Gypsum Plasters for the Ceramic Industries
Published in Asian Ceramics, December 1999

The history of plastic forming of clay is almost as old as man himself.  For a long time, ceramic pieces were shaped in wood or terra cotta molds.Then came slip casting, which is now a well-established forming process.   The process was in use as early as 1740 in England for the production of tableware pieces.   In sanitaryware, since the invention of the flushing toilet at the end of the 19th century, the slip casting process with plaster molds has been used extensively.

Plaster molds have a range of unique properties for slip casting :

  • a smooth surface is obtained and model detail is finely and faithfully reproduced
  • a wide range of absorption characteristics can be obtained
  • ware release from the mold surface is easy
  • it is a low cost process

In tableware, the development of the semiautomatic roller head machine around 1950 for the shaping of cups and flatware was followed in 1965 by the introduction of fully automatic jiggering and jollying machines which allowed a high output rate but required some new characteristics from plaster molds.

Since then, both in sanitaryware and tableware,  the technology has developed tremendously  with :

  • a change from traditional to automated rapid production lines,
  • flexibility and at the same time a need for uniformity and better shop performance.

Now that we are at turn of the millenium, and although alternative techniques of shaping have been experienced, plaster mold technology is still the future of the whiteware industries.

Changes in the ceramic industry have made it increasingly important to produce plasters with better performances and new properties. Fortunately, gypsum plaster, as you will see, is a highly versatile product which can be modified in many ways, and today high-quality molding plasters are available with

  • excellent uniformity from batch to batch,
  • controlled permeability and water uptake characteristics with extended life time.

What is gypsum plaster ?

Gypsum is a hydraulic binder based on calcium sulfate. This powder material, mixed with water, will produce a slurry, which can be given virtually any form. A crystallization reaction between plaster and water will take place and the slurry will progressively harden and form a crystalline structure, with only a small change in dimension. Finally, a solid is obtained called set gypsum.

The setting reaction was investigated from a scientific point of view by Lavoisier in 1768 but this setting reaction had been used for many years before, on a large scale, in building industry and artistic moldings. Ancient Egyptians already used plaster as a jointing material for pyramids and as a chemically inert surface for painting.

From a chemical point of view, plaster is made of calcium sulfate hemihydrate CaSO4 0.5H20. Hemihydrate (HH) does not occur naturally but can be obtained from a natural product widely distributed, called calcium sulfate dihydrate (DH) or gypsum rock CaSO4 2H2O (DH).  It is the most common sulfate mineral. It is a sedimentation rock, formed by evaporation of inland lakes, and it exists in various states of purity. Common impurities are clay, chalk, dolomite, silica and metallic oxides. Depending on impurities natural gypsum can be white, pink, red and take various shapes. The name plaster of Paris usually refers to the hemihydrate obtained from the purest materials, and used for molding applications.

Gypsum stone mining is relatively easy due to the low hardness of the material. Extraction takes place in mines or in open quarries. Worldwide production of gypsum rock is around 100 million tons per year, with world reserves estimated today at more than 3 billion tons.

To produce HH from the DH stone, the following steps are involved:

  • calcining to eliminate a part of the water of crystallization from the gypsum rock
  • grinding to obtain a powder with a suitable particle size distribution
  • homogenizing and mixing with various additives to modify the properties according to customer requirements

From a chemical point of view, calcining can be summarized in the following diagram:

Gypsum plaster is a typical multiphase system containing different proportions of anhydrite and hemihydrate as well as residual dihydrate, depending on the calcination conditions.

Calcium sulfate hemihydrate occurs in two forms  : beta and alpha HH

Beta production is the usual way of producing hemihydrate : gypsum dihydrate powder is calcined at a low temperature (about 140 °C) in kettles or rotary kilns. This calcining takes place under normal atmospheric pressure and is a quick and inexpensive process

The alpha HH is obtained by calcining lumps in an autoclave using high-pressure steam. This process, which is much longer, was first operated on an industrial scale in the USA about 60 years ago. It requires special equipment and a good knowledge in calcining technology.

In fact, alpha and beta HH are not true polymorph structures. The 2 products have the same chemical composition and the same crystalline structure. The alpha and beta HH only differ in crystallite size.

During calcination in a beta process, the water of crystallization is liberated as steam. This will break the gypsum crystals and produce a porous aggregate of small particles. This breaking does not occur during autoclaving and the alpha HH produce consists of larger crystallites with a smaller specific surface area.

At first sight, this difference seems unimportant, but for the user, the difference is tremendous. When using plaster as a molding material, the user will first have to prepare a slurry with water.

In the following diagram the differences for the user are summarized. The outstanding feature is excess of water over the stoechiometry. This excess water will have to be eliminated by drying and will generate the porous structure of the set plaster.

Water demand for Alpha and Beta Hemihydrates

Theory

Beta Plaster
P/W = 125 %

Alpha Plaster
P/W = 300 %

CaSO4, 0,5 H20 + 1,5 H20
100 g    +    18.6 g

CaSO4, 0,5 H20 + 6,45 H20
100 g     +   80 g

CaSO4, 0,5 H20 + 2,7 H20
100 g     +     33.3 g

CaSO4, 2 H20
118,6 g

CaSO4, 2 H20 + 4,95 H20
118,6 g            61,4 g

CaSO4, 2 H2 0 + 1,2 H20
118,6 g             14,7 g

Characteristics of plasters

 We can highlight the characteristics :

  • of the HH powder itself
  • of the slurry obtained after mixing plaster and water
  • of the set plaster, for example a plaster mold used in ceramics

For the powder itself, the technically relevant properties are :

  • the granulometry of the powder (average particle size and distribution of sizes of particles)
  • the phase composition : % of HH, % of anhydrite and % of residual DH, giving a good account on the quality of the calcining
  • the bulk density, usually near 0.5 for beta plasters and more than 1 for alpha plasters, important for transport of the powder and storage in hoppers.

 After mixing plaster and water, the main characteristics of the slurry include :

  • the rheology of the mix (fluidity , slump)
  • the setting time linked with crystal growth and hardening of the crystal structure
  • the expansion during setting, due to mutual repulsion of growing crystals

After complete crystallization and the drying procedure the main characteristics of the mold are :

  • the surface aspect (essentially detail reproduction and absence of defects such as pinholes).
  • the mechanical strength, essentially resistance to impact and resistance to wear
  • the porosity of the mold, a key feature in the casting process

The plaster to water ratio P/W will have a tremendous influence on nearly all of these characteristics.

A low plaster to water ratio , 130 % for example, which is usual for beta plasters will produce:

  • low mechanical strength
  • high total porosity (near 50 %)
  • low expansion (around 1mm/m)

A higher plaster to water ratio, 160 % for example, which is usually obtained with a mixture of alpha and beta plasters, will have :

  • high mechanical strength
  • lower porosity
  • higher expansion, typically 2mm/m

In sanitaryware, usual plaster to water ratios are between 130% and 150%

In tableware, mixing ratios start with 130% for casting and go up to 220% in roller head technology.

Meeting the requirements of the whitewares industries.

The requirements of the sanitaryware and tableware producers include essentially:

* for the mold shop, at the chosen P/W mixing ratio:

  • the right rheology: the slurry should be liquid enough to be poured like a liquid and fill in all the details of the mold but thick enough to avoid the settling of the slurry before the setting time.
  • efficient setting kinetics : with one batch of slurry, enough pouring time to cast all the molds and a quick setting to demold quickly will optimize the rotation of the molds in the mold shop.
  • a controlled expansion, especially in tableware for turning molds, as the molds must fit exactly into the spindle heads. In casting, molds made from several fitting parts should also have a low expansion to avoid seam problems on the tableware pieces.

* in sanitaryware, for the casting department :

  • molds strong enough for handling in a wet state (a wet mold showing only around 50 % of the mechanical strength of a dry mold).
  • the right water uptake, starting from the first fill, adapted to monocasting or multiple casting, according to the organization of the casting department.
  • a lifetime as long as possible. The limitation usually comes from the water uptake which becomes weaker and weaker due to the progressive dissolution of the molds.

* in tableware, for the roller head technology :

  • a high air permeability to avoid plastic deformation of the body during the shaping process, especially for bone china pieces.
  • a life as long as possible. The decreasing water uptake of the molds can be a limitation in casting, but it is more usually the deterioration of the working surface due to dissolution and humid abrasion which will limit the lifetime of the molds.

To concentrate on these basic requirements, what can be optimized by the ceramic producers themselves and what can be achieved by the plaster producer?

  • The user will probably try to determine the best plaster to water ratio P/W:   this is always a compromise between mechanical strength, wear resistance of   the humid molds and water uptake capacity.

Each plaster has its own P/W ratio, depending on its granulometry and on the % of alpha HH that has been added.  From the recommended value given by the plaster producer, small modifications are possible, for example increasing the P/W ratio will improve the wear resistance: a better lifetime will be obtained, essentially with a very abrasive ceramic body.

But the fluidity of the slurry will decrease: this can only be done to a limited extent.  An advantage is that the settling will be reduced.  But the main effect is that P/W ratio increase will diminish the water uptake capacity.  It will also increase the price per mold.  So, for a given plaster, the possibilities to modify the P/W value are quite limited.

The user has also limited possibilities to modify the setting time of the slurry with his blending procedure: water temperature, mixing time and speed will modify the crystallization rate.  A good soaking procedure before start of blending, will also limit the number of pinholes

Some skilled technicians even discovered that they could modify, on a small scale, the water uptake capacity of plaster molds with the appropriate blending procedure.

* What the plaster producer can do to accommodate the requirements of the sanitaryware and tableware producers is:

  • select a good quality gypsum rock : the higher the purity, the better the    strength and the better the surface aspect of the molds.  Only the purest    deposits of gypsum rock are well adapted for molding plasters.  Especially    metallic salts and silica must be avoided in molding plasters: they will produce   a quick deterioration of the surface of the molds.
     
  • by controlling calcination he can produce a consistent product with a high    percentage of HH, virtually 0% residual DH and small quantities of anhydrite.   This will allow a good consistency from batch to batch and avoid fast setting   plasters, which are difficult to use in an industrial process of casting.
     
  • by controlling the grinding,  the plaster producer will improve the surface    smoothness of the plaster molds and improve their wear resistance. This is    especially important in tableware
     
  • by mixing alpha and beta plaster, the choice of possible mixing ratios is    improved without side effects on the rheology. A range of P/W ratio usually    between 125 % to 220 % is then accessible.  The high values are an advantage   when dealing with abrasive ceramic bodies in the roller head technology
     
  • by mixing additives and plaster, a number of characteristics can be modified   according to the specific requirements of each customer : the setting kinetics   can be controlled within wide limits: it is for example possible with additives to   have a long pouring time followed by a very quick hardening or to have a very    progressive thickening . The expansion value can also be considerably reduced.   This will limit the stresses inside the molds and improve their lifetime.

And finally some additives will allow to modify the water uptake capacity.  The P/W ratio will determine the total porosity of a plaster mold.  But, with some specific additives, it is possible to obtain a different distribution of pores at a given P/W ratio: for example produce small pores instead of large pores, which will modify to a large extent the water uptake strength and allow, for instance, multicasting.

Microchemistry to control microporosity has been in use for a number of years.  Most of the additives used act as crystallization modifiers. 

In sanitaryware, it is possible for each casting process and for each production organization to determine the optimum pore size distribution of the plaster molds and to obtain it with a specific formulation.

This is also true in tableware for the roller-head shaping technology: a specific formulation will produce the high air permeability values necessary for the shaping of bone china for example.

Gypsum is an extremely versatile material.  Most chemicals and any impurity can influence the crystallization process of gypsum.  This has been a disadvantage for a long time, users complaining about the inconsistency of gypsum products.

Today the situation is different: better knowledge of the crystallization of gypsum, of the benefits of alpha HH and of the role of additives as well as a better control of the manufacturing process of gypsum products can really be the basis of a new fruitful cooperation between gypsum manufacturers and sanitaryware and tableware producers.  At stake are higher quality products, improved productivity and reduced costs.

Gabriel Seng - Lafarge Prestia - August 1999

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