Fired clay products
The technique of firing clay to produce bricks and tiles for building construction is more than 4000 years old. It is based on the principle that clayey soils (containing 20 to 50 % clay) undergo irreversible reactions, when fired et 850 -1000° C, in which the particles are bonded together by a glassy ceramic material.
A large variety of soils are suitable for this process, the essential property being plasticity to facilitate moulding. While this depends on the clay content, excessive proportions of clay can cause high shrinkage and cracking, which is unsuitable for brickmaking. The qualities of fired clay products vary not only according to the type and quantity of other ingredients of the soil, but also to the type of clay mineral. For the production of good quality bricks and tiles, careful testing of soils is necessary.
Burnt brick production has reached a high level of mechanization and automation in many countries, but traditional small-scale production methods are still very widespread in most developing countries. Thus there is a great variety of non-mechanized and mechanized methods for clay winning, preparation, moulding, drying and burning, which can only be dealt with briefly in this manual.
• Clay deposits are found at the foot of hills or on agricultural land close to rivers (which naturally generates conflicting interests between the use of land for brickmaking and for agriculture).
• The criteria for choosing a suitable location are the quality of clay, availability of level ground and closeness of a motorable road for transports.
• Hand-digging in small and medium-sized production plants is usually done to a depth of less than 2 m. (After excavation of large areas, they can be returned to agricultural use.)
• Mechanical methods, using drag-line and multi-bucket excavators, are required for large-scale brickmaking plants. These methods require proportionately less excavating area, but make deep cuts in the landscape.
• This includes sorting, crushing, sieving and proportioning, before the material is mixed, wetted and tempered.
• Sorting is done by picking out roots, stones, limestone nodules, etc., or in some cases by washing the soil.
• Crushing is required because dry clay usually forms hard lumps. Manual pounding is common, but laborious. However, simple labour-intensive crushing machines have been developed (see ANNEX).
• Sieving is needed to remove all particles larger than 5 mm for bricks, or 0.6 mm for roof tiles.
• Proportioning is required if the clay content or grain size distribution is unsatisfactory. In some cases, rice husks, which serve as a fuel, are added to the clay, in order to obtain lighter and more uniformly burnt bricks.
• Thorough mixing is needed and a correct amount of water. Since manual mixing (traditionally by treading with bare feet) is laborious and often unsatisfactory, motor-powered mixers are preferred. The effort of mixing can be greatly reduced by at/owing the water to percolate through the clay structure for some days or even months. This process, known as "tempering", allows chemical and physical changes to take place, inproving its moulding characteristics. The clay must be kept covered to prevent premature drying.
• Moulding is done by hand or by mechanized methods.
• Hand-moulding methods make use of simple wooden moulds: the clay is formed into a clot, thrown into the mould, and the excess cut off.
• There are two traditional techniques for releasing the brick from the mould: a. the slop-moulding method, by which the mould is kept wet and the clay is mixed with more water, and b. the sand-moulding method, by which the clot is rolled in sand to prevent the clay from sticking to the mould.
• Bricks made by slop-moulding are vulnerable to slumping and distortion, while sand-moulding produces firmer, well-shaped bricks. Where sand is not available, finely ground clay can also be used, according to a technique developed at the ITW (Intermediate Technology Workshop in the United Kingdom).
• With table moulds (as developed by ITW, United Kingdom, and Central Building Research Institute, India), less effort, more accurately shaped bricks and higher outputs are achieved. While the moulding is done in the same way as with simple wooden moulds, the bricks are ejected by means of a foot-operated lever.
• Roofing tiles are made with specially shaped moulds, but principally in the same way as bricks. The main difference is that other material characteristics, with regard to uniformity, particle size and clay content, are needed.
• Mechanized brickworks use machines which extrude the clay through a dye to form a clay column, which is wirecut into brick-sized pieces. This method produces denser and stronger bricks, which can also be perforated.
• An intermediate solution is brick and tile moulding with mechanical compression. Two machines produced in Belgium (CERAMAN and TERSTARAM) were specially designed for this purpose, but are also used to make air-dried, stabilized soil bricks. Mechanical compression allows for considerably lower moisture contents, thus shortening the drying period.
• Green bricks are likely to be crushed in the kiln, under the weight of those piled on top; they can shrink and crack during firing; the water driven off can condense on cold bricks away from the heat source; or steam is developed, building up excessive pressures within the bricks; and, finally, too much fuel is required to drive out the remaining water. Hence, thorough drying is vital.
• Drying should be relatively slow, that is, the rate at which moisture evaporates from the surface should not be faster than the rate at which it can diffuse through the fine pores of the green brick. Air should have access to all sides of the bricks, so that they must be stacked with sufficient gaps between them.
• Natural drying is done in the open under the sun, but a protective covering (eg leaves, grass or plastic sheeting) is advisable to avoid rapid drying out. If it is likely to rain, drying should be done under a roof. But traditionally, bricks are only made in the dry season.
• Artificial drying (as in large mechanized plants) is done in special drying chambers, which make use of heat recovered from the kilns or cooling zones.
• Drying shrinkage is inevitable, and causes no special problems if below 7 % linear shrinkage. 10 % linear shrinkage should not be exceeded, thus, if necessary, the clay proportion must be reduced by adding sand or grog (pulverized brick rejects).
Typical clamp in India: The crushed coal, being screened in the foreground, is the fuel used. On the right are green bricks stacked for drying (Photo: K. Mukerji)
• There are two types of kilns for burning bricks: intermittent and continuous kilns.
• Intermittent kilns include clamps and scove kilns (traditional field kilns), updraught and downdraught kilns. Their fuel efficiency is very low, but they are adaptable to changing market demands. They vary in size from 10000 to 100000 bricks.
• Continuous kilns include various versions of the Hoffmann kiln (particularly the Bull's trench kiln) and the high-draught kiln. These are very fuel efficient. Tunnel kilns, in which the bricks are passed through a stationary fire, are too sophisticated and capital-intensive to be considered here.
• Clamps are basically a pile of green bricks interspersed with combustible material (eg crushed coal, rice husks, cow dung). Some holes are left at the base of the clamp, where the fire is lit. The holes are closed and the fire allowed to burn out, which can take a few days or several weeks. The bricks near the centre of the clamp will be the hardest. Sorting out is necessary, as about 20 to 30 % are not saleable. These are refired or used in the clamp base, sides or top.
• Scove kilns, plastered on all sides with mud, are principally the same as clamps, except that tunnels are built across the base of the pile, in order to feed additional fuel. This is the best method for burning wood.
• Updraught kilns (also known as Scotch kilns) function in the same way as scoves, except that the tunnels and walls are permanent.
• Downdraught kilns have a permanent arched roof. The hot gases from the fuel burnt ant the sides of the kiln, rise to the arched roof and are drawn down between the bricks by the chimney suction, through the perforated floor and out through the chimney.
• The Hoffmann kiln, which was originally circular but now more commonly oval, is a multi-chamber kiln in which the combustion air is preheated by cooling bricks in some chambers, and passes through the firing zone, from which the exhaust gases preheat the green bricks. While the cooled bricks are removed from one side of the empty chamber, green bricks are stacked on the other side. The fuel is fed from the top, through holes in the permanent arched roof. The daily output is about 10 000 bricks.
• The Bull's trench kiln operates on the principle of the Hoffmann kiln, except that the expensive arched roof is omitted and the exhaust gases are drawn off through 16 m high moveable metal chimneys with a wide base, which fit over the openable vent holes set in the brick and ash top of the kiln. The fuel, generally crushed coal, is fed in through the holes on the top. Depending on the size of the kiln, daily outputs can be between 10 000 and 28 000 bricks, 70 % of which being of high quality.
• The high-draught kiln is a further development of the Bull's trench kiln, whereby temporary cross-walls of green bricks leave openings on alternate sides, thus making the hot air travel a longer distance in a zigzag fashion, achieving a larger transfer of heat from a given quantity of fuel (wood and coal). Fans are installed to provide the necessary draught. Daily outputs of 30 000 bricks are possible.
• Wood, coal and oil are the main types of fuel used. Coal is suitable for all purposes, while wood is less suited for clamps and oil is not used for clamps, downdraught, Bull's trench and high-draught kilns.
Note: Figures in brackets mean that the fuel is not suitable for that kiln.
• Solid or perforated bricks of all shapes and sizes for standard masonry constructions, including foundations, floors, and load-bearing walls, arches, vaults and domes.
• Roof tiles of various shapes and sizes for roof slopes ranging between 1: 3 (18°30') and 1: 1 (45°).
• Floor tiles and facing bricks for waterproof and durable surface finishes, and for improving appearance.
• Special products, such as engineering bricks which have high densities and compressive strengths: refractory bricks, with high heat resistance,used for lining kilns and furnaces; acid resisting bricks and tiles to withstand chemical attack; pipes and channel elements for various purposes.
• Specially shaped, hollow clay blocks for composite reinforced concrete beam slabs (for ceilings and roofs).
• Brick rejects can be used to construct kiln walls, as a filler in wall or floor cavities, as an aggregate in concrete, or, when finely ground, underfired rejects produce a pozzolana (surkhi) and others produce grogs for brickmaking.
• Fired clay products can have high compressive strengths, even when wet, and are thus resistant to impact and abrasion.
• The porosity of fired clay permits moisture movement, without significant dimensional changes. Brick and tile constructions can "breathe".
• Solid bricks have a high thermal capacity, required for most climates, except for the predominantly humid zones; perforated bricks can be used (with perforations running vertically) for cavity walls, which provide thermal insulation, or (with perforations perpendicular to the wall face) for ventilation or screen walls.
• Fired clay products provide excellent fire-resistance.
• Bricks and tiles are weather resistant and can remain without any surface protection, thus saving costs. However, exposed brickwork is often considered unfinished and hence not always accepted.
• Poor quality and broken bricks are useable for other purposes, hence no wastage.
• The production process can be extremely labour-intensive and thus create many jobs, even for unskilled workers.
• Relatively high fuel consumption of the firing process. In many countries, where firewood is used, large forest areas have disappeared causing serious ecological damage. Where firewood is still available, it is usually extremely expensive, but this is also true for other fuels. Therefore, good quality fired clay products tend to be expensive.
• Simple field kilns do not always produce good quality and uniform bricks, and generally operate with very low fuel efficiency. Capital investments for fuel efficient kilns that produce good bricks are often too high for small-scale producers. They are also not justified, if continuous or large supplies of bricks are not required.
• A common defect of bricks is "lime blowing" (or "lime bursting"), a weakening or breaking of bricks, which is caused by the hydration of quicklime particles, derived from limestone in brickmaking clays.
• Another defect is "efflorescence", which appears temporarily on the surface of the brick, and is caused by soluble salts inherent in the clay or process water.
• Fuel efficiency is primarily dependent on the design of the kiln: continuous kilns retain the heat longest and utilize the heat from the cooling bricks, while the green bricks are preheated by the exhaust gases. Intermittent kilns have to heat up the entire heap anew, each time a batch is fired.
• Firewood should not be used up faster than it can be regrown. Hence plantations of fast-growing trees are vital. Considering their lower calorific value, larger numbers of fast-growing trees are needed than slow-growing trees. However, such plantations can be difficult to maintain in dry regions or when the rains fail.
• Agricultural wastes and other biomass, such as rice husks, coffee husks, papyrus, are useful and cheap (partial) substitute fuels. Mixing them with the clay helps to burn the bricks uniformly, avoiding unburnt cores.
• The Bull's trench and highdraught kilns have a fuel efficiency comparable to sophisticated, mechanized kilns. They are also cheaper to build than the Hoffmann kiln. It is, therefore, worth considering using the first batch of bricks from a clamp to build a more fuel efficient kiln, whereby the size is tailored to suit the local market demands. A certain minimum size is nevertheless needed to provide the requisite draught.
• Lime blowing can be minimized by reducing the particle size of the raw mix and firing at 1000° C. The addition of 0.5 to 0.75 % of common salt (sodium chloride) before firing has also proved effective. After firing, the bricks can be soaked in water for 10 minutes, during which the lime is slaked. The process, called "docking", is not always successful.
• Improvements are possible and greatly needed in all phases of brick manufacture, so that a good deal of research is still required to find simple, inexpensive methods for proper clay preparation, fast and uniform moulding, and - most important of all - maximum fuel efficiency.
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