Calcium Silicate: Properties, Structure, Production, Uses

The calcium silicate is the name that is assigned to a group of chemical compounds formed from calcium oxide (CaO) and silica (SiO 2 ). The general formula of these compounds is xCaO • ySiO 2 • zH 2 O.

They are white or yellowish-white solids. They can be anhydrous, that is, without water (H 2 O) in their structure, or they can contain it. They are part of several types of minerals in nature.

Calcium silicate mineral. Dave Dyet http://www.shutterstone.com http://www.dyet.com / Public domain. Source: Wikimedia Commons.

Calcium silicates are insoluble in water but when they join with it, they form hydrated gels (materials such as gelatin) which, after curdling, are very hard, resistant and almost waterproof.

This has led to their use in the construction industry, as they are used in cement, bricks and moisture-insulating panels. 
They are also part of materials to heal perforations in the teeth and have even been studied for use in the regeneration of bones, that is, as a biomaterial.

They have been proposed to reduce the pollution generated by some metallurgical industries. They are also used as friction generators in vehicle brakes and clutches.

Article index

  • one

    Structure

  • two

    Nomenclature

  • 3

    Properties (edit)

    • 3.1

      Physical state

    • 3.2

      Molecular weight

    • 3.3

      Melting point

    • 3.4

      Density

    • 3.5

      Solubility

    • 3.6

      pH

    • 3.7

      Other properties

  • 4

    Obtaining

  • 5

    Applications

    • 5.1

      In obtaining bricks

    • 5.2

      In Portland cement

    • 5.3

      To immobilize radioactive garbage

    • 5.4

      As an insulator for buildings

    • 5.5

      To reduce pollution in the metallurgical industry

    • 5.6

      In biomaterials

    • 5.7

      In the biodentine

    • 5.8

      Other apps

  • 6

    References

Structure

Calcium silicate can contain a variable amount of calcium oxide (CaO) and silica (SiO 2 ). Its general formula is:

xCaO • Ysio 2 • zH 2 O

where x, y, and z are numbers that can have various values.

The amount of CaO must be between 3% and 35% (by weight on dry basis) and the SiO 2 content must be between 50-95% (by weight on dry basis). 
They can be anhydrous (without water in its structure, that is, z = 0 in the formula) or they can be hydrated (with water is its conformation).

Nomenclature

  • Calcium silicate
  • Silicic acid calcium salt
  • Calcium oxide and silicon

Properties (edit)

Physical state

Very fine white or off-white solid.

Molecular weight

Calcium metasilicate CaO • SiO 2 or CaSiO 3 = 116.16 g / mol

Melting point

Calcium Metasilicate CaSiO 3 = 1540 ° C

Density

Calcium Metasilicate CaSiO 3 = 2.92 g / cc

Solubility

Insoluble in water and ethanol.

pH

A mud prepared with 5% calcium silicate can have a pH of 8.4-12.5.

Other properties

Calcium silicate can be hydrated (with water in the molecule) or anhydrous (without water in the molecule) with various proportions of calcium in the form of calcium oxide CaO and silica in the form of silica dioxide SiO 2 .

It has a high water absorption capacity. Calcium metasilicate (CaO • SiO 2 or CaSiO 3 ) stands out for its brilliance and whiteness, low humidity, low volatile content and good oil absorption.

Calcium Metasilicate CaSiO 3 . Ondřej Mangl / Public domain. Source: Wikimedia Commons.

Among the calcium silicate hydrates, those formed by adding water to Ca 2 SiO 5 and Ca 3 SiO 5 are distinguished . The hydration products of these two compounds are the most abundant in certain types of cement.

Obtaining

Calcium silicate is made in various ways by reacting siliceous material (such as diatomaceous earth) and calcium compounds (such as calcium hydroxide (Ca (OH) 2 ).

Calcium silicate can be prepared for example by calcining calcium oxide (CaO) with silica (SiO 2 ) at elevated temperatures.

When the reaction is carried out at a 1: 1 molar ratio (this means that there are the same number of CaO molecules as SiO 2 ), the calcium metasilicate CaSiO 3  or CaO • SiO 2 results :

CaO + SiO 2 + heat → CaSiO 3

Applications

In obtaining bricks

With calcium silicate units or bricks for construction are made. They are obtained with fine siliceous material and quicklime or hydrated lime. Inert pigments can be added to give the brick a different color.

Units are molded under pressure and cured in an autoclave (steam oven) at 170 ° C for 4-6 hours. During curing some of the lime reacts with the siliceous material to form a calcium silicate hydrate, which holds the brick together.

Calcium silicate bricks. Holger.Ellgaard / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0). Source: Wikimedia Commons.

However, calcium silicate bricks tend to expand and shrink more than clay bricks, which can sometimes cause cracking of the masonry.

This has attracted attention and they have been considered potentially dangerous.

In Portland cement

Calcium silicates are part of Portland cement, which is a widely used material in the construction industry.

Portland cement is a hydraulic cement that is produced by pulverizing materials formed mainly by hydrated calcium silicates and calcium sulfate CaSO 4 (gypsum).

Surface with cement. Cement contains calcium silicates in its composition. Author: Pexels. Source: Pixabay.

It hardens quickly due to the hydration reaction that generates a hydrated calcium silicate gel. This results in a strong, dense and low permeable material (that does not let water through).

The silicates it contains are tricalcium silicate Ca 3 SiO 5 or 3CaO.SiO 2  and dicalcium silicate Ca 2 SiO 4 or 2CaO.SiO 2 .

To immobilize radioactive garbage

Calcium silicates in cement can vary in their percentage by weight. The composition of Portland cement can change depending on the type of construction structure for which it is intended.

Some types of this cement are used to immobilize radioactive waste so that it does not cause harm to people or the environment.

As an insulator for buildings

Calcium silicate is used to obtain mineral foam boards or insulating mineral boards.

Calcium silicate sheets. Achim Hering / CC BY (https://creativecommons.org/licenses/by/3.0). Source: Wikimedia Commons.

These serve to insulate the walls from moisture. CaO and SiO 2 are mixed with water and 3-6% cellulose is added, which improves the flexibility and stability of the edges.

The resulting sludge is poured into molds and then heated with steam at high pressure and temperature in a special steam oven called an autoclave.

The result is a rigid, very fine-pore foam that is cut into sheets or boards and treated with special additives so that it can repel water.

Calcium silicate foam is used in the construction sector, especially to insulate walls and improve protection against humidity, being useful especially in the renovation of old buildings.

To reduce pollution in the metallurgical industry

Dicalcium silicate Ca 2 SiO 4 or 2CaO.SiO 2 found in slag or waste from steel production has been used to precipitate dissolved metals in acid effluents from other metallurgical processes.

Precipitating means that the dissolved metal becomes part of a solid compound that goes to the bottom of the container and can be collected.

Some wastes from the steel industry have calcium silicates useful for precipitating metals from acidic solutions. Author: Skeeze. Source: Pixabay.

The Ca 2 SiO 4 present in steel slag reacts with water and produces Ca (OH) 2 which has the ability to neutralize the acidity of acid solutions of metals from other processes:

2 Ca 2 SiO 4 + 4 H 2 O → 3CaO. 2SiO 2 .3H 2 O + Ca (OH) 2

In addition to neutralizing, the calcium silicate compound can adsorb some of the M 2+ metal ions by exchange with the calcium ion Ca 2+ . Here is an outline:

≡Si-O-Ca + M 2+ → ≡Si-OM + Ca 2+

The solid compound that contains the metal can then be used for another purpose and is not discarded. This is an example of industrial ecology.

In biomaterials

Calcium silicate ceramics began to be tested as biomaterials since 1990. They have been studied for their potential use in the regeneration of bone tissue because they possess superior bioactivity than other materials.

This is attributed to the fact that they have silicon (Si), which plays an essential role in the mechanisms that lead to the formation of new bone.

Calcium silicate-based cements have the ability to induce the formation of a calcium phosphate / apatite coating when immersed in biological fluids and promote tissue regeneration.

Calcium silicates can function as a base for biomaterials that allow repairing bones. https://www.scientificaimations.com/ / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

For these reasons it is being considered as a suitable material for bone repair.

In the biodentine

Calcium silicate is part of biodentin. This is a material that is used to repair tooth perforations, bone resorptions and as a filling for the end of the roots of the teeth.

Biodentin is a low-porosity, bioactive cement that has greater mechanical strength or hardness than other materials and resembles dentin.

Calcium silicates are part of materials used to cover perforations in teeth. Jak / Public domain. Source: Wikimedia Commons.

It is made up of tricalcium silicate (Ca 3 SiO 5 ), dicalcium silicate (Ca 2 SiO 5 ), calcium carbonate (CaCO 3 ) and zirconium oxide. 
When mixed with water, the calcium silicates form a sticky hydrated gel that after a time solidifies creating a hard structure.

It exerts a positive effect on the cells of the dental pulp and accelerates the formation of bridges in the dentin, where the strength of its bonds, its microhardness and resistance to compression stand out.

Tube with calcium silicate to cure teeth. Shaimaa Abdellatif / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

Other apps

Calcium silicates are also used as anti-caking agents and filter aids.

Calcium metasilicate CaSiO 3 is used in ceramics, in devices where friction is required, such as vehicle brakes and clutches, and in obtaining metals.

Due to its high brightness and whiteness, CaSiO 3 is used in fillers for paints and plastics.

References

  1. Ropp, RC (2013). Group 14 (C, Si, Ge, Sn, and Pb) Alkaline Earth Compounds. Calcium Silicates. In Encyclopedia of the Alkaline Earth Compounds. Recovered from sciencedirect.com.
  2. FAO (2015). Calcium Silicate. Specifications prepared at the 80 th JECFA (2015) and FAO JECFA Monographs published in 17. Recovered from fao.org.
  3. Harrisson, AM (2019). Constitution and Specification of Portland Cement. Calcium Silicate Hydrate. In Lea’s Chemistry of Cement and Concrete (Fifth Edition). Recovered from sciencedirect.com.
  4. Gellert, R. (2010). Inorganic mineral materials for insulation in buildings. Calcium silicate foam and mineral foam. In Materials for Energy Efficiency and Thermal Comfort in Buildings. Recovered from sciencedirect.com.
  5. Goudouri, OM. et al. (2016). Cgaracterizing the degradation behavior of bioceramic scaffolds. Apatite / wollastonite scaffolds. In Characterization and Design of Tissue Scaffolds. Recovered from sciencedirect.com.
  6. Rani, P. et al. (2019). Dental pulp capping nanocomposites. Biodentin. In Applications of Nanocomposite Materials in Dentistry. Recovered from sciencedirect.com.
  7. Ingham, JP (2013). Concrete products. Calcium silicate units. In Geomaterials Under the Microscope. Recovered from sciencedirect.com.
  8. Ojovan, MI and Lee, WE (2005). Immobilization of Radioactive Wastes in Cement. Hydraulic Cements. In An Introduction to Nuclear Waste Immobilization. Recovered from sciencedirect.com.
  9. Ramachandra Rao, S. (2006). Resource Recovery and Recycling from Metallurgical Wastes. Calcium Silicate as Precipitant for Dissolved Metals. In Waste Management Series. Recovered from sciencedirect.com.
  10. Prati, C. and Gandolfi, MG (2015). Calcium silicate bioactive cements: Biological perspectives and clinical applications. Dent Mater, 2015 Apr; 31 (4): 351-70. Recovered from ncbi.nlm.nih.gov.

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