Cement plaster (Cement render)

Cement plaster (Cement render)

Cement plaster is a mixture of suitable plaster, sand, Portland cement and water which is normally applied to masonry interiors and exteriors to achieve a smooth surface. Interior surfaces sometimes receive a final layer of gypsum plaster. Walls constructed with stock bricks are normally plastered while face brick walls are not plastered. Various cement-based plasters are also used as proprietary spray fireproofing products. These usually use vermiculite as lightweight aggregate. Heavy versions of such plasters are also in use for exterior fireproofing, to protect LPG vessels, pipe bridges and vessel skirts.

Cement plaster was first introduced in America around 1909 and was often called by the generic name adamant plaster after a prominent manufacturer of the time. The advantages of cement plaster noted at that time were its strength, hardness, quick setting time and durability.

Applications

Heat-resistant plaster is a building material used for coating walls and chimney breasts and for use as a fire barrier in ceilings. Its purpose is to replace conventional gypsum plasters in cases where the temperature can get too high for gypsum plaster to stay on the wall or ceiling in Stucco and Plasterwork.

Early 19th Century plasterer at work – painting by John Cranch. (1751–1821) 19th century stucco plasterwork from House of Borujerdies in Kashan, Iran.

Art

Plaster may also be used to create complex detailing for use in room interiors. These may be geometric (simulating wood or stone) or naturalistic (simulating leaves, vines, and flowers). These are also often used to simulate wood or stone detailing found in more substantial buildings.

In modern days this material is also used for False Ceiling. In this, the powder form is converted in a sheet form and the sheet is then attached to the basic ceiling with the help of fasteners. It is done in various designs containing various combinations of lights and colors. The common use of this plaster can be seen in the construction of houses. Post-construction, direct painting is possible (which is commonly seen in French architecture), but elsewhere plaster is used. The walls are painted with the plaster which (in some countries) is nothing but calcium carbonate. After drying the calcium carbonate plaster turns white and then the wall is ready to be painted. Elsewhere in the world, such as the UK, ever finer layers of plaster are added on top of the plasterboard (or sometimes the brick wall directly) to give a smooth brown polished texture ready for painting.

Interior Design

Plaster (often called stucco in this context) is a far easier material for making reliefs than stone or wood, and was widely used for large interior wall-reliefs in Egypt and the Near East from antiquity into Islamic times (latterly for architectural decoration, as at the Alhambra), Rome, and Europe from at least the Renaissance, as well as probably elsewhere. However, it needs very good conditions to survive long in unmaintained buildings – Roman decorative plasterwork is mainly known from Pompeii and other sites buried by ash from Mount Vesuvius.

Plaster may be cast directly into a damp clay mold. In creating this piece molds (molds designed for making multiple copies) or waste molds (for single use) would be made of plaster. This “negative” image, if properly designed, may be used to produce clay productions, which when fired in a kiln become terra cotta building decorations, or these may be used to create cast concrete sculptures. If a plaster positive was desired this would be constructed or cast to form a durable image artwork. As a model for stonecutters this would be sufficient. If intended for producing a bronze casting the plaster positive could be further worked to produce smooth surfaces. An advantage of this plaster image is that it is relatively cheap; should a patron approve of the durable image and be willing to bear further expense, subsequent molds could be made for the creation of a wax image to be used in lost wax casting, a far more expensive process. In lieu of producing a bronze image suitable for outdoor use, the plaster image may be painted to resemble a metal image; such sculptures are suitable only for presentation in a weather-protected environment.

Plaster expands while hardening then contracts slightly just before hardening completely. This makes plaster excellent for use in molds, and it is often used as an artistic material for casting. Plaster is also commonly spread over an armature (form), made of wire mesh, cloth, or other materials; a process for adding raised details. For these processes, limestone or acrylic based plaster may be employed, known as stucco.

Products composed mainly of plaster of Paris and a small amount of Portland cement are used for casting sculptures and other art objects as well as molds. Considerably harder and stronger than straight plaster of Paris, these products are for indoor use only as they rapidly degrade in the rain.

In dentistry, plaster is used for mounting casts or models of oral tissues. These diagnostic and working models are usually made from dental stone, a stronger, harder and denser derivative of plaster which is manufactured from gypsum under pressure. Plaster is also used to invest and flask wax dentures, the wax being subsequently removed by “burning out,” and replaced with flowable denture base material. The typically acrylic denture base then cures in the plaster investment mold. Plaster investments can withstand the high heat and pressure needed to ensure a rigid denture base. Moreover, in dentistry there are 5 types of gypsum products depending on their consistency and uses: impression plaster, model plaster, dental stones.

In orthotics and prosthetics, plaster bandages traditionally were used to create impressions of the patient’s limb (or residuum). This negative impression was then, itself, filled with plaster of Paris, to create a positive model of the limb and used in fabricating the final medical device.

Fire protection

Plasters have been in use in passive fire protection, as fireproofing products, for many decades.

The finished plaster releases water vapor when exposed to flame, acting to slow the spread of the fire, for as much as an hour or two depending on thickness. It also provides some insulation to retard heat flow into structural steel elements, that would otherwise lose their strength and collapse in a fire. Early versions of these plasters have used asbestos fibers, which have by now been outlawed in industrialized nations and have caused significant removal and re-coating work. More modern plasters fall into the following categories:

3D printing

Powder bed and inkjet head 3D printing is commonly based on the reaction of gypsum plaster with water, where the water is selectively applied by the inkjet head.

Safety issues

The chemical reaction that occurs when plaster is mixed with water is exothermic. When plaster sets, it can reach temperatures of more than 60 °C (140°F) and, in large volumes, can burn the skin.

People can be exposed to plaster of Paris in the workplace by breathing it in, swallowing it, skin contact, and eye contact. The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for plaster of Paris exposure in the workplace as 15 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday. The National Institute for Occupational Safety and Health (NIOSH) has set a Recommended exposure limit (REL) of 10 mg/m3 total exposure and 5 mg/m3 respiratory exposure over an 8-hour workday.