Composition of Drywall and Joint Compound

Drywall—the familiar panels of white rock-like material covered with paper and used in nearly all wall surfaces—was invented in 1916 by the U.S. Gypsum company as a cheap and easy replacement for troweled plaster wall construction. Earlier forms of the product, known as plasterboard, were manufactured as early as 1888 in Kent, England, but it was the U.S. Gypsum company that created the product with wrapped paper edges and gave it the now-familiar name of Sheetrock. Initially, the product was only mildly successful, as most builders continued to prefer traditional plaster wall construction. But after World War II, when hundreds of thousands of homes were being swiftly built in the boomer economy of the late 1940s and 1950s, drywall became the standard for residential wall construction. It remains the most popular material for finishing wall surfaces.

The Composition of Drywall

Drywall panels, also known as wallboard, plasterboard, gypsum board, or Sheetrock, is commonly thought to be simply mineral gypsum sandwiched between layers of paper. But while gypsum does indeed comprise the bulk of drywall panels, there are actually a number of other ingredients included:

Cellulose

Up to 10 percent of drywall's composition is comprised of cellulose, found in the paper facing on both sides of the drywall panel. Increasingly, though, drywall is being faced not with paper but with mold-resistant fiberglass mat. Examples include USG Sheetrock Mold Tough and GP's DensArmor Plus. In fact, if you ever use a fiberglass joint tape such as FibaTape, you are using the same type of material used for facing panels of mold-resistant drywall.

Gypsum Plaster

Comprising between 70 to 90 percent of sheet drywall, gypsum has long been the mainstay of drywall products. Otherwise known as calcium sulfate, gypsum is cheap to mine, is fire-proof, and it provides superior sound-deadening properties. Fire-rated type X drywall has fiberglass added to the gypsum. Unlike asbestos, which is known to cause an often-fatal illness called mesothelioma, gypsum dust is not a grave health hazard. 

However, the layer of rock-like material in drywall panels is not pure gypsum but is more accurately known as gypsum plaster. This material is manufactured by first heating the raw gypsum powder to drive off the water. The material is then slightly rehydrated and mixed with various fibers and additives to improve strength, speed the hardening, hinder mildew and mold, and provide other benefits. These materials are included in only trace amounts, by volume, but they are essential to creating the gypsum plaster core of drywall panels:

• Paper and/or fiberglass fibers
• Plasticizers
• Starch
• Finely ground mica crystal as an accelerant
• EDTA or other chelating agents
• Anti-mildew agents, such as boric acid
• Wax emulsion or silanes to hinder water absorption
• Potassium sulfate

Drywall panels are created by sandwiching a core layer of wet gypsum plaster between heavy paper or fiberglass surface sheets, then hardening them in drying chambers. Once dry, the material becomes strong and rigid, and it is ready to use as a building material.

Joint Compound Composition

In addition to the large drywall panels, a wall system also depends on the joint compound, or mud, that covers the tape sealing the joints between panels. Because this compound dries to a finish that looks very much like gypsum, it's often assumed that joint compound is a liquified gypsum material. It is not. Instead, the composition of drywall mud includes:

• Calcium carbonate: Otherwise known as ground limestone, this is the main mineral in drywall mud. 
• Talcum: Ultra-fine talcum powder is what makes sanding drywall joints such an unpleasant chore. This ultra-fine mineral, familiar in baby powder, is used in a joint compound because its plate-shaped particles lie flat and resist cracking. Talc is the element that helps your mud sand down as smooth as glass. Between 5 percent and 15 percent of joint compound, by volume, is made up of talc.

Hazardous Materials in Drywall?

There has been a good deal of recent publicity regarding hazardous chemicals found in drywall panels, mostly in products manufactured in China. For example, sulfur has been found in Chinese drywall, but not in most U.S.-made drywall.

An excessive level of sulfur in drywall has been linked to corrosion of electrical wiring and plumbing pipes. Strontium has been found in both Chinese drywall and U.S.-made drywall, although the concentrations are much higher in Chinese products.

Some drywall products also contain various agents to fight mold and mildew, and these agents can cause difficulty for sensitive individuals.