Gypsum plaster is chemically similar to drywall compound, but it is applied using different techniques.
Plaster is a building material much like cement or mortar but primarily used to finish walls. There are many types of plaster, including gypsum, lime, cement and earth. Builders can use these materials over or in place of conventional gypsum wallboard finishes, and they can produce either a smooth finish or a textured one. Each type of plaster has its own special finishing requirements.
Lime
Lime plaster is made of a mix of calcium hydroxide, sand and water, and was the traditional plaster in Europe and North America until the early 20th century. This white plaster is caustic when wet and dries very slowly. It's not used in most new construction, but may be required in some historic restoration work. Lime plaster is also used in some "green" building projects. Lime plaster finish coats include only very fine sand, to produce a smooth, even surface. In homes with interior lime finishes, wait for several days before painting because this material can leach alkaline substances after it appears to be dry.
Gypsum
Gypsum plaster largely replaced lime for interior work during the 19th and early 20th century. This faster-drying material starts to set up as soon as it is mixed, making it easier and less time-consuming to apply. Gypsum plaster is very fine, and made from heated calcium sulfate. It's a traditional component of both lime and gypsum plaster finishes, and it can be applied over gypsum wallboard. Gypsum is relatively soft and not water resistant. As with lime, finish coats tend to include finer materials than base plaster coats. However, some modern plaster finishes include coarse aggregate for texture. Gypsum plasters may also be textured with a rake or trowel after application.
Cement
Cement plaster is also called stucco, and may be used on exterior or interior finishes, though it's more common on the outside of homes. Properly applied and maintained, it can last for many years. If this material cracks, water between the plaster and the wall can cause the finish coat to fall off in chips and sheets. Cement plaster takes dyes easily and is more likely to be colored than lime or gypsum plasters. This type of plaster is poorly suited to very smooth applications, but it does well when it includes texturing elements. According to the Portland Cement Association, stucco can be made to resemble wood, brick or stone by using dyes and stamping techniques.
Earth
Earth plaster is uncommon in North America and is used primarily in "green" building projects. This primarily clay-based plaster has also been used in traditional buildings in Africa, Asia and among Native American peoples. It can be colored, using natural iron oxides and other pigments, but these tend to be relatively subtle and in the brown to red range. Some earth plaster is reddish-purple. This plaster is usually applied on earthen or straw-bale structures, but it can be used over wallboard. It requires a coarser scratch coat, usually with some straw or other aggregate mixed in, followed by a smoother top-coat of mostly clay. Earth plasters provide a natural feel, but may crack in climates where humidity changes rapidly, and they may feel perpetually damp in wet climates.
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