Clay Bricks-

Chapter 3

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Bricks are blocks of tampered clay molded to suitable shape and size while it is in plastic condition, dried in the sun and burnt so as to make them strong, hard and durable.

Brick is a popular material of construction because of its local availability; strength, durability, and insulating property against heat and sound.

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Production Technology

Raw Material

Clay raw material for brick making comes from a wide variety of sources including bedded mud rock formations, hydrothermal altered volcanic rocks, river alluvium and colluvial soils.

Clays for the manufacture of bricks and other structural clay products such as tiles, hollow-blocks and sewer pipes differ from most industrial minerals.

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The quality of the bricks is highly dependent on the:

• The mineral composition of the different raw material inputs, • The energy fuel required for the calcinations of the bricks,• The type of the kiln used for firing, etc.

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Production Process

There are generally four basic stages in brick manufacturing:

Clay preparation, Molding, drying and Firing.

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Reactions occurring on firing brick clays:

Initially, at temperature up to 2000C, any moisture remaining in the ware after drying will be driven off.

Between 450-6000C the clay minerals lose their structural water (dehydration).

Any organic matter present may burn off at any temperature between 300 and 7000C.

Quartz undergoes a phase change at about 5700C.

Any carbonate minerals present will decompose to the oxide and carbon dioxide at temperature of 600-9000C. Release of carbon dioxide may cause an increase in porosity of the body.

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All the reactions described above occur below the conventional firing range of ceramic bodies.

Verification, or glass formation, may start at any temperature above 9000C, depending on the composition of the body. The body contract due to the formation of liquid (firing shrinkage) and the porosity is reduced. The solid phase reaction of aluminosilicates (clays) at temperature higher than 8000C results in the formation of mullite.

The firing cycle comprises a 24-hour ramp up to the soak temperature, which is followed by 12 hours at 10500C and completed by a 24-hour cooling period.

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Tests

Some of the laboratory tests, which are found essential to study the behavior of bricks include:

Compressive strength, water absorption, durability, efflorescence, etc. Their durability, strength, aesthetic value makes them preferable for the construction.

Engineering bricks usually have strength of more than 50MPa but all others should have strength of greater than 5MPa

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5. Constraints

The brick technology is highly dependent on energy sources. Production of bricks requires a calcinations temperature in the range of 900-1200oC and the optimum temperature is variable on the type of input raw material.

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Raw Materials

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 by-products from paper making, 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).

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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.

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The manufacturing process

The initial step in producing brick is crushing and grinding the raw materials in separator and a jaw crusher.

Next, the blend of ingredients desired for each particular batch is selected and filtered before being sent on to one of three brick shaping processes-extrusion, molding, or pressing, the first of which is the most adaptable and thus the most common.

Once the bricks are formed and any subsequent procedures performed, they are dried to remove excess moisture that might otherwise cause cracking during the ensuring firing process.

Next, they are fired in ovens and then cooled.

Finally, they are dehacked, automatically stacked, wrapped with steel bands, and pepped with plastic corner protectors.

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Grinding, sizing, and combining raw materials

1.Preparation

Each of the ingredients is conveyed to a separator that removes oversize materials. A jaw crusher with horizontal steel plates then squeezes the particles, rendering them still smaller.

Materials of the correct size are sent to storage silos, and over-sized material goes to a hammer mill, which pulverizes it with rapidly moving steel hammers. The hammer mill uses another screen to control the maximum size of particles leaving the mill, and discharge goes to a number of vibrating screens that separate out material of improper size before it is sent on to the next phase of production.

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2. ExtrusionWith extrusion, the most common method of brick forming, pulverized material and water are fed into one end of a pug mill, which uses knives on a rotating shaft to cut through and fold together material in a shallow chamber.

The blend is then fed into an extruder at the far end of the mill. The extruder usually consists of two chambers. The first removes air from the ground clay with a vacuum, thereby preventing cracking and other defects. The second chamber, a high-pressure cylinder, compacts the material so that the auger can extrude it through the die.

After it is compressed, the plastic material is forced out of the chamber through a specially shaped die orifice. The cross-section of the extruded column, called the pug, is formed into the shape of the die. Sections of desired length are cut to size with rotating knives or stiff wires.

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In molding, the soft, wet clay is shaped in a mold (usually a wooden box).

The interior of the box is often coated with sand, which provides the desired texture and facilitates removing the formed brick from the mold. Water can also be used to assist release.

Pressing, the third type of brick forming, requires a material with low water content. The material is placed in a die and then compacted with a steel plunger set at a desired pressure. More regular in shape and sharper in outline than brick made with the other two methods, pressed bricks also feature frogs.

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3. Chamfering the brick

Chamfering machines were developed to produce a furrow in brick for such applications as paving. These machines use rollers to indent the brick as it is being extruded. They are sometimes equipped with wire cutters to do the chamfering and cutting in one step. Such machines can produce as many as 20,000 units per hour.

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4. Coating

The choice of sand coating, also applied as the brick is extruded, depends on how soft or hard the extruded material is. A continuous, vibrating feeder is used to coat soft material, whereas for textured material the coating may have to be brushed or rolled on. For harder materials a pressure roller or compressed air is used, and, for extremely hard materials, sand blasting is required.

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5. Drying

Before the brick is fired, it must be dried to remove excess moisture. If this moisture is not removed, the water will burn off too quickly during firing, causing cracking. Two types of dryers are used. Tunnel dryers use cars to move the brick through humidity-controlled zones that prevent cracking. They consist of a long chamber through which the ware is slowly pushed. External sources of fan-circulated hot air are forced into the dryer to speed the process.

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6. Firing

After drying, the brick are loaded onto cars (usually automatically) and fired to high temperatures in furnaces called kilns. In general, the cars that moved the bricks through the drying process are also used to convey them through the tunnel kiln. These cars are pushed the kilns continuously maintained temperature zones at a specific rate that depends on the material. The source of fuel can be gas, sawdust, or coal.

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Bricks from industrial wastes

The traditional brick manufacturing process damages the environment severely as it uses top soil as raw material and generates pollutant during the manufacturing process.

On the other hand, many industrial sectors produce huge quantities of solid wastes and deposit them on the ground.

Fly ash from the thermal power station, red mud from aluminum industry, phosphor-gypsum for phosphatic fertilizer industry and sludge from tanneries are few examples of such solid wastes. Such wastes, if not managed properly, may pollute the ground water resources. However, there are technologies which utilize industrial wastes as a raw material for the brick manufacturing.

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Vertical Shaft Brick Kiln Technology (VSBK)

The vertical shaft brick kiln technology originally developed in China is an energy efficient, environment friendly and economically viable means to produce quality bricks.

The VSBK consists of one or more shafts located inside a rectangular brick structure. The shafts are around 1 meter wide with nominal length of 1, 1.5 1.75 or 2m. The inside surface of a brick wall is sometimes lined with refractory bricks. The gap between the shaft wall and outer wall is filled with insulating materials like clay, rice husk, fly ash, etc.

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The shaft is loaded from top with a number of batches of bricks for firing. Each batch normally contains four layers of bricks set in a predetermined pattern. The stack of brick rests on square support bars (which can be removed or inserted) resting on a pair of horizontal beams across the arches in the unloading tunnel.

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Main advantages of VSBK technology:

•It represents a very energy-efficient, low-cost method of firing bricks. The firing quality is high with very low wastage. •The kilns require very little maintenance once constructed and are less complicated to build.•The kiln is very compact not requiring a large area of land and can be built near to the clay source. The kilns can also be demolished and rebuilt using the same material, as the clay source is depleted. •The construction cost is low.•The kiln is not affected by variation in the weather as long as dry green bricks are available.•More than one shaft allows for firing flexibility to cope with seasonal and economical variations in brick production and demand.•The kiln is highly suitable for use where fuel is incorporated into the brick.

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Technical Specification

The following technical data of ordinary ceramic bodies also act as standard values for good brick clays:•Plasticity 15-35%•Linear drying shrinkage 2-9%•Linear firing shrinkage 0.5-3%•Linear total shrinkage <12%•Loss on ignition 0.5-3%

In most industrialized countries, bricks are aerated (that is made more porous to enhance their insulating properties) by adding polystyrene or saw meal to the body before firing. Polystyrene and saw meal burn out and leave pores behind which add to the insulating properties.

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Physical values for fired ordinary ceramic products

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Various types of fired brick products