Drywall

You can find more information on our web, so please take a look.

Background

Drywall is a construction material consisting of thin panels of gypsum board. The board is composed of a layer of gypsum rock sandwiched between two layers of special paper. Drywall makes for a much more efficient method of construction than the common earlier technique of applying wet plaster to a gypsum lath.

In addition to being easy to install, drywall provides a measure of fire protection to buildings. Gypsum contains large amounts of water bound in crystalline form; 10 square feet (1.0 sq m) of gypsum board contains over 2 quarts (2 1) of water. When exposed to fire, the water in the gypsum board vaporizes; the temperature of the panel remains at 212°F (100°C) until all of the water is released, protecting the underlying wood framework. Even after all of the water evaporates, the gypsum itself will not burn and continues to provide substantial fire protection.

Plaster made from gypsum has been used as a construction material for thousands of years. In fact, plaster applied at least 4,000 years ago to walls inside the Great Pyramids of Egypt is still in good condition. Today drywall panels are widely utilized in modern construction around the world.

Raw Materials

The primary component of drywall is the mineral gypsum. It is a light-density rock found in plentiful deposits worldwide. Each molecule of gypsum (or dihydrous calcium sulfate) is composed of two molecules of water (H 2 0) and one of calcium sulfate (CaSO 4 ). By weight the compound is 21% water, but by volume it is nearly 50% water.

Because the water present in gypsum is in crystalline form, the material is dry. Although ice, another form of crystalline water, becomes a liquid at room temperature, the water bound in the gypsum molecules remains solid unless it is heated to 212°F (100°C), at which point it changes to a gaseous state and evaporates.

Gypsum, called gypsos by the ancient Greeks, is one of the most useful minerals known to man. In its pure form it is white, but impurities often give it colors like gray, brown, pink, or black. Ancient Assyrians called it alabaster and made sculptures from it. Today, pulverized gypsum is used for a wide variety of applications. It is an ingredient in some brands of toothpaste and is used as a filler in products such as paint, cosmetics, and drugs. Automotive window glass is secured in a bed of gypsum while it is being polished. Gypsum is applied to farmland as a fertilizer and soil conditioner. An excellent source of calcium, it is used to fortify foods such as breads. It is even used to create simulated snowstorms in motion pictures.

Gypsum that has been crushed and heated to remove 75% of its water content is known as plaster of Paris. When water is added to this fine white powder, the resulting material is easily molded into any desired shape. Upon drying, the reconstituted gypsum regains its rock-like qualities while retaining the desired shape. Besides its use in making gypsum board, this material is used to make sculptures, pottery, dishes, bathroom fixtures, and casts for broken bones.

Millions of tons of gypsum are mined each year in North America, and gypsum board is the principal product in which it is used. Besides the newly mined material, up to 20% of the gypsum used to manufacture drywall can be recycled from waste generated at the manufacturing plant or at construction sites. Gypsum produced as a byproduct of the flue-gas desulfurization process at electric power plants provides an economical, environmentally sound raw material for making high-quality gypsum board.

Two types of paper are used in the production of most drywall, and both types are made from recycled newspaper. The ivory manila face paper, when properly primed, readily accepts most paints and other types of wall finishing products. The gray back paper can be laminated with aluminum foil to produce a special type of drywall that resists the flow of water vapor in environments like bathrooms. Specialized varieties of gypsum board might be made with different types of paper; for instance, some papers are made to be moisture resistant to various degrees, while another type of highly absorbent paper is designed to accept a thin coat of plaster veneer after installation.

The Manufacturing
Process

Fabrication of drywall consists of placing the gypsum core material between two layers of paper, drying the product, and finishing it into panels of standard size.

Blending of additives

• 1 Depending on the variety of wallboard being produced, certain additives are blended with the plaster of Paris that will form the core of the drywall. Each additional ingredient amounts to less than onehalf of one percent of the amount of gypsum powder. Starch is added to help the paper facings adhere to the core, and paper pulp is added to increase the core's tensile strength (resistance to lengthwise pressure). Unexpanded vermiculite is added when producing fire-resistant grades of gypsum board; in some cases clay is also added.
• 2 Water is added to the plaster of Paris mixture to form a slurry of the proper consistency. An asphalt emulsion and/or a wax emulsion is added to achieve the desired level of moisture resistance in the final product. A foaming agent such as a detergent is included, and during the mixing process air is entrained into the material. The finished gypsum panel will be over 50% air; this minimizes the board's weight and makes it easier to cut, fit, and nail or screw to the framing. Glass fibers are added to the wet core material when making firerated gypsum board.

Making the sandwich

• 3 The gypsum slurry is poured onto a layer of paper that is unrolling onto a long board machine. Another layer of paper unrolls on top of the slurry. The sandwich then passes through a system of rollers that compact the gypsum core to the proper thickness. The most common thicknesses are 0.37 inch (9.5 mm), 0.5 inch (12.7 mm), and 0.62 inch (15.7 mm).

Finishing the edges

• 4 Automated assembly lines in gypsum board plants range from 300-800 feet (93-247 m) long. As the drywall continues along the conveyor belt, the edges are formed. Various shapes of edges are possible, depending on the final use of the panel. Options include the traditional square edge, a tongue and groove type, tapered and/or beveled edges, and even rounded edges.
• 5 The face paper is wrapped snugly around each edge and sealed to the back paper.

Cutting the panels

• 6 By the time the edges have been shaped, the plaster core has set sufficiently for a knife to slice the continuous strip into standard panel sizes. The board, generally 48 inches (1219 mm) or 54 inches (1572 mm) wide, is usually cut into panels that are 8 feet (2400 mm) or 12 feet (3600 mm) long.

The drying process

• 7 The panels are transferred to a conveyor line that feeds them through a long, drying oven. At one plant, for example, the gas-fired oven is 470 feet (143 m) long. Panels enter the oven at 500°F (260°C) and are exposed to gradually decreasing levels of heat during the 35-40 minutes they travel through the system. Humidity and temperature are carefully controlled in the dryer.

The finished product

• 8 After emerging from the drying oven, the dry wall panels are visually inspected before being bundled into "lifts" of 30 or 40 boards and transferred to the warehouse to await shipment. Each board is labeled with a UPC bar code that is used for warehouse inventory, billing, and price scanning at the retail level.

Product Evolution

Since the invention of gypsum board at the turn of the century, there has been gradual progress in making it lighter in weight while improving its performance characteristics. In the late 1950s, standard gypsum board (not fire-rated) weighed 2 pounds per square foot (9.8 kg per sq m); the various kinds of standard gypsum board now average about 1.6 pounds per square foot (8 kg per sq m). This not only makes handling and installation easier, but decreases shipping costs as well.

Fire-resistant and moisture-resistant gypsum boards were developed in the late 1950s and early 1960s. Another innovation came in 1988 with the development of controlled density (CD) ceiling board. In this product, the core is compressed in such a way as to create thin, dense layers of gypsum on both sides of a standard density core. Although CD board is 0.5 inch (12.7 mm) thick, it is more resistant to sagging than conventional gypsum board that is 0.62 inch (15.7 mm) thick.

Another area of investigation involves better ways of disposing of wallboard waste. During building construction drywall scrap is generated, both as trimmings from panels cut to fit required shapes and as damaged panels that cannot be used. An estimated 1.7 million tons (1.5 billion kg) of gypsum board waste material was deposited in land-fills in the United States in 1990. Research has begun in the area of pulverizing this material and using it as a soil treatment rather than simply discarding it. It appears that the effects are very similar to those achieved with gypsum products manufactured specifically for agricultural use.

wuxing contains other products and information you need, so please check it out.

Where To Learn More

Periodicals

"TU Electric Pioneers FGD-Gypsum Production for Use in Wallboard." Power, April 1988, pp. 33-34.

White, Edwin H. and Mark E. Burger. "Construction Drywall as a Soil Amendment." BioCycle, July 1993, pp. 70-71.

Other

Standing, H.A. "The Story of Gypsum." Domtar Gypsum Company, 1-800-DOM-TAR1.

— Loretta Hall

Gypsum Drywall

To give you a basic overview of gypsum drywall recycling, we’ve compiled answers to commonly-asked questions about the process.

What is Gypsum Drywall?

A description of gypsum drywall must first begin with a discussion of gypsum itself. Gypsum, a naturally occuring mineral, is composed of calcium sulfate (CaSO4) and water (H2O). Also referred to as hydrous calcium sulfate (CaSO4.2H2O), gypsum is mined from deposits formed by ancient seabeds as a raw material for many different manufacturing, industrial, and agricultural uses. Over 80% of the gypsum mined is used in manufactured products such as drywall. Gypsum possesses many attributes that make it an attractive construction material. Calcined gypsum can be wetted to form a paste that can be directly applied to a structure’s surface or that can be molded into a desired shape; the gypsum hardens upon drying. Gypsum is naturally fire resistant.

Gypsum drywall, often referred to as gypsum wallboard or sheet rock, replaced gypsum plaster as the major material used for interior wall surfaces because of its ease of installation. Gypsum drywall consists of approximately 90% gypsum and 10% paper facing and backing. Drywall is manufactured by first calcining the gypsum, a process that heats the mineral to remove part of the water (resulting in CaSO4.1/2H2O). The stucco that is formed is then rehydrated by mixing with water, and the slurry created is spread onto a moving continuous sheet of paper and sandwiched between another layer of paper. This continuous sheet of wallboard is allowed to harden for several minutes, cut into panels and sent to a kiln for final drying. It is trimmed to the dimensions required, bundled, and is then ready for shipment. Drywall comes in many different types and sizes to meet specific construction needs.

Can Gypsum Drywall be recycled?

The mineral gypsum has many uses in today’s society. In addition to the manufacture of drywall for building construction, gypsum is also widely used as a soil amendment, in the production of cement, and as an ingredient in the manufacture of many types of commercial products. Since the gypsum makes up approximately 90% of the weight of a piece of drywall, if the gypsum can be recovered from the drywall, the majority of the material can be recycled. Scrap gypsum drywall is currently being recycled in several locations in North America. Examples include:

• The manufacture of new drywall
• Use as an ingredient in the production of cement
• Application to soils and crops to improve soil drainage and plant growth
• A major ingredient in the production of fertilizer products
• An additive to composting operations

Despite its successful use in many locations, most drywall in North America is still disposed in landfills. Challenges to widespread recycling include collection and separation, low landfill disposal fees, and the need for more education of potential end users of the recycled material.

For more information on how gypsum drywall can be recycled, see the Markets page.

Can Gypsum Drywall create odor problems when disposed in landfills?

When gypsum drywall is disposed in landfills, a series of biological and chemical reactions can occur that have the potential for adverse environmental impacts. When drywall in a landfill gets wet, some of the sulfate from the gypsum dissolves into the water. If this “leachate” reaches the groundwater, contamination with sulfate may result. The US federal secondary drinking water standard for sulfate is 250 mg/L. Concentrations above this level have been observed in the groundwater at unlined C&D debris landfills. The sulfate also contributes to the high total dissolved solids (TDS) concentrations observed in groundwater at many C&D debris landfills.

Another issue results form the biological conversion of dissolved sulfate to hydrogen sulfide (H2S). H2S is a foul-smelling gas (rotten eggs). It is produced under wet, anaerobic conditions, such as those that often occur in landfills. The presence of organic matter such as yard trash or cardboard is needed for the microorganisms to thrive, but even the paper on the drywall itself provides enough organic matter for the biological reactions to proceed. H2S has been observed over a tremendously large concentration range at C&D debris landfills. The human nose can detect H2S concentrations at relatively low concentrations (250 ppm). Usually by the time the gas reaches the atmosphere, it is usually diluted to below most currently accepted safety concentrations. Landfill operators should exercise extreme caution, however, any time undiluted C&D debris landfill gases might come in contact with a worker (e.g. excavation activities, confined spaces). The odor problem alone has been enough to result in strict actions being required on the part of some landfill operators. Some C&D debris landfill operators have been required to install gas collection and recovery systems, and others have resorted to placing odor masking agents around their sites. In the Vancouver area, gypsum drywall was banned from disposal in landfills as a result of odor problems.

Any comments regarding this website will be welcomed. Please feel free to contact us at [email protected].

For more information, please visit gypsum board manufacturing plant.

More Information:

• Markets
  
• Processing
• Environmental and Permitting Issues