Special Bricks | CE 011

Special Bricks

Special bricks are those made by some special technique and/or with some special material to fulfill some special purposes. These bricks differ from conventional or modular bricks in respect of anyone or more of size, material, quality or manufacturing process. Compressed mud block, hollow clay bricks, hollow concrete block, solid mortar block, fly ash bricks, fire bricks or refractory bricks, acid resistance bricks, terracotta etc. are examples of special bricks.

Compressed Mud Blocks

Compressed mud blocks are made up of the same clay by which bricks are manufactured, have the same size as that of bricks but are not called bricks since these are not burnt. It is just the normal and readily available clay by which the compressed mud blocks are made. But an improved result in strength is obtained if 3 to 5% slaked lime is mixed with the clay. Other ingredients like different types fibers may also be used along with lime. Sometimes 10 to 20% fly ash is also mixed with mud for a good result. Well mixed clay of desired ingredients and proportion is poured into the block/tray of a pressing machine and then pressed by a ram. The intensity of pressure is in the tune of 20 - 25 MPa i.e., 3,000 – 3,500 psi. A variety of machines are available in market for production of compressed mud blocks. The options include single unit or multi-unit production, manually, fuel operated or electric operated machine etc. The production cost of compressed mud block is much less than the conventional brick and, hence, it is a good building material for low cost construction.

Hollow Clay Bricks

Hollow clay bricks are specially made light weight burnt clay bricks with thorough multiple perforations from one end to other. Since these bricks are made in very controlled condition, these are very well and uniformly burnt. Hollow bricks are available in a variety of sizes depending on place of manufacturing but a widely used size is 400 x 200 x 150 mm i.e., 16 x 8 x 6 in. These are normally used to make filler walls in RCC framed structure buildings but are not used as a load bearing wall. Due to having perforations, hollow bricks are very effective as a heat proof and acoustic proof material. Since it is light weight material, it creates lesser dead load in structure which in turn reduces the overall construction cost but at the same time offers lesser resistance to earthquake shocks.

Hollow Concrete Block

Hollow concrete blocks, also known as concrete masonry units or CMU are alike hollow clay bricks but with some differences. These blocks are made up of concrete made up of ordinary Portland cement, sand and 6-10 mm or quarter to 3/8 inch stone chips. Sometimes, cement, cinder and stone chips or only cement and cider are also used instead. In that case, they are also called as cinder block. For a better ductility, steel meshes may also be inserted. Hollow concrete blocks basically contain a pair of hollow cores from top to bottom. Apart from that, two sides of blocks are made like wings so that after joining two blocks with mortar, another hollow portion may be created. Common nominal size of these blocks is 400 x 200 x 200 mm or 16 x 8 x 8 inch but this size also varies a little from place to place or country to country. The thickness of the block walls is normally 40-50 mm or 1.5 to 2 inches. Similar to hollow clay bricks, these blocks are also light weight, heat proof and acoustic proof.

Solid Mortar Block

At present, the supply of conventional or traditional bricks has been greatly reduced due to paucity of suitable raw materials and manufacturing hazards like scarcity of coal. This deficiency of supply and demand has been mitigated by the production of solid mortar blocks which are now widely used instead. The manufacturing facility of these blocks over bricks is that these can be made under the shed in a factory – without any fire – and without caring the seasonal obligations. These blocks are made up of cement, sand and 6-12 or quarter to half inch size stone chips. Stone dust and cinders are also used sometimes as a fine aggregate. In that case, these are also called cinder blocks. Nominal size of these blocks are 400 x 200 x 300 mm or 400 x 200 x 200 mm or 400 x 200 x 100 mm i.e., 16 x 8 x 12 inch or 16 x 12 x 8 inch or 16 x 12 x 4 inch. Here also, the variations in size are noticed from place to place or from country to country.  

Fly Ash Bricks

Huge amount of fly ash are produced as a wastage material of thermal power plants. Bricks that are made by utilizing this fly ash are called fly ash bricks. A mixture of fly ash (90 – 95%) and lime or cement (5–10%) is first compressed and then dried to produce fly ash bricks. These bricks have lower engineering properties compared to conventional bricks.

Fire Bricks

Fire bricks are special types of bricks used for lining of walls of furnace, boilers, chimneys or combustion chamber to protect those from high heat and temperature. It is made up of fire clay or refractory clay burning at high temperature in special types of kilns. Fire bricks are normally white or yellowish in color, weighing about 2500 kg/cum, crushing strength of about 12 – 15 N/mm² and water absorption of about 5 to 7 per cent.

Acid Resistant Bricks

Acid resistant bricks or acid proof bricks, also known as chemical resistant bricks are used to protect structures from acid attack, chemical attack or corrosion and hence are not needed for structures of normal environment. It is mainly used in acid producing factories or pharmaceutical factories or alike as a lining wall. However, it sometimes appears as weak against hydrofluoric acid and strong alkali. It is manufactured from silica shale (a special type of sedimentary rock) by dint of prolonged baking of mix paste in very high temperature. Acid resistant bricks have the same nominal size as that of conventional bricks but have a varieties of colours ranging from cream to greyish to grey to brown to red. It’s worth mention here that these bricks are very costly – double or more than double than that of conventional bricks.

Terracotta Bricks

Terracotta bricks are made up of a special type of soil - brown to red in colour and available only in some selected places. In our vicinity, such kind of soil is available in a place called Bankura. In earlier days, terracotta soils were used to make handcrafted decorative items and wall tiles for lining in part of external and internal walls. At present, different types of terracotta bricks – some of which are solid and some are hollow - are available on line but are not so easily available in local market. These bricks are manufactured by various renowned companies in a similar manner as that of conventional bricks but quality control – from mixing to burning - is done in very efficient manner. Present terracotta bricks are available in a variety of sizes. Although costs of these bricks are at par or a little higher than those of conventional bricks, transportation cost becomes very high and hence ultimately appears to be uneconomical for construction of common buildings of general people.

AAC Block / AAC Bricks

The full form of AAC Block or AAC Brick is Autoclave Aerated Concrete Block or Autoclave Aerated Concrete Brick. These are also known as ACC (autoclave cellular concrete) Blocks or ALC (autoclave lightweight concrete) Blocks. These are actually precast blocks, made up of lightweight and porous foam concrete, invented in Sweden in 1920. Cement, quartz sand, water, calcined gypsum and aluminium powder are basic raw materials for the production of foam concrete. Pulverized fly ash is also mixed sometimes as an alternate or additional material. At present, different renowned companies manufacture AAC blocks and sell these all over the world under various brand names. AAC blocks are available in various sizes – length and width are normally 600 x 200 mm or 24 x 8 inches but the thickness may vary from 100 to 300 mm or 4 to 12 inch. AAC blocks are very light weight, heat proof, cold proof, acoustic proof and soft. Since it is soft, it can be cut into pieces of any size with cutting tools. Since it is bigger in size, construction time required to fill up an area becomes much less in comparison to conventional bricks. Being lightweight, it reduces the overall dead load of the structure. The quality of these blocks becomes very good since these are produced in factory under good quality control. Cost of these blocks are at par or even lower than conventional bricks but since it is not available everywhere, transportation cost may be well worth accountable. Whatever be the economical aspect, the ground reality is that it is still not popular or even known to the general people and use of it is still limited to high rise buildings of metropolitan cities.

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