Gypsum and gypsum products

This section is about the history of gypsum, gypsum stone, production of products from gypsum.

History.

It's been know from written records that gypsum, being a bonding element of as old as history of civilization, was first used in plaster mixed with lime in Çatalhöyük in b.c. 9000. On the other hand, it was used in city buildings and roads during period of Sumerian and Assyrian (b.c. 7000), pharaoh's tomb in Egypt (b.c. 2800) and as bonding agent in pyramids. During old Greek and Roman civilations (b.c. 350-400), it was seen that gypsum was used as plaster on wall and wood.

In Europe the use of gypsum come up in b.c. 200s and and it was nationally announced in Paris in the end o f 1200s. It was seen that wood made constructions were not affected from fire in London fire and the use of gypsum was imposed obligatory in Paris.

It was seen that gypsum had a special place in Islamic architecture In wide geography from Atlantic to Indian ocean gypsum was used in all plastering decoration of all mosques. Similarly, in Seljuk and Ottomanian works the wide use of gypsum was remarked.

Application, maintenance, forming and easy to process; having moisture regulation and isolation (heat and sound protection) characteristics, fire resistance and it low cost, consumption of ready to use gypsum based building elements are booming in developed countries.

Gypsum rock.

Gypsum rock is the main raw material from which the gypsum plaster is made. Gypsum rock is find in the nature as gypsum (CaSO4.2H2O), anhydride (gypsum rock without water (CaSo4)), and selenite, bassanit, alabaster - is CaSO4 component composed of Ca++, SO4- ions' components and containing water or without water in its crystals depending on formation environment.

Gypsum rocks are used in different forms and industry areas depending on their crystal structure, but in this article we will talk about a component CaSO4.2H2O called as gypsum and having 2 molecules of water in its crystals.

The used purce gypsum rock (CaSO4.2H2O) has, theoretically, 32.5% of CaO, 46.6% of SO3 and 20.9% of crystal water.

Production of plasters from gypsum.

The most suitable of gypsum rock minerals found in nature is gypsum. As gypsum can easily be dehydrated under heat effect (looses water in its crystals), gypsum is economical and advantageous.

Although stated in many sources that obtaining gypsum powder from gypsum rock by heating fine grinded mineral under 120-160 centigrade degrees, but in real life it is NOT true. To produce High Quality gypsum powder many instruments should be used together in fine coordination.

Theoretically;

Gypsum powder is obtained by

CaSO4.2H2O + heat <---> CaSO4.1/2 H2O + 3/2 H2O

reaction.

This rection is reversable, and when it is treated with water can absorb 3/2 of water that it lost and becomes hardened.

As it had been mentioned above, although its theoretically expressed like this, matters that determine quality of gypsum production are instruments like temperature, vacuum, speed, time and etc must be studied well and running of them synchronously of them based on experience.

These instruments if not used correctly phase balance of gypsum is changed, quality of gypsum will be degraded towards anhydrite (gypsum without water), bonding structure of gypsum weakens, strength and adherence decrease and when applied to a wall there appears cracks and molders.

Depending on the calcination methods, three types of hemihydrate can be obtained from gypsum rock, such as;/p>

• 1-Beta(β) hemihydrate: Theoretically it is obtained from a calcination of gypsum under 120-160 °C. Crashed in the form crystals like short round riziform. They have law consistency and require high amount of water. They have large surface area and setting time, and thy have porous and sponge like structure.
• 2- Alfa(α) hemihydrate: It is obtained by cooked gypsum rock under high steam pressure. They are formed in long needle shaped crystals. They have high consistency and require fewer water, have high strength, production capacity of it are limited.
• 3-Anhydrite: It is obtained by evaporating all two molecules of water content under high temperatures. It does not require water and has no gypsum value. They can be used as additive materials.

Gypsum plaster and types.

With the wide use of gypsum in building industry, big enterprises are started to be established and depending on this developed various technologies and kiln types for the gypsum powder production.

Our concern herewith is Beta(β) hemihydrate, which is widely used in building industry and have high production capacities.

Construction gypsum, which is obtained by open process, instead of closed process, by applying heat by mixing anhydrite III, anhydrite II and β-hemihydrate. Under normal conditions, β-hemihydrate have 5.5-6 minutes of initial setting time according to gypsum/water ratio. However, depending on area of use in building industry setting times can be longer or extended or shortened by adding chemicals in it. By using these characteristics of β-hemihydrate, according to TSEN 13269-1-2 standard “Description of gypsum and requirements”, 3 different type of gypsum plaster, such as, gypsum plaster (B4), gypsum strengthen with fiber (C1) and finishing plasters (C2) can be produced depending on area of application, setting times, strength, spreading coefficient and grain siz.

Factors that determine the quality in the production of these gypsum products are not the brand value of used chemicals nor their quality, but they are engineering calculation made when building production process from grinding, calcination, filtration, mixing and packing, and experience, ability and knowledge of the team that runs this process.

Gypsum products;

• 1-Gypsum plaster (B4).

a- Hand plasters: They, which are also known as gypsum plaster with perlite in building industry, is a gypsum plaster produced according to TSEN13269-1-2 requirements for hand plastering.

b- Spray plasters: It is a gypsum plaster produced according to TSEN13269-1-2 requirements to spray by means of plaster spraying machine.

• 2- Gypsum strengthen with fiber (C1).
• 3- Finishing plasters (C2).

Test and controls.

There are quality tests, determined by TSEN 13269 1-2 standarda, that need to be applied during production and application.

They, which differ from each other according to the type of produced gypsum, are spreading, initial setting time, final setting time, hydration, adherence and strength test.

As stated above, to meet the requirements of standards is not related with the use of chemicals, but it’s the issue of calcination of gypsum under suitable conditions.