Gypsum is a soft mineral made of calcium sulfate and water (CaSO₄·2H₂O) that serves as the core ingredient in drywall, plaster, and cement. It’s one of the most widely used materials in building interiors, and its popularity comes down to a few key properties: it’s lightweight, easy to cut and shape, naturally fire-resistant, and inexpensive to produce. In 2024, roughly 160 million metric tons of gypsum were mined worldwide across 78 countries.
What Gypsum Actually Is
Gypsum is a naturally occurring mineral found in sedimentary rock deposits around the world. Chemically, it’s calcium sulfate with two molecules of water locked into its crystal structure. That trapped water, called “water of crystallization,” is the single most important feature of the material for construction purposes. It can be driven off with controlled heating, and when water is added back to the resulting powder, it re-forms into a solid. This is the basic chemistry behind plaster, joint compound, and the gypsum core inside every sheet of drywall.
In its raw state, gypsum is porous, soft, and lightweight. You can scratch it with a fingernail. It exists in several natural forms: massive rock gypsum, translucent crystals called selenite, and fibrous varieties known as satin spar. For construction, though, the mineral is typically crushed, heated to remove some of its water content, and then processed into products.
Drywall and Plasterboard
The most visible use of gypsum in construction is drywall, also called gypsum board or plasterboard. A standard sheet consists of a gypsum core sandwiched between two layers of heavy paper. It’s used for nearly all interior walls and ceilings in residential and commercial buildings because it installs quickly, can be cut with a utility knife, and nails or screws directly into wood or metal framing.
Drywall comes in several types and thicknesses designed for specific situations:
- Standard 1/2-inch (12.7 mm) board is the most common choice for walls in homes and offices.
- 5/8-inch (15.9 mm) Type X board is a fire-rated product required in garages, stairwells, and walls between units in multi-family buildings. It has a denser core and higher compressive strength.
- Moisture-resistant board uses a water-resistant facing and core designed for bathrooms, kitchens, and other damp areas. These boards absorb no more than 5% of their weight in water.
Once drywall is hung, the seams between sheets are covered with tape and joint compound. Two main types of joint compound exist. Drying-type compound hardens as water evaporates into the air, which means it’s sensitive to temperature and humidity during application. Setting-type compound hardens through a chemical reaction instead, so it firms up reliably even in cold or humid conditions. The tradeoff is that setting compound dries rock-hard and is significantly more difficult to sand smooth.
Gypsum in Cement
Every bag of Portland cement contains a small percentage of gypsum, typically around 3 to 5%. Without it, cement would harden almost instantly when mixed with water, a problem called “flash setting” that would make the material nearly impossible to work with.
Gypsum prevents this by reacting with certain compounds in the cement to form a mineral called ettringite, which coats the cement particles and slows down the initial hydration reaction. This gives workers a usable window of time to pour, spread, and finish the concrete before it begins to set. The amount of gypsum added directly influences both the setting time and the final strength of the cured concrete. Adding around 5% gypsum to certain cement types can accelerate the development of a solid internal structure within the first 24 hours while still providing enough working time during the initial pour.
Why Gypsum Is Fire-Resistant
Gypsum’s fire resistance is arguably its most valuable property in construction, and it comes directly from that water trapped inside the crystal structure. A standard sheet of 5/8-inch Type X drywall contains roughly 21% water by weight, all locked up as part of the mineral itself.
When fire heats one side of a gypsum wall, that energy goes into boiling off the crystallization water rather than raising the temperature on the other side. The water converts to steam and escapes through the board’s surface, absorbing a large amount of heat energy in the process. The temperature of the gypsum stays close to the boiling point of water (100°C/212°F) until the dehydration is complete. This buys critical time, often 45 to 60 minutes or more depending on the assembly, before fire can penetrate through to the other side of the wall. The gypsum itself is non-combustible and produces no toxic fumes.
Natural vs. Synthetic Gypsum
Construction gypsum comes from two sources. Natural gypsum is mined from underground or surface deposits. The United States is the world’s largest producer, mining an estimated 22 million metric tons in 2024, followed by Iran (16 million tons), Oman (14 million tons), China (12 million tons), and Spain (11 million tons).
Synthetic gypsum, often called FGD gypsum, is a byproduct of coal-fired power plants. When scrubbers clean sulfur dioxide from the exhaust gases (a process called flue gas desulfurization), the captured sulfur reacts with limestone to produce calcium sulfate dihydrate, the same compound as natural gypsum. Global FGD gypsum production reached an estimated 255 million tons in 2020, with Asia accounting for 55% of that output, Europe 22%, and North America 18%. The United States alone produces around 29 million tons of FGD gypsum annually.
FGD gypsum is typically more than 90% pure calcium sulfate dihydrate, which often makes it purer than mined gypsum. It costs less to produce and diverts an industrial waste product from landfills. Most major drywall manufacturers now use a blend of natural and synthetic gypsum, and in some plants, synthetic gypsum makes up the majority of the raw material.
Recycling Gypsum
Gypsum is one of the few construction materials that can theoretically be recycled indefinitely. The dehydration and rehydration cycle that makes plaster work in the first place also means old gypsum products can be ground up, reheated, and reformed into new ones. EU member states have set a target of recycling 70% of gypsum from construction and demolition waste.
In practice, hitting that target is difficult. Demolition waste is the biggest challenge because gypsum boards are typically contaminated with screws, paint, joint compound, insulation, and other materials that are hard to separate. Current processing methods allow about 20 to 30% recycled gypsum to be incorporated into new plasterboard production, with a goal of pushing above 30%. The recycled material also tends to produce weaker products, with compressive strengths ranging from just 1.0 to 3.1 megapascals in some studies, partly because grinding old boards produces very fine particles that require more water to mix and result in a more porous final product.
Construction offcuts from new builds, by contrast, are much cleaner and easier to recycle. Many drywall manufacturers already accept clean scrap from job sites and feed it back into production.
Other Construction Uses
Beyond drywall and cement, gypsum shows up throughout the building industry. Plaster of Paris, made by heating gypsum to remove three-quarters of its water, is used for ornamental moldings, ceiling medallions, and decorative cornices. Gypsum plaster applied directly to masonry walls remains common in parts of Europe, the Middle East, and South Asia as an alternative to cement-based renders. It produces a smoother finish and sets faster, though it’s not suitable for exterior walls or wet environments.
Gypsum-based self-leveling floor underlayments are poured over subfloors to create a flat surface before installing tile or hardwood. These products take advantage of gypsum’s ability to flow easily when mixed with water and then set into a hard, dimensionally stable layer. Gypsum blocks, essentially solid gypsum formed into masonry-sized units, are used for non-load-bearing interior partition walls in some European countries, offering built-in fire resistance without additional finishing.