composition and applications, there are notable differences between the two materials. One significant difference lies in their solubility. Calcium sulfate is relatively insoluble in water, making it suitable for applications where water resistance is required, such as in construction materials. Plaster of Paris, on the other hand, is highly soluble in water, allowing it to be easily mixed and molded into various shapes before hardening.

Another difference is the setting time. Calcium sulfate has a slower setting time compared to Plaster of Paris. This property makes it more suitable for applications where longer working times are required, such as in construction projects that involve large-scale installations. Plaster of Paris, with its quick setting time, is preferred in situations where rapid setting and hardening are necessary, such as in medical casts or art projects that require immediate demolding.

Furthermore, the strength and hardness of the two materials differ. Calcium sulfate, being less soluble and slower setting, generally exhibits higher strength and hardness compared to Plaster of Paris. This makes it more suitable for load-bearing applications in construction, where durability and structural integrity are crucial. Plaster of Paris, with its lower strength, is better suited for non-load-bearing applications, such as decorative elements or temporary structures.


In conclusion, Calcium Sulfate and Plaster of Paris are both valuable materials with distinct attributes that make them suitable for various applications. While Calcium Sulfate is insoluble, slower setting, and exhibits higher strength, Plaster of Paris is highly soluble, quick setting, and has lower strength. Understanding the differences between these materials allows us to choose the most appropriate one for specific projects, whether it be in construction, art, medicine, or other industries.

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