Gypsum

The popularity of gypsum has made the questions-’what is Gypsum’ and ‘what is Gypsum used for’ relevant.

Gypsum is a naturally occurring mineral composed of hydrated calcium sulfate and appears soft white or grey in colour. It is formed mainly in layered sedimentary deposits and has a variety of uses in many industries like building, sculpting, gardening, and ornaments. It is an inert and safe mineral that has existed for millions of years since you can find it being used in Egyptian pyramids as well. It is the most common sulfate mineral.

Where is Gypsum Found? 

Gigantic gypsums are formed beneath layers of sedimentary rock, along with halite, anhydrite, sulfur, calcite, and dolomite. Thick beds and layers of rocks are commonplace for finding gypsum. Ocean water is high in calcium and sulfate minerals so gypsum is also found in lagoons as the water in the ocean can slowly evaporate and be replenished with new sources of water. When water evaporates, the mineral is left behind. Evidence of gypsum dunes has been found on the planet Mars as well. 

What is the Chemical Formula for Gypsum? 

The main components of gypsum are calcium sulfate (CaSO4) and water (H2O). Its chemical name is Calcium Sulphate Dihydrate and the chemical formula of gypsum is represented as CaSO4.2H2O.  Gypsum and Anhydrite (CaSO4) are very similar chemically just that gypsum has 2 water molecules and Anhydrite does not contain any water molecules. 

Physical and Chemical Properties of Gypsum Mineral 

Gypsum was known as Spear Stone in old English since it takes a crystal-like form, projecting out of a rock like a spear. You can mill mix gypsum with water to get its original rock-like shape, and it can be hardened. Its recycling loop can be termed as a “closed recycling loop” since you can recycle it for a number of times and it never loses its quality. It is moderately soluble in water, and its solubility reduces with the rise in temperature, contrary to the behaviour of other salts. Here are some of its important chemical and physical properties at a glance:

Chemical Classification

Sulfate

Chemical Composition

Hydrous Calcium Sulfate, CaSO4.2H2O

Mohs hardness

2

Specific gravity

2.2

Color

Colourless, red, brown, clear, yellow, white, grey

Diaphanous property

Transparent to translucent

Texture

Silky, Sugary, Vitreous

Cleavage

Perfect

Crystal system

Monoclinic

Diagnostic properties

Specific gravity, cleavage, low hardness


Different kinds of Gypsum 

Since gypsum is found all over the world, its shape and texture also vary depending on which part of the world it is found. It is found in approximately 85 countries, and the biggest amount of gypsum is produced in North America. Gypsum has found its use in diverse fields, based on which it can be classified in the below categories:

  • Its Different Forms are:

    • Rock in dull colour

    • Alabaster which is its fine-grained variety

    • Gypcrete or gypcrust, which is a hard layer formed on the soil

    • Selenite Satin Spur has a fibrous texture with a silky lustre.

  • Types of Gypsum Products:

    • Type I – Impression Plaster 

    • Type II – Dental plaster used in model material

    • Type III – Dental stone used in mounting material

    • Type IV – Die stone or high storage stone

    • Type V – High strength which is used when a high expansion is needed as die material like in fabricating base metal alloy castings

How is Gypsum Processed 

Gypsum rock is first mined or quarried, then crushed and ground to a fine powder. It then goes through a process called calcining where heat at 350 degrees is supplied to the gypsum powder which removes 3/4th of the water molecules. Hemihydrate is the name of calcined gypsum which is then used in gypsum board, gypsum plaster, and other products. Its selection and preparation (like cleaning) determine the quality of plaster produced. The chemical reaction of this process can be given as:

(CaSO4, 2 H2O) + heat = (CaSO4, ? H2O) + 1.5 H2O

Uses of Gypsums – Some of the Important Uses of Gypsums Include:

  • Constructing buildings

  • In making pottery and moulds

  • In dental appliances to make casts and moulds and impression material

  • Manufacturing plaster of Paris

  • Conditioning soil

  • Hardening material in cement

  • Filler ingredient in many foods

FAQ (Frequently Asked Questions)

1. What Variety of Gypsum is Used for Ornamental Purposes?

The 2 varieties of gypsum which are used in making ornaments are “satin spar” and “alabaster.”

2. Is Gypsum Harmful?

It is not a toxic material and in case you ingest it, just wash out your mouth and drink plenty of water. It may irritate the eye or cause discomfort to the respiratory system, but all the effects are short term.

3. Where is the Largest Gypsum Quarry?

Gypsum rock is found all through the world and the biggest quarry is found in Nova Scotia, in Eastern Canada, and is owned by National Gypsum.

4. Is There Any Alternative to Natural Gypsum?

Flue Gas Desulphurisation Gypsum, or FGD Gypsum can be used as an alternative to natural gypsum. In coal-fired power stations, their by-products give rise to these gypsums.

5. What are the 2 Ways of Calcification of Gypsum?

 There are 2 ways gypsum is calcined to produce different types of gypsum:

    1. Beta plaster is produced when calcifications happen at atmospheric pressure. It gives Type I and Type II gypsums.

    2. Alpha plaster is produced when calcification happens at elevated pressure. It gives Type III, type IV, and Type V gypsums.