Technology[edit | edit source]

General

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 at 850 - 1000° C, in which the particles are bonded together by a glassy ceramic material.

Advantages
+ Fired clay products can have high compressive strengths, even when wet' making them resistant to impact and abrasion. The excellent condition of many ancient brick constructions clearly demonstrates the durability of fired clay products.
+ 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.

Problems
- 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 kilos 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 possible defect of burnt bricks is "efflorescence", which appears temporarily on the surface of the brick, and is caused by soluble salts inherent in the clay or process water.

Procedures

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. These operations are well documented in a number of publications (see Select Bibliography) and the reader is advised to refer to them for details.

Soil Selection

  • A large variety of soils are suitable for this process, the essential property being plasticiy 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.
  • Soil selection is not only a matter of experience, simple field tests and subsequent laboratory tests are vital.

Clay Testing

  • The list of tests is long and not all are needed for each soil type and use.
  • The main field tests are by sight, smell, touch, by making balls, ribbons and threads, by sedimentation in a glass jar and by dropping.
  • Laboratory apparatus is needed for particle size analysis by sieving, for determining shrinkage, plasticity, dry strength, compressibilty, optimum moisture content and cohesion.
  • As a rough guideline, the minimum clay contents required for the production of:
    - bricks is 40 %, and for
    - tiles is 60 % clay.

Experience and expert advice is required to determine the optimum clay content, as high percentages can lead to shrinkage and cracking.

Clay Winning

  • 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). It must, however, be remembered that the fertile topsoil required for agriculture is not used for brickmaking. These 30 to 50 cm of soil have to be removed before excavating the clay for brickmaking.
  • 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.

Clay Preparation

  • 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.
  • Sieving is needed, but laborious. However, simple labour-intensive crushing machines have been developed to get particles less than 5 mm for bricks and less than 0.5 mm for roof tiles.
  • Proportioning of different clays is required if the clay content or grain size distribution is unsatisfactory. However, in some cases, rice or coffee husks or saw dust, which serve as a fuel, are added to the clay, in order to obtain lighter burnt bricks and also to prevent the freshly moulded, highly plastic clay from cracking during the drying process.
  • 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 allowing 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, improving its moulding characteristics. The clay must be kept covered to prevent premature drying.

Moulding

  • All fired clay products require some form of compaction, either by dynamic compaction (throwing, tamping) or static compaction (with mechanical or hydraulic equipment). The principal methods are:
  • Hand moulding systems: using wooden moulds, moulding tables or manually operated presses; and
  • Mechanized systems: using motorized presses with mechanical or hydraulic energy transmission, or extruders, which can be partly hand operated or fully automatic.

Drying

  • 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; therefore, if necessary, the clay proportion must be reduced by adding sand or grog (pulverized brick rejects).

Burning

  • 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 10 000 to 100 000 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 very sophisticated and capital-intensive.
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Authors Eric Blazek
License CC-BY-SA-3.0
Language English (en)
Related 0 subpages, 17 pages link here
Aliases Clay Brick and Tile Moulding Equipment 3
Impact 315 page views
Created April 7, 2006 by Eric Blazek
Modified December 10, 2023 by Felipe Schenone
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