Plaster : so old a history for such a high-tech material !
The oldest traces of plaster renders are 9,000 years old, and were found in Anatolia and Syria. We also know that 5,000 years ago, the Egyptians burnt the gypsum in open-air fires, then crushed it into
powder, and finally mixed this powder with water to make jointing material for the blocks of their monuments, such as the magnificent Cheops Pyramid for example…
The Greeks also used gypsum, in particular as window for their temples when it was of a transparent quality ("selenite gypsum"). The writer Theophraste (372-287 BC) described quite precisely the fabrication of plaster as it was
done at that time in Syria and Phenicia. Throughout the centuries, expertise was gained in many parts of the World with gypsum calcination and the use of plaster (mostly as render for walls and ceilings and as jointing
compound for walls). In the 1700's, Paris was already the "capital of plaster" ("Plaster of Paris") since all the walls of wooden houses were covered with plaster, as a protection against fire. The King of
France had enforced this rule after the big London fire literally destroyed this city in 1666.
The first scientific study on plaster was realized in 1768 by a French scientist named Lavoisier. In 1887, Le Chatelier, another prominent French scientist, was the first to study
the hydration mechanisms of plaster. And then, plaster gradually became a fascinating research subject for scientists. Today, Lafarge Central Laboratory (in France) has a cutting-edge in R&D in gypsum and plaster, employing some of the best experts world-wide and boasting the
most comprehensive range of high-tech analysis equipment. It also cooperates with many prestigious universities all around the world, discovering future uses of gypsum and plaster...
Well, this is another story, and a top secret one !… 
By the end of the 19
th century, plaster was used in the construction
industry in a very massive way, in Paris and many
other cities around Europe.
During the 20th
century, plaster was found to be of great use outside of the construction industry. For example in the ceramic industries
(sanitary ware, tableware, giftware…), in dentistry, in metal casting, jewelry, in medical applications, in cosmetics, animal food, and many more applications… It looks like
each decade brings new uses for this material.
Large gypsum deposits near Paris have long been
mined to manufacture… "Plaster of Paris". And Lafarge has had a plant nearby those deposits for a long time. Therefore it can really be said that Lafarge Prestia is "at the very origin of Plaster of Paris".
From Gypsum to Plaster of Paris
Gypsum is a sedimentary rock, which settled through the evaporation of sea water trapped in lagoons. According to the nature of its impurities, gypsum can show various colors, ranging from white to brown, yellow, gray and pink. Gypsum selection and preparation (cleaning, classifying) are
key factors to produce the best plasters. The chemical reaction is :
(CaSO4, 2 H2O) + heat = (CaSO4, ˝ H2O) + 1.5 H2O
Several processes are available to calcine gypsum into Plaster of Paris. We can 1st: Calcination under atmospheric pressure to produce Beta plaster ; 2nd: Calcination under elevated pressure to produce Alpha plaster.
Controlling some critical calcination parameters is essential to master the growth of the plaster crystals. And the performance of the plaster depends a lot on its crystals' sizes and shapes.
Therefore process know-how and control are key-expertise for plaster manufacturers.
Grinding plaster will determine the particle size distribution of the powder. Different applications call for different granulometrical distributions, and Lafarge Prestia uses different types of grinding
processes which enable to serve of markets. Finally, the plasters will be mixed with additives and fillers, in order to adjust their rheological characteristics (setting times, fluidity, viscosity),
their hardening kinetics, their permeability (through pore size distribution), their mechanical strengths, their resistance to abrasion, etc… Lafarge Prestia knows not only how to formulate for all types of
applications, but also which additives to choose in order to delay the ageing process of the plaster in bags, a distinctive competitive-edge... 
distinguish two categories :
Last but not least, plasters will be bagged. And there also, after a lot of studies,
Our process combines the efficiency of fully automated production lines and the flexibility of
manufacturing cycles that allow to reduce batch sizes and shorten production lead-times, thus enabling us to respond to order requirements with increased rapidity.
Alpha and Beta plasters
Beta plasters present micro-crystals that will yield high porosity and average low
mechanical strength only when cast with water, as they need high excess of water in slurry (typically: water quantity will be 80% of plaster quantity).
Alpha plasters present well formed crystals that will yield low porosity and high mechanical strength when cast with water, as they need low excess of water in slurry (typically: water quantity will be 33% of plaster quantity only).
Many of the plasters prepared for industrial use will be a mix of Beta and Alpha plasters, with additives to optimize the performance and control the ageing process. Our
Gypsum dehydration
Plaster manufacture
Plaster of Paris is a calcium sulfate hemi-hydrate : (CaSO4, ˝ H2O) derived from gypsum, a calcium sulfate dihydrate (CaSO4 , 2 H2O), by firing this mineral at relatively low temperature and then reducing it to powder. Calcination of the gypsum at higher temperatures produces different types of anhydrites (CaSO4), as shown on the table below
Plaster : such a high-tech material…
In many of their recent applications, the plasters used have
not so much in common with the plasters used in the construction industry and in plasterboards… Alpha comes from the calcination of gypsum under water pressure in an autoclave furnace whereas Beta is fired at atmospheric pressure.
Further more, additives and fillers have to be used to optimize the final performance of the formulated plasters. In some plasters of Lafarge Prestia range for example, as many as 12 components will be mixed !
Formulation expertise is the key to success in our business but the process control know-how is critical as well. Choosing the right raw materials, preparing them (cleaning and classifying them by size),
adjusting the process parameters to make the calcination conditions both optimum and stable, controlling the early-ageing process are critical know-how. Lafarge Prestia is lucky to benefit from the technical support of the Lafarge group
Laboratory in France (probably the biggest R&D Laboratory dedicated to building materials in the world) which employs some of the world experts in plasters. They give us invaluable
technical tips that definitely give us a technological edge in the new products we now develop for the future, in more and more different applications. However, who would complain when addressing the challenges of our Future ?
Because they have to deliver specific performances, they will be processed in specific ways, in order to optimize their crystal size, particle size distribution,
particle surface tension, hardening kinetics, thixotropic behavior, and -quite often- a mix of alpha and beta plasters will be used. 
Plasters have been used for more than 9,000 years. It is a Lafarge Prestia goal to be recognized as a landmark in their contemporary history by developing
new uses for plasters in high-tech applications. Easily said ! But what a tough daily responsibility for our researchers and production team !
Well, no one, at Lafarge Prestia !
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