Gypsum is a widely used mineral defined as calcium sulfate dihydrate (\(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}\)). This substance has two molecules of water chemically bonded within its crystalline structure. Found in sedimentary rock formations, gypsum is highly valued for construction materials, agriculture, and various industrial processes. Understanding its physical properties, especially density, is necessary for its processing and application. This article explores the density values of gypsum and how changes to its chemical state affect this measurement.
The Specific Density Range of Mineral Gypsum
The intrinsic density of pure, solid mineral gypsum—the dihydrate form—is consistently measured within a narrow range. For the crystalline mineral itself, known as the particle density, the value typically falls between 2.31 and 2.33 grams per cubic centimeter (g/cm\(^3\)). This density measurement is based on the mass of the material excluding any voids or air space. In the standard unit for construction and geology, this range translates to approximately 2,310 to 2,330 kilograms per cubic meter (kg/m\(^3\)).
The minor variations in this range are usually attributable to small amounts of mineral impurities or slight imperfections in the natural crystal lattice. This value represents the maximum possible density for the raw, hydrated mineral in its solid state. This particle density is distinct from the bulk density of crushed gypsum powder or manufactured products, which will always be lower because of the air trapped between the individual particles. For example, crushed gypsum may have a bulk density closer to 1,290 to 1,600 kg/m\(^3\).
How Hydration State Impacts Density
The most significant factor influencing gypsum’s density is the amount of water locked within its crystal structure, a state controlled by heating, a process called calcination. Raw gypsum, the dihydrate, is chemically transformed by removing some or all of this water, resulting in entirely new materials with different densities. When the dihydrate is heated to temperatures around 120 to 180 degrees Celsius, it loses about 75 percent of its chemically bound water.
This partial dehydration converts the gypsum into calcium sulfate hemihydrate, commonly known as Plaster of Paris (\(\text{CaSO}_4 \cdot 1/2\text{H}_2\text{O}\)). Although mass is lost by driving off water, the resulting hemihydrate crystal structure packs more tightly than the original dihydrate structure. The particle density of the hemihydrate is therefore higher, often reported around 2.76 g/cm\(^3\) for the alpha form, in contrast to the dihydrate’s 2.32 g/cm\(^3\).
If the material is heated further to higher temperatures, all of the water is removed, creating anhydrous gypsum, or anhydrite (\(\text{CaSO}_4\)). Anhydrite represents the densest form of calcium sulfate, with a particle density typically ranging from 2.8 to 3.0 g/cm\(^3\). This increase in density across the three states—dihydrate, hemihydrate, and anhydrite—is a result of the rearrangement of the remaining atoms into progressively more compact crystal lattices.
Practical Applications Where Gypsum Density Matters
The density of gypsum products is a critical specification in construction and manufacturing, affecting several performance characteristics. In manufactured gypsum board, or drywall, density plays a role in the product’s effectiveness as a fire-resistant barrier. Gypsum contains approximately 21 percent chemically combined water by weight, which is released as steam when exposed to high heat.
This process of water release absorbs thermal energy, effectively slowing heat transfer and retarding the spread of fire. Higher-density gypsum boards, particularly those designated as Type X, often have a more compact core, which can prolong the time the board remains structurally intact during a fire. Density also directly influences the acoustic performance of a wall or ceiling assembly. Sound dampening in building materials relies heavily on mass, so a denser gypsum product provides greater resistance to the transmission of airborne sound.
Density is also a factor in determining the structural weight of an entire building. While the particle density of the mineral is high, manufacturers intentionally create porosity, or trapped air pockets, within the final product core to reduce its overall bulk density. This manufacturing choice provides a balance between the desired performance characteristics, such as sound and fire resistance, and the need to keep the material light enough for easy installation and to minimize the load on the building’s structural frame.