Composition of Bricks – What are bricks made of ??

Composition of Bricks – Bricks are rectangular units of construction material. Bricks are used in masonry construction, walls, and pavements. It is used as a substitute for stone, where the stone is not readily available. Brick chips are often used as coarse aggregate in the concrete mix.

Abstract: This Technical Note presents fundamental procedures for the manufacture of clay brick. The types of clay used, the three principal processes for forming brick and the various phases of manufacturing, from mining through storage, are discussed. Information is provided regarding brick durability, color, texture (including coatings and glazes), size variation, compressive strength, and absorption.

Key Words: absorption, clays, color, cooling, compressive strength, de-hacking, drying, durability, firing, forming, hacking, manufacturing, mining, preparation, shales, size variation, texture.

Composition of Bricks – What are bricks made of ??

For the preparation of bricks, clay or other suitable earth is molded to the desired shape after subjecting it to several processes. After drying, it should not shrink and no crack should develop. The clay used for brick making consists mainly of silica and alumina mixed in such a proportion that the clay becomes plastic when water is added to it. It also consists of small proportions of lime, iron, manganese, sulfur, etc. The proportions of the various ingredients are as follows:


Percentage in brick

Silica (SiO2) 55%
Alumina (Al2O3) 30%
Iron Oxide (Fe2O3) 8%
Magnesia (MgO) 5%
Lime(CaO) 1%
Organic Matter 1%

Clay is one of the most abundant natural mineral materials on earth. For brick manufacturing, clay must possess
some specific properties and characteristics. Such clays must have plasticity, which permits them to be shaped or
molded when mixed with water; they must have sufficient wet and air-dried strength to maintain their shape after
forming. Also, when subjected to appropriate temperatures, the clay particles must fuse together.

Natural clay minerals, including kaolin and shale, make up the main body of brick. Small amounts of manganese, barium, and other additives are blended with the clay to produce different shades, and barium carbonate is used to improve brick’s chemical resistance to the elements. Many other additives have been used in brick, including byproducts from papermaking, ammonium compounds, wetting agents, flocculents (which cause particles to form loose clusters) and deflocculents (which disperse such clusters). Some clays require the addition of sand or grog (pre-ground, pre-fired material such as scrap brick).

A wide variety of coating materials and methods are used to produce brick of a certain color or surface texture. To create a typical coating, sand (the main component) is mechanically mixed with some type of colorant. Sometimes a flux or frit (a glass containing colorants) is added to produce surface textures. The flux lowers the melting temperature of the sand so it can bond to the brick surface. Other materials including graded fired and unfired brick, nepheline syenite, and graded aggregate can be used as well.

Chief Ingredients of Brick and Their Functions

Silica (Sand) and Alumina (Clay), these two are the most prominent ingredients in brick clay. When mixed with water in proper proportions, it gains plasticity. The plastic mass can be easily molded and dried. It should not go through cracking, shrinkage or warping.


Alumina is the main constituent of clay. It acts as a cementing material in raw brick. Brick clay is plastic due to the presence of alumina. This plasticity ensures that bricks can be molded. An excess amount of alumina in clay may cause the bricks to shrink, warp or crack on drying and burning as any other cementing material.


Good quality bricks contain 50-60% silica. It is present in both free and combined form. As frees sand, it remains mechanically mixed with clay. In combined form, it reacts with alumina to form aluminosilicates. Silica prevents raw bricks from cracking, shrinking and warping. The higher the proportion of sand, the more and shapely and uniform in texture will be the brick. Although, excess silica destroys cohesion between the brick clay particles and makes brick brittle and weak. The durability of bricks largely depends upon the proper proportion of silica and alumina.


Bricks should contain a little amount of finely powdered lime. It enables silica (of a required portion) to melt at the furnace temperature of 1650oC and binds the particles of brick together resulting in strong and durable bricks. At about 1100o C, lime acts as a catalyst to elevate the furnace temperature to 1650oC at which silica fuses. This slightly fused silica works as a strong cementing material. Excess lime in brick clay will cause the vitrification of bricks. It causes bricks to melt, as more than the required amount of silica will fuse. The bricks then lose their shape and become disfigured.

Iron Oxide

Bricks contain a small quantity of Iron Oxide. Iron Oxide acts a flux like lime, thus helps silica to fuse at low temperatures. It imparts a red color to bricks upon burning. Iron also increases the durability and impermeability of the bricks.


A small proportion of magnesium decreases shrinkage and gives a yellow tint to the bricks. An excess amount of it causes bricks to decay.

Harmful Ingredients of Brick

Excess lime melts the bricks and disfigures it. If CaCO3 exists (in the purest form, i.e., if it contains at least 95% CaO) in lime-lump in brick clay, it converts into quicklime on burning. When these bricks come in contact with water, quicklime slakes and expands. And causes disintegration of bricks.

Alkalis are mainly salt of Sodium (Na) and Potassium (K). It acts as a flux in the kiln and causes fusion, warping, and twisting of bricks. Alkalis absorb moisture from the atmosphere and cause dampness & efflorescence in bricks (because of the presence of hygroscopic salts, e.g., CaCl2, MgCl2, etc.).

Pebbles, Stones & Gravels
Their presence does not allow thorough mixing of earth, thus the bricks produced are weaker. Such bricks cannot be broken at the desired section and they break very irregularly.

Iron Pyrites (FeS)
Iron Pyrites causes crystallization & disintegration of bricks while burning. It discolors bricks in the form of black slag.

Organic Matter
Organic matter in bricks makes bricks porous resulting in low density and weaker bricks.

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.

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