Burnt-clay bricks

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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.

Table 3. 10 Characteristics of Masonry Units

Figure 3.16 Mould for brick forming.

Figure 3.17 Stacking pattern for brick drying.

Figure 3.18 Kiln for 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 900C). 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.

Process Substances Chemically
Burning Limestone - Quick lime CaC03 - CaO + C02
Slaking Quick lime - Slaked lime CaO + H20 - Ca(OH)2
Hardening Slaked lime - Limestone Ca(OH)2 + CO2 - CaC03 + H20

Slaked lime is mainly used in building because it is fat, i.e., it makes workable mortar and rendering and plaster mixes. A lime mortar becomes stiff initially by evaporation or loss of water to absorptive materials such as bricks, but subsequent hardening depends on the chemical reaction with carbon dioxide from the air (carbonation) reforming the original calcium carbonate (limestone).

Non-hydraulic lime is also produced from limestones with a high content of magnesium carbonate. It is less easily slaked, but some of the magnesium oxide remaining unslaked may carbonate and produce greater strength than high-calcium lime.

Hydraulic lime is produced by mixing and grinding together limestone and clay material, and then burning it in a kiln.

It is stronger but less fat or plastic than non-hydraulic lime. During the burning the calcium oxide from the limestone will react with siliceous matter from the clay forming dicalcium silicate. This compound may react with water forming 'mineral glue'- tricalcium disilicate hydrate. The reaction is slow and may take weeks or months, but after some time a very good strength is achieved.

The reaction forming dicalcium silicate requires a very high temperature to be complete. In practical production a lower temperature of 1200C is used leaving some of the ingredients in their original state. Due to the temperature the limestone will lose the carbon dioxide and thus form quicklime. If a correct amount of water is added the quicklime will slake forming a fine powder. Note, however, that excess water will lead to premature hardening due to hydraulic reaction.

Cement

Portland cement hardens more quickly and develops considerably higher strength than hydraulic lime. This is because cement contains tricalcium silicate. However, the manufacturing process is much more complicated than that of lime. The ingredients are mixed in definite and controlled proportions and then ground to a very fine powder. The fine grinding is necessary since the formation of tricalcium silicate can only take place in a solid state and therefore only the surface of the particles in the mix are accessible for the chemical reaction which requires a temperature of 1250 to 1900C to be completed.

During the burning the small particles of limestone and clay are sintered together to clinker. After cooling this is ground to cement powder, a small amount of gypsum being added during the grinding. The finer the cement particles, the larger the surface area which is available for hydration by water and the more rapid the setting and hardening occurs. Cement is normally sold in 50 kg bags but occasionally is available in bulk at a lower price.

Ordinary portland cement is the least expensive and by far the most widely used type of cement. It is suitable for all normal purposes.

Rapid-hardening portland cement is more finely ground and thus has a faster chemical reaction with water and develops strength more rapidly. It has the same strength after 7 days that ordinary Portland cement has after 28 days. Early hardening may be useful where early stripping of form work and early loading of the structure is required.

Low-heat portland cement develops strength very slowly. It is used in very thick concrete work where the heat generated by the chemical reactions in ordinary Portland cement would be excessive and lead to serious cracking.

Chemistry of Cement The main components of standard portland cement are:

The manufacturing process aims to produce a material with a high content of tricalcium silicate, usually 55 to 62% of the crystals in the clinker. Other crystals formed are: about 15% dicalcium silicate, (the same component as the hydraulic binder in hydraulic lime), 8 to 10% tricalcium aluminate and 9% tetracalcium aluminate ferrite. Since cement sinters during the burning it is very important that no calcium oxide, quicklime, remains in the finished product. The quicklime will remain embedded in the clinker even after very fine grinding and not be available for slaking until the hardening process of the cement has gone quite far. When finally the quicklime particles are slaked they expand and break the structure already developed. The proportion of limestone in the initial mix must therefore be within 0.1%.

When cement is mixed with water the chemical reactions which are so important for the hardening start. The most important is the forming of tricalcium disilicate hydrate, 'mineral glue', from hydrated calcium oxide and silica.

2(3CaO SiO2) + 6H20 = 3CaO 2SiO2 3H20 + 3Ca(OH)2 and

2(2CaO SiO2) + 3H20 = 3CaO 2SiO2 3H20 + Ca(OH)2

The reaction between dicalcium silicate and water is slow and will thus only contribute to the strength of the concrete after considerable time. Aluminate would interfere with these processes, hence the addition of gypsum at the end of the manufacturing process. The gypsum forms an insoluble compound with the aluminate.

In the process of hydration the cement will chemically bind water corresponding to about one-quarter of its weight. Additional water evaporates leaving voids, which reduce the density and therefore strength and durability of the end products.

Pozzolana

A pozolana is a siliceous material which, in finely divided form, can react with lime in the presence of moisture at normal temperatures and pressures to form compounds possessing cementious properties. Unfortunately the cementitious properties of pozzolana mixtures are highly variable and unpredictable.

A wide variety of materials, both natural and artificial may be pozzolanic. The silica content constitutes more than half the weight of the pozzolana. Volcanic ash was the first pozzolana used when the Romans made concrete from it for many large and durable buildings. Deposits of volcanic ash are likely to be found wherever there are active or recently active volcanoes. Other natural pozzolana are derived from rock or earth in which the silica constituent contains the mineral opal and from the lateritic soils commonly found in Africa. Artificial pozzolana includes fly ash from the combustion of coal in thermo-electric power plants, bumt clays and shales, blast furnace slag formed in the process of iron manufacture, and rice husk ash and the ash from other agricultural wastes.

The energy requirement for the manufacture of portland cement is very high. By comparison, lime and hydraulic lime can be produced at less than half the energy requirement, and natural pozzolana may be used directly without any processing. Artificial pozzolana requires some heating, but less than half as much as is required for lime production.

Pozzolana and lime can be produced with much less sophisticated technology than portland cement. This means that pozzolana can be produced at relatively low cost and requires much less foreign exchange than cement. However, it takes two to three times the volume of pozzolana required to make a concrete with the same strength as with portland cement and this adds to the cost for transport and handling.

The main use of pozzolanas is for lime-pozzolana mortars, for blended pozzolanic cements and as an admixture in concrete mix. Replacing up to 30% of the portland cement with pozzolana will produce 65 to 95% of the strength of portland cement concrete at 28 days. The strength nominally improves with age since pozzolana reacts more slowly than cement, and at one year about the same strength is obtained.


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