Fired clay units are made by forming the unit from moist clay by pressing, extrusion or casting followed by drying and firing (burning) to a temperature usually in the range 850–1300°C. During the firing process complex chemical changes take place and the clay and other particles that go to make up the brick are bonded together by sintering (transfer of ions between particles at points where they touch) or by partial melting to a glass.
During the drying and the firing processes the units generally shrink by several per cent from their first-made size and this has to be allowed for in the process. Some clays contain organic compounds, particularly the coalmeasure shales and the Oxford clay used to make Fletton bricks. Some clays are deliberately compounded with waste or by-product organic compounds since their oxidation during firing contributes to the heating process and thus saves fuel. The burning out of the organic material leaves a more open, lower density structure.
The ultimate example of this is ‘Poroton’, which is made by incorporating fine polystyrene beads in the clay. Wood and coal dust can be used to achieve a similar effect in some products.
Forming and Firing
Soft Mud Process
The clay is dug, crushed and ground then blended with water using mixers to make a relatively sloppy mud. A water content of 25–30% is required for this method. In some plants other additives may be incorporated such as a proportion of already fired clay from crushed reject bricks (grog), lime, pfa, crushed furnace clinker and organic matter to act as fuel.
In the well-known yellow or London Stock brick, ground chalk and ground refuse are added. The mud is formed into lumps of the size of one brick and the lump is dipped in sand to reduce the stickiness of the surface. In the traditional technique the lump is thrown by hand into a mould and the excess is cut off with a wire. This gives rise to the characteristic ‘folded’ appearance of the faces of the brick caused by the dragging of the clay against the mould sides as it flows.
Nowadays most production is by machine, which mimics the hand-making process. These bricks usually have a small frog (depression) formed by a raised central area on the bottom face of the mould. Because of the high drying shrinkage of such wet mixes and the plasticity of the unfired (green) brick the size and shape of such units are fairly variable. This variability adds to their ‘character’ but means that precision brickwork with thin mortar beds is not feasible.
The finished brick is also fairly porous, which improves its insulation properties, and, paradoxically, its effectiveness as a rain screen, but limits its strength.
Stiff Plastic Process
The clay is dug, crushed and ground then blended with water using mixers to make a very stiff but plastic compound with a water content of 10–15%.
This is then extruded from the mixer and cut into roughly brick-shaped pieces and allowed to dry for a short period before being pressed in a die. The clay is very stiff so when ejected straight from the mould it retains very precisely the shape of the die.
The low moisture content means that the shrinkage is low and therefore the size is easier to control and the drying time is relatively short. Another advantage is that the unfired brick is strong enough to be stacked in the kiln or on kiln cars without further drying. This type of unit will usually have at least one shallow frog and may have frogs in both bed faces.
The process is used to produce engineering bricks, facing bricks, bricks with very accurate dimensions and pavers.
Wire Cut Process
Clay of intermediate consistency is used, with a moisture content of 20–25%, and the clay is extruded from a rectangular die with the dimensions of the length and breadth of the finished unit.
The ribbon of clay, the ‘column’, is then cut into bricks by wires set apart by the height of the unit plus the allowance for process shrinkage. The cutting machines are usually arranged such that the group of cutting wires can travel along at the same speed as the column while a multiple cut is made. This means that the process is fully continuous and the cut is perpendicular to the face and ends of the unit. A plain die produces a solid column with just the characteristic wire-cut finish and these bricks will have no depressions in their bed faces.
In this process it is easy, however, to include holes or perforations along the length of the column by placing hole-shaped blockages in the die face. This has the following advantages:
- Reduction in the weight of clay required per unit so transport costs at every stage of the production and use of the units and all clay preparation costs, i.e. for shredding, grinding, mixing, etc., are also reduced.
- A reduction in the environmental impact by reducing the rate of use of clay deposits and therefore the frequency of opening up new deposits.
- Reduction in the mass and opening up of the structure of the units thus speeding up drying and firing, cutting the fuel cost for these processes and reducing the capital cost of the plant per unit produced.
- Oxidation of organic matter in the clay is facilitated by increasing the surface area to volume ratio, and reduces the chance of blackhearting.
- The thermal insulation is improved. This has a modest effect for UK-Standard size bricks but the improvement can be substantial for large clay blocks.
- The units are less tiring to lay because of their lower weight.
Because of these factors and the very large proportion of the production cost spent on fuel, most clay units are perforated at least to the extent of 10–25% by volume. It should be stressed that there is a penalty in that the clay must be very well ground and uniform in consistence for successful production of perforated units.
Any lumps or extraneous air pockets can ruin the column. To improve consistency the mixers are commonly heated and the front of the extruder is de-aired (evacuated) to minimise air bubbles. Figure 1 illustrates the production of a typical three-hole perforated brick by stiff plastic extrusion and wire-cutting.
This is one of the simplest processes for forming bricks. In the UK only Lower Oxford clay (or shale) is used; this comes from the Vale of Aylesbury and runs in a band towards the east coast. It contains about 7% natural shale oil, which reduces the cost of firing but does give rise to some pollution problems.
The clay is dug and then milled and ground to pass through a 2.4 mm or 1.2 mm sieve without altering the water content markedly from that as-dug (8–15%). The coarser size is used for common bricks and the finer for facings. The powdered clay is then fed into powerful automated presses, which form a deepfrogged, standard size brick known as a Fletton (named after one of the early manufacturers).
Unmodified Flettons are limited to a single barred pink/ cream colour. Flettons intended as facing bricks will either be mechanically deformed to give a patterned surface (rusticated) or have an applied surface layer, such as coloured sand, fired on.
Drying and Firing in Hoffman Kilns
The Hoffman kiln is a multi-chamber kiln in which the bricks remain stationary and the fire moves. It is mainly used for the manufacture of Flettons.
In the classic form it consists of a row of chambers built of firebricks in the form of short tunnels or arches. In the most efficient form the tunnels form a circle or oval shape and are connected together and to a large central chimney by a complex arrangement of ducts. A single ‘fire’ runs round the circle and at any one time one chamber will be being loaded or ‘set’ ahead of the fire and one will be being unloaded behind the fire.
The bricks are stacked in the kiln in groups of pillars termed ‘blades’, which leaves lots of spaces between units to enable the gases to circulate freely. Chambers immediately in front of the fire will be heating up using the exhaust gases from the hottest chamber, and those further ahead will be being dried or warmed by gases from chambers behind the fire that are cooling down.
Figure 2 illustrates the broad principles of the system, showing only the ducts in use for the fire in one position. In practice the ducts are positioned to ensure a flow through from the inlets to the outlets.
The whole process is very efficient, particularly as a large proportion of the fuel is provided by the oil in the bricks themselves. Because of the organic content of the bricks the firing has to be carried out under oxidising conditions during the last phases in order to burn out the oil.
If this is not done the bricks have a dark unoxidised central volume, known as a blackheart, which can give rise to deleterious soluble salts. During this phase some fuel is added to maintain the temperature. This is essentially a batch process and the average properties of the contents will vary a little from chamber to chamber.
Additionally, the temperature and oxidising condition will vary with position in the chamber, thus some selection is necessary to maintain the consistency of the product.
Drying and Firing in Tunnel Kilns
Tunnel kilns are the complement of Hoffman kilns in that the fire is stationary and the bricks move through the kiln as stacks on a continuous train of cars on rails (Fig. 3).
In practice a long insulated tunnel is heated in such a way that the temperature rises along its length, reaches a maximum in the centre and falls off again on the other side. To maximise efficiency only the firing zone is fuelled and hot gases are recycled from the cooling bricks and used to heat the drying and heating-up zones of the kiln.
Most extruded wire-cuts and stiff plastic bricks are now fired in such kilns, which are continuous in operation. Stocks and other mud bricks may also be fired this way after a pre-drying phase to make them strong enough to withstand the stacking forces.
Clamps are the traditional batch kilns, comprising a simple insulated refractory beehive-shaped space with air inlets at the base and a chimney from the top and fired using solid fuel.
These are the modern version of the clamp, where the units are fired in batch settings using oil or gas as a fuel. They are now only used for the production of small runs of specially shaped brick ‘specials’, some examples of which are shown in Fig. 3.
Clay bricks probably have the widest range of strengths of any of the manufactured masonry materials, with compressive strengths ranging from 10 MPa for an under-fired soft mud brick to as much as 200 MPa for a solid engineering brick.
The compressive strength is measured by a crushing test on whole units with the stress applied in the same direction as the unit would be loaded in a wall. Solid and perforated units are tested as supplied, but frogs are normally filled with mortar as they would be in a wall.
The European test method EN772-1 (2000) uses either mortar capping or face grinding to achieve even loading. The quoted strength is the average of six to ten determinations of stress based on the load divided by the area of the bed face.
The flexural strength and modulus of elasticity are not normally designated test parameters in unit standards, but it is important to obtain data for finite element analysis models. A standard three-point bending method is included in RILEM LUM A.2 (1994), with linear elastic analysis used for the calculation of the maximum flexural stress.
Other important properties are the dimensions, water absorption and porosity, initial rate of water absorption, density and soluble salts content, which can be measured by methods specified in EN772-16 (2000), EN772-7 (1998), EN772-11 (2000), EN772-13 (2000) and EN772-5 (2001), respectively.
In these standards the dimensions are measured for individual units with allowed individual tolerances for each replicate. Water absorption and porosity are measured in the same way as for mortar, except that the preferred saturation technique is to boil the units in water for 5 hours.
The initial rate of water absorption (IRWA or suction rate) is measured to give some idea of the effect of the unit on the mortar. Units with high suction rates need very plastic, high water:cement ratio mortars whereas units with low suction rates need stiff mortars. IRWA is determined by standing the unit in 3 mm depth of water and measuring the uptake of water in 60 seconds. The IRWA is calculated from:
wi = (m2 − m1)/Lb
where: wi = initial rate of absorption (IRWA); m1 = initial mass of the unit/specimen; m2 = mass after 60 seconds of water absorption; L = length of the bed (mortar) face to ± 0.5%; and b = width of the bed (mortar) face to ± 0.5%.
The result is normally given in units of kg/m2 /min. The content of water soluble salts is measured by standard wet chemical analysis techniques or by modern instrumental techniques such as flame photometry. The elements and compounds of concern are sulphates, sodium, potassium, calcium and magnesium.
Table 1 gives typical values and ranges for some of the key properties of clay bricks. In most brickwork, bricks are loaded upon their normal bed face but often they are loaded on edge or on end. Typical examples are headers and ‘soldiers’ in normal walls, stretchers in arches and reinforced beams and headers in reinforced beams. While unfrogged solid bricks show a small variation in strength for loading in different directions, owing to the change in aspect ratio (height:thickness), perforated, hollow or frogged units may show marked differences, as illustrated in Table 2.
Taking the simplest geometry as an example it can be seen from Fig. 4 that the minimum crosssectional area of the 5-slot unit resisting the load will be 80% on bed but 71% on edge and 25% on end. The ratios of the strengths in Table 2 follow approximately the ratios of the areas.
Other factors such as the slenderness of the load-bearing sections and the effect of high local stresses at rectangular slot ends complicate the behaviour, and may explain the variations between different types.
It can also be shown that porosity in the form of vertical perforations results in a smaller reduction in the strength of a material than does generally distributed porosity, and is the more efficient way of reducing the weight. This is illustrated in Fig. 5. More detailed information on clay brick properties is contained in BRE Digest 441 parts 1 and 2 (1999).
Calcium Silicate Units
Calcium silicate units are manufactured from firm mixtures of lime, silica-sand and water. Aggregates such as crushed rocks or flints may be incorporated to alter the performance and appearance, and pigments may be used to vary the colour.
Common colours are whites, blacks, buffs and grey-blues. Reds are produced but they seldom have the richness of fired clay units. There is only one basic process, in which the mixture is pressed to high pressures in a die in a static press, ejected, set on cars and then placed.