GYPSUM – PLASTER of PARIS
Post 278 ⇒ by Gautam Shah →
Gypsum mineral is found as layered sedimentary deposits in association with halite, anhydrite, sulphur, calcite and dolomite. It is a very common sulphate mineral found in all regions of the world, and is of great commercial importance.
In well-developed crystals the mineral is selenite. The fibrous massive variety, called a satin spar is translucent and opalescent, and is used in jewellery. The fine-grained rock like variety is called alabaster, which is carved and polished for statuary and decorative items. Gypsite is the earthy and powdery variety.
Gypsum is in use by man, in the form of plastering and cementing (binding) material and in the form of alabaster for beads and other decorative items, for more than 12000 years.
The word Gypsum derives from the Greek, gyps =burned or baked (calcined) mineral, or the Greek word gypsos =chalk or plaster. Gypsum as a rock-material was known in Old English as spear stone due to its crystalline projections. Selenite was used in Greek, alluding to the pearly lustre (moon light) on cleavage fragments.
‘Plaster was discovered in Catal-Huyuk in Asia in an underground fresco, and in Israel Gypsum floor screeds were found from 7000 B.C. During the time of the Pharaohs, Gypsum was used as mortar in the construction of the Cheops Pyramid (3000 B.C.). In the Middle Ages and the Renaissance, decorations and artistic creations were made of plaster.’
Modern uses of Gypsum include, as sulphur donating additive in fertilizers, filler in paper and textiles, and setting time retarder in Portland cement. Substantial part of Gypsum in calcined form is used as plaster of Paris, Keene’s cement, board products, and ceiling tiles and blocks.
Common Gypsum is composed of Hydrated Calcium Sulphate (CaSO4 ·2H2O). Gypsum plaster is a white cementing material made by partial or complete dehydration of the mineral gypsum, sometimes with additional setting time retarding or hardening agents. Applied in a plastic state (with water), it sets and hardens by chemical recombination of the gypsum with water.
PLASTER of PARIS
Plaster of Paris is a hemi-hydrate of calcium sulphate, composition CaSo4, ½ H2O made by calcining the mineral gypsum, at temperatures 120°–180° C (248°–356° F), creating a composition of CaSo4, 2H2O. The hemi hydrate calcium sulphate, in commercial forms contain many impurities, which govern the chemical behaviour. The powdered hemi-hydrate is mixed with water to form a paste or slurry, the calcining reaction is reversed and a solid mass of interlocking gypsum crystals with moderate strength is formed. The mixing of water with plaster causes an exothermic chemical reaction that releases heat. This heat helps to harden the Plaster of Paris, allowing it to set. Upon setting there is a slight contraction. This dimensional consistency makes it suitable for casts and its release. Different types of Plaster of Paris are available for different applications that have varying setting time, required quantity of mixing water, and hardness on drying. These characteristics are controlled by the calcination, additives, impurities, granular size-distribution, amount and ambient temperature conditions.
Rapid setting qualities may set even before complete hydration has taken place and so may not leave any time for tempering and rendering. Insufficiently hydrated mass attracts moisture later on, resulting in expansion, warping and peel off. Such failures can occur in low setting qualities, when for any reason the amount of hydration water and mixing efforts are insufficient. Particle size distribution is an important factor governing the behaviour of the Plaster of Paris. Too many fine particles quicken the setting without allowing full hydration of the mass. Sand and mineral lime particles improve the wettability of the initial mass and workability during casting or rendering. Hydraulic Lime on getting carbonated can increase the surface hardness at a later date.
It is called Plaster of Paris because Paris became a centre for production of plaster material during the 1700s. It was than mainly used for coating masonry surfaces, as it set at a much faster rate than Lime plaster. It was also used to cover wooden surfaces to make them fire resistant. Plaster of Paris releases water vapour when exposed to a flame, making it ideal as a fire resistant material.
Plaster of Paris is used to make ceiling boards and ornamental cornices. It is also used for making casts and moulds for sculptures, forensic investigations, dentistry, jewellery and for immobilizing broken bones. Gypsum plasters are now used as leveller coating before painting, micro cracks and crevices filler.
Large surfaces of Plaster of Paris require some inter-mass reinforcing and substrate keying. Jute, cotton, threads, coir, hair, viscose, fibreglass, are some of the commonly used fibres for reinforcement. Masonry surfaces of bricks, rough stones and cement plasters offer sufficient substrate bonding. Wood, Metal and Polished stone surfaces require substrate keying through wood, metal lattices, netting or laths. Very thick sections or massive items require some technique for discharge of heat during hydration. This is done layer by layer application, use of cold water mixing, or colder atmosphere application. An entrapped heat quickens the rate hydration of wetted outer mass, but blocking the hydration of rest of the inner core mass. It may also crack the mass due to localized expansion.
For specific hard finish surfaces such as in commercial Gyp or Gypsum boards, the gypsum is completely dehydrated at high temperature, and with use of chemicals such as alkali sulphate, alum, or borax. For surface integrity fibres (short staples), semi digested paper pulp, lime or clay may be added to the plasters during manufacture. Gypsum boards are reinforced with paper or synthetic films on one or both sides.
Many famous historical paintings in Europe are painted on a thin layer of wet plaster or Gesso. It is a white paint like coating of a binder gum mixed with chalk, gypsum, and sometimes a colourant, or any combination of these. It is preparatory surface for drawing or impressing outlines of artwork. The coating was applied on wood panels, canvas, sculptures and masonry walls and ceilings. Gypsum is used as a coating material over papier-mache forms.