Basic chemistry

Calcium silicate: properties, structure, production, uses

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

They are white, white or yellowish solids. They can be anhydrous, meaning no water (H 2 O) in their structure, or they can contain it. They are part of many types of minerals in nature.

Calcium silicates are insoluble in water, but when joined to it they form hydrated gels (materials such as gelatin) which, when curdled, are very hard, resistant and almost impermeable.

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 cure perforations in teeth and have even been studied for use in bone regeneration, that is, as a biomaterial.

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

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 different values.

The amount of CaO should be between 3% and 35% (by weight, on a dry basis) and the SiO 2 content should be between 50-95% (by weight, on a dry basis). They can be anhydrous (no water in their structure, ie z = 0 in the formula) or they can be hydrated (with water is their conformation).

Nomenclature

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

properties

physical state

Very fine white or off-white solid.

Molecular weight

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

Fusion point

CaSiO 3 metasilicate of calcium = 1540 ° C

Density

Metasilicate of calcium CaSiO 3 = 2.92 g / cc

Solubility

Insoluble in water and ethanol.

pH

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

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

Obtaining

Calcium silicate is obtained in several 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 in a molar ratio of 1:1 (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

Forms

In obtaining bricks

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

Units are pressure molded 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.

However, calcium silicate bricks tend to expand and shrink more than clay bricks, which can sometimes cause cracks in masonry.

This drew attention and was considered potentially dangerous.

In Portland cement

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

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

It hardens quickly due to the hydration reaction generated by a hydrated calcium silicate gel. This results in a material that is strong, dense and poorly permeable (which does not allow the passage of water).

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

Immobilize radioactive waste

Calcium silicates in cement can vary in percent by weight. The composition of Portland cement can change depending on the type of building structure it is intended for.

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

As insulation for buildings

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

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

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 foam with very fine pores that is cut into sheets or boards and treated with special additives to be able to repel water.

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

To reduce pollution in the metal industry

The dicalcium silicate Ca 2 SiO 4 or 2CaO.SiO 2 found in slag or steel production residues 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 sinks to the bottom of the container and can be collected.

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 acidic metallic solutions 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 absorb some of the metal ions M2 + exchanging it with the calcium ion Ca2 + . Here is an outline:

≡Si-O-Ca + M 2+ → ≡Si-OM + Ca 2+

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

In biomaterials

Calcium silicate ceramics have been tested as biomaterials since 1990. They have been studied for their potential use in bone tissue regeneration due to their superior bioactivity compared to other materials.

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

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

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

In biodentin

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

Biodentin is a bioactive cement with low porosity, which has greater mechanical resistance or hardness than other materials and is similar to dentin.

Related:   Carbon atom: characteristics, structure, hybridization

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

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

Other apps

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

CaSiO 3 calcium metasilicate is used in ceramics, in devices where some friction is required, such as vehicle brakes and clutches, and in the production of metals.

Due to its high gloss and whiteness, CaSiO 3 is used to fill paints and plastics.

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