Gypsum board is made of a mineral core of calcium sulfate dihydrate sandwiched between two sheets of paper or fiberglass. That core makes up the vast majority of the board’s weight, but a surprisingly long list of additives, bonding agents, and engineering choices turn a simple mineral into one of the most common building materials in the world.
The Mineral Core
The heart of every gypsum board is calcium sulfate dihydrate, a white crystalline mineral with the chemical formula CaSO₄·2H₂O. The “dihydrate” part is important: two water molecules are locked inside every unit of the crystal structure. Those trapped water molecules account for roughly 21% of the mineral’s weight, and they’re the reason gypsum board performs so well in a fire (more on that below).
Raw gypsum comes from two sources. The first is mined natural gypsum, quarried from sedimentary deposits. The second is synthetic gypsum, a byproduct of coal-fired power plants that scrub sulfur dioxide from their exhaust. In 2018, U.S. producers mined about 21 million metric tons of crude gypsum while roughly 400 facilities generated an estimated 30 million metric tons of synthetic gypsum. Together, domestic mined and synthetic sources covered about 88% of U.S. consumption that year, with imports filling the rest.
To make the core, raw gypsum is heated to drive off some of its water, creating a powder called calcium sulfate hemihydrate (commonly known as plaster of Paris). This powder is then mixed with water and additives on the production line, where it recrystallizes back into calcium sulfate dihydrate and hardens into a solid slab.
The Paper or Fiberglass Facing
A bare slab of gypsum would be brittle and hard to handle. The outer layers give it structural integrity. Standard gypsum board uses paper facers on both sides. The front face is a smooth, multilayer sheet designed for painting or finishing. The back face is a coarser liner. Both can be made from 100% recycled fiber content, which helps manufacturers meet environmental targets.
Some specialty boards replace paper with fiberglass mat facers. Fiberglass resists moisture and mold better than paper, making it a common choice for boards used in bathrooms, basements, or exterior sheathing. Impact-resistant panels take this further by combining a high-density core with extra-durable paper or fiberglass facers plus an additional fiberglass reinforcing mesh layer.
How the Core Bonds to the Paper
Starch is the glue that holds the whole sandwich together. During manufacturing, modified starch is mixed into the wet gypsum slurry. As the board moves through a drying oven, moisture evaporates from the inside outward, and the starch molecules migrate toward the interface between the gypsum core and the paper. Once they arrive, they swell, crosslink, and form hydrogen bonds at gypsum contact points, creating a strong three-dimensional adhesive network. This bonding process works in both dry and moist conditions, which is why you can’t easily peel the paper off a finished board.
Additives That Control Weight and Strength
A solid block of gypsum would be far too heavy for walls and ceilings. Manufacturers lighten the core by whipping in tiny air bubbles using foaming agents, typically surfactants similar to those used in detergent production. These chemicals lower the surface tension of the wet slurry so that vigorous mixing traps stable air pockets throughout the core. The result is a density reduction of 30% to 35% compared to solid gypsum, bringing finished boards into a weight range that one or two workers can handle.
To compensate for the strength lost by adding all that air, manufacturers reinforce the core with short glass fibers, usually about 12 millimeters long. Research has shown that fibers shorter than that don’t improve mechanical strength effectively. Two common types are standard E glass fibers and alkali-resistant glass fibers coated with zirconium oxide to prevent degradation in the alkaline gypsum environment.
Additives for Fire and Heat Resistance
Gypsum board is widely used as a fire barrier, and the reason traces back to those two water molecules locked in every crystal. When a fire heats one side of a gypsum wall, the crystals begin to dehydrate at temperatures between 125°C and 225°C (roughly 257°F to 437°F). The water releases as steam, and because this reaction absorbs a large amount of energy, it dramatically slows the transfer of heat to the other side of the wall.
For boards rated as Type X (the designation for enhanced fire resistance), manufacturers boost this effect by adding lightweight minerals like vermiculite, mica, or perlite. These materials act as thermal insulators while keeping the board light enough to install easily. Even small amounts, typically less than 5% by weight, can meaningfully improve fire resistance. Some formulations achieve a two-hour fire rating, meaning the wall holds back flames and heat for that long in standardized testing.
Specialty Formulations
Not all gypsum board is the same white panel you see at a home improvement store. Manufacturers adjust the recipe to suit different environments. Moisture-resistant boards incorporate water-repellent compounds in the core, often waxes or silicone-based additives, that reduce the amount of water the gypsum can absorb. These are sometimes called “green board” because of their traditional green paper facing, though fiberglass-faced moisture-resistant panels are increasingly common.
Mold-resistant boards pair moisture-resistant cores with fiberglass facers, removing the paper that mold feeds on. Abuse-resistant boards designed for high-traffic areas like school hallways use denser cores and reinforced facers. Despite these variations, the ASTM C1396 standard requires all gypsum boards to meet specific benchmarks for core hardness, flexural strength, nail pull resistance, water absorption, and other physical properties before they can be sold.
From Slurry to Finished Board
On a production line, the process moves fast. Calcined gypsum powder is mixed with water, starch, foaming agents, and any specialty additives in a continuous mixer. The resulting slurry is poured onto a moving sheet of facing paper, the back paper is laid on top, and the sandwich passes through forming plates that set the board’s thickness. The gypsum recrystallizes and hardens within minutes. The continuous ribbon of board is then cut to length and sent through a drying kiln, where the excess water evaporates and the starch migrates to form its bond with the paper.
The finished product is deceptively simple: mineral, paper, starch, air, and a handful of performance additives. But the chemistry happening inside that flat white panel is what makes it lightweight enough to carry, strong enough to span between studs, and capable of holding back fire for hours.