Kiln Construction and Brick Firing
Kiln Construction and Brick Firing
It is during the firing that the bricks receive their strength. In the presence of high heat, the alkalies in the clay, together with small amounts of oxides of iron and other metals, are joined in chemical union with the alumina and silica in the clay to form a dense and durable mass.
A kiln is a furnace or oven in which bricks are fired or heat treated to develop hardness. Where brickmaking is done on a large scale, the firing operation is performed in a continuous-process kiln referred to as a tunnel kiln. In making brick on a small scale, firing is a periodic operation wherein the bricks are placed in the kiln, the fire started and heat developed, and then, after several days of firing, the fuel is cut off from the fire and the entire kiln and its load are allowed to cool down naturally.
The kiln is filled with well-dried bricks, stacked in the same manner as during the drying. The top of the stack in the kiln is then sealed with mud. Some openings are left through which combustion gases can escape. Pieces of sheet metal are provided to slide over the openings to control the rate at which the fire burns.
Although a range of fuels can be used in this kiln, wood or charcoal are the most common. When the kiln is at the prime heat for firing, a cherry-red hue develops (corresponding to a temperature range of 875 to 900°C). This condition is held for about 6 hours. Sufficient fuel must be available when the burning starts since the entire load of bricks might be lost if the fires were allowed to die down during the operation. Firing with wood will require four to five days.
During the firing the bricks will shrink as much as 10%. As they are taken out of the kiln they should be sorted to different grades, the main criteria being strength, irregular dimensions, cracks and sometimes discoloration and stain.
Binders
When binders are mixed with sand, gravel and water, they make for a strong and long lasting mortar or concrete.
Binders can be broadly classified as non-hydraulic or hydraulic. The hydraulic binders harden through a chemical reaction with water making them impervious to water and therefore able to harden under water. Portland cement, blast-furnance cement (super sulphated), pozzolanas and high- alumina cement belong to the hydraulic binders. High-calcium limes (fat or pure limes) are nonhydraulic since they harden by reaction with the carbon dioxide in the air. If, however lime is produced from limestone containing clay, compounds similar to those in portland cement will be formed, i.e., hydraulic lime.
Lime
Non-hydraulic lime is high-calcium limes that are produced by burning fairly pure limestone, essentially calcium carbonate, so as to drive off the carbon dioxide leaving calcium oxide or quicklime. The burning process requires a temperature of 900 to 1 100° C. Quicklime must be handled with great care because it reacts with moisture on the skin and the heat produced may cause burns. When water is added to quicklime considerable heat is evolved, expansion takes place breaking down the quick lime pieces to a fine powder and the resulting product is calcium hydroxide, also called hydrated lime, or slaked lime.
After drying the powder is passed through a 3mm sieve, and poured into bags for storage (in dry conditions) and distribution.
Burnt-clay bricks
Burnt-clay bricks have good resistance to moisture, insects and erosion and create a good room environment. They are medium in cost and have medium-to-high compressive strength.
Bricks can be made with sophisticated factory methods, simple labour-intensive methods or a range of mechanized technologies in between. The labour-intensive production methods are most suitable for rural areas where the demand for bricks is limited. The bricks produced by hand will have relatively lower quality, especially compressive strength, and will tend to have irregular dimensions. However, they are economical and require little capital investment or transportation cost. Bricks made in this manner have been used in buildings which have lasted for centuries. Their longevity has depended on the quality of the ingredients, the skill of the artisans and the climate in which they were used.
Brick Making
Four main ingredients are required for brick making: suitable clay and sand, water, fuel and manpower. The clay must be easily available, be plastic when mixed with small amounts of water, develop strength upon drying and develop hard and durable use-strength when burned.
Suitable soils contain 25 to 50% clay and silt and 50 to 75% coarser material as determined by the simple sedimentation test. The soil must be well graded. Another test consists of rolling out by hand on a flat surface a long cylinder with a 10mm diameter from moistened soil and then picking it up by one end and letting it hang unsupported. A soil is adequate for brick making if the piece of cylinder that breaks off is between 50 and 150mm long. In the bar shrinkage test, using a mould 300mm long and 5Omm wide and deep, a suitable soil should show no cracking or only a little on the surface and shrink less than 7%, i.e., less than 20mm.
The clay is obtained by chipping it out of a clay bank and when necessary, mixing it with sand to a mixture that will not crack during drying. Water is gradually added to make the clay plastic.
In making bricks, the mould must be cleaned periodically with water. Before each brick is formed, the mould is sprinkled with sand. A lump or clot of clay just slightly larger than required for a brick is rolled into a wedge shape and then in sand before it is thrown, point down, into the mould. Thrown correctly, the mould will be completely filled and the excess clay is then shaved off the top with a bowcutter. The sand in the mould and on the clot helps release the newly formed brick.
The bricks should be left to dry for about three days in the place where they were made. They will then be strong enough to be stacked, as shown in Figure 3.17, for at least one week of further drying. Clay tends to become lighter in colour when dry and, when sufficiently dried, the brick, upon being broken in half, will show no color differential throughout the section area. During drying the bricks should be protected from rain.
Advantages of Brick Construction
There are many advantages when bricks are used as part of the construction.
- Aesthetic: Bricks offer natural and a variety of colors, including various textures.
- Strength: Bricks offer excellent high compressive strength.
- Porosity: The ability to release and absorb moisture is one of the most important and useful properties of bricks, regulating temperatures and humidity inside structures.
- Fire protection: When prepared properly, a brick structure can give a fire protection maximum rating of 6 hours.
- Sound attenuation: The brick sound insulation is normally 45 decibels for a 4.5 inches brick thickness and 50 decibels for a 9-inch thick brick.
- Insulation: Bricks can exhibit above normal thermal insulation when compared to other building materials. Bricks can help regulate and maintain constant interior temperatures of a structure due to their ability to absorb and slowly release heat. In this way, bricks can produce significant energy savings—more than 30 percent when compared to wood construction.
- Wear-resistance: A brick is so strong that its composition provides excellent wear resistance when compared to wood.
Categorizing Bricks by Raw Materials
In modern construction practices, bricks are categorized according to their component materials and method of manufacture. Under this classification, there are five common types:
- Burnt clay bricks
- Sand lime bricks (calcium silicate bricks)
- Concrete bricks
- Fly ash clay bricks
- Firebrick
Burnt Clay Bricks
Burnt clay bricks are the classic form of brick, created by pressing wet clay into molds, then drying and firing them in kilns. This is a very old building material—the type of brick found in many of the ancient structures of the world. In appearance, these bricks are solid blocks of hardened clay, usually reddish in color.
Burnt clay bricks are typically sold in four classes, with first-class offering the best quality and most strength. These high-grade burnt clay bricks have no noticeable flaws, but they’re also going to cost more.
When these bricks are used in walls, they require plastering or rendering with mortar. Uses for burnt clay bricks include:
- Masonry walls
- Foundations
- Columns
Sand Lime Bricks
Sand lime bricks (also known as calcium silcate bricks) are made by mixing sand, fly ash and lime. Pigments may also be added for color. The mixture is then molded under pressure to form bricks; the materials bond together by a chemical reaction that occurs as the wet bricks dry under heat and pressure. These bricks are not, however, fired in kilns in the same manner as burnt clay bricks. Sand lime bricks can offer some advantages over clay bricks such as:
- Their color appearance is gray instead of the regular reddish color.
- Their shape is uniform and presents a smoother finish that doesn’t require plastering.
- These bricks offer excellent strength for load-bearing structures.
- When pigments are added, the bricks can be used for ornamental purposes.
- Less mortar is required during construction.
- Edges are straight and precise, making construction easier.
- Bricks do not effloresce salts and minerals.
The uses for sand lime bricks include:
- Structural foundations and walls
- Exposed brick walls and pillars
- Ornamental uses (when pigments are added)
Concrete Bricks
This types of bricks are made from solid concrete and are growing in popularity among homeowners. Concrete bricks are usually placed in facades, fences, and provide an excellent aesthetic presence. These bricks can be manufactured to provide different colors if pigments are added during production. Concrete bricks should not be used in below-ground applications.
Common uses for concrete bricks include:
- Fences
- Internal (hidden) brickwork
Fly Ash Clay Bricks
Fly ash clay bricks are byproduct of coal burning—fired at about 1,000 degrees C. Because a high volume of calcium oxide, this type of brick is sometimes described as self-cementing, since it expands when exposed to moisture. This tendency to expand, however, can also produce pop-out failure. Fly ash clay brick has the advantage of being lighter in weight than clay or concrete brick.
Typical uses for fly ash clay brick include:
- Structural walls
- Foundations
- Pillars
- Anywhere that improved fire resistance is required
Fire Bricks
Also known as refractory bricks, these are manufactured from specially formulated earth with a high aluminum oxide content. After burning, these bricks can withstand very high temperatures without their shape, size, or strength being affected.
Common used for this type of brick include:
- Lining of chimneys and furnaces
- Pizza ovens and outdoor brick barbecues
Categorise of bricks by shapes
In the history of professional construction practices, brick is one of the oldest of all building materials. It is also arguably the most durable since there are brick walls, foundations, pillars, and road surfaces constructed thousands of years ago that are still intact. Today, bricks are most often used for wall construction, especially as an ornamental outer wall surface.
Brick Defined
Officially, the term brick is used to denote a building unit made of shaped clay, but in modern times it is used to refer to any stone- or clay-based building unit that is joined with cementitious mortar when used in construction. Typically, bricks are about 4 wide, 8 inches long, with a variety of thicknesses. Larger stone- or clay-based building units of the type used in foundations are usually called blocks.
How Bricks Are Categorized
There are several ways that brick can be categorized. For example, you can divide brick into the types used for facing (exposed and visible on the exterior of a structure) vs. backing bricks (which are used structurally and are hidden from view). Another means of categorizing brick is according to how they are manufactured: unfired (brick that is air-cured) and fired (brick that is baked in ovens to harden it). Bricks can be also categorized according to their typical use: common bricks or engineering bricks. For purposes of residential construction, it is usually common bricks that are of most interest, since engineering bricks are more often used in civil engineering projects, such as road or bridge construction, or sewers construction.
Bricks can also be categorized according to their shape. Some common shapes include:
- Brick veneers: These bricks are thin and used for surface cladding.
- Airbricks: These bricks contain large holes to circulate air and lessen weight. They are used on suspended floors and cavity walls.
- Perforated bricks: These bricks contain many cylindrical holes drilled throughout the brick. They are very light in weight.
- Bullnose brick: These are bricks moulded with round angles.
- Paving bricks: These bricks contain a good amount of iron. They are used in underfoot paving applications.
- Capping bricks: These bricks are used to cap the tops of freestanding walls.
- Hollow bricks: About one-third of the weight of the normal bricks, these are used mostly in partition walls where load-bearing is not required.