HOW FIRING STRENGTHENS CLAY TO CREATE DURABLE BUILDING MATERIALS
Moon is a baked brick, and has been a primary form of construction material for many millennia, starting as early as Ancient Sumer and Egypt.
Due to these attributes they have become an essential tool in construction projects that take place worldwide.
It is a process of converting a soft, easily soluble material like clay into a hard, compact and strong structure as a brick by means of certain scientific phenomenon, which occurs in a kiln.
Hence, getting knowledge requires the understanding of the nature of clay, water content in the material and the potential effects of high temperatures on the chemistry of clay.
When the bricks are made and exposed to heat several changes occur physically and chemically to produce a materials that is so strong and can be able to survive environmental conditions for centuries.
The simple idea behind baked bricks is that the clay minerals, when heat is applied, change in certain characteristics. Clay is fine-grained mineral formed from the breakdown of rocks, that mainly consists of silicates and aluminosilicates.
Clay together with water can be modeling illustrating the capability of molding it into bricks hence facing brick. However, in natural form which is clay the material is highly porous and very susceptible in water or mechanical stresses.
The baking, or firing, process is what transforms the bricks from hard, but weak structures into what they are by changing the chemicals and texture of the clay.
That, according to the author, are the following: The first step in the process of strengthening bricks through baking is removal of water.
In raw clay, there is submerged water, the water actually between the clay grains and the hydrogen water that is a part and parcel of the molecules and crystal of clays.
If the shaped bricks are subjected to a kiln and heated in gradual manner free water is the first to disappear at temperatures below 100oc(212of)
This step is important because at the final stages of firing, if there is free water remaining within the brick it is possible to have defined cracking or warping due to the expansion of steam.
This slow heating procedure enables the free water to come out of the brick without impairing its construction in anyway.
Subsequently, as the temperature increases further and it reaches between 200 and 600 °C (392 and 1112 °F) the chemically bound water evaporated from the clay.
This process which is called dehydroxylation leads therefore to the destruction of the clay minerals especially kaolinite to a more stable one.
Finally at this level the clay becomes anhydrous inferring that within its molecular formation it cannot hold water. Unfortunately, this change is permanent, and the clay cannot turn it back into a plastic state, even by immersing in water.
The loss of chemically bound water is another reason for the improvement seen in strength and the bricks itself, because all additional water will no longer be drawn into the material and the product will be less likely to break down prodigiously from the effects of weathering.
The greatest changes to the bricks’ strength occur in the stage of vitrization, which occurs at a temperature of 900 to 1200°C (1652 to 2192°F).
Vitrification is the process of fusion of some of the constituents of clay major of which are silica (SiO2) and alumina (Al2O3).
These materials when heated turn into a glass like phase and then immobilising the remaining solid particle with the help of aggregation.
This glassy phase it is what endows baked bricks with their natural hardness and their ability to withstand mechanical stresses.
The level of vitrification just depends on how much of the clay has become fully fused to glass like substance and thus the kind of clay used and the temperature at which it was fired at are factors that would determine the degree of vitrification of the end product.
Besides sintering, the formation of a new mineral phase during firing plays an important role in building the strength of baked bricks. Mullite is a crystalline aluminosilicate material that forms a significant chemical reaction that takes place during firing.
Mullite develops at temperatures exceeding 1000 C or 1832 F and has been extensively used for its high tensile strength and thermal shock resistance.
Combustion thoughts demonstrate that mullite considerably reinforces the mechanical properties of the brick structure and its solidity against compression and bending loads together with thermal expansion.
The formation of mullite and other crystalline phases contributes to understanding why bricks baked in a kiln can support not only the mass of large constructions but also the actions of a natural environment and temperature changes when designing buildings.
Another aspect that results into increase strength of baked bricks is yielded with the densification of the material during firing. At higher temperatures the clay particles get close packed, thereby decreasing the pore space of the brick.
Porosity is the opposite of density and consequently when porosity is reduced, a material will have got less voids or more solidity and strength. The latter makes the brick even less porous concerning water, which is essential to withstand erosion and freeze-thaw effects.
Depression of porosity during the baking process has correlation with the level of vitrification; the glassy phase surrounds the solid particles and covers up the open porosity.
The firing process also assists in overcoming the remaining of some organic and volatile matters that the raw clay may contain. Essentially, if organic materials such as plants tissue are used in laying the bricks to interlock, then these can leave areas of weakness within the brick itself, provided these organic materials have not being fully combusted when the bricks were being fired.
This means that for everyday use, products such as glass stoppers and sealing wax are most effective at high temperatures above 400°C (752°F), because these higher temperatures drive off the organic materials and what remains is a more uniform and dense material.
The following purification process also further enhances the strength and quality of the brick because any material which is likely to weaken the brick is also removed.
Likewise, during firing salts or other soluble compounds present in the clay are expelled thus minimizing efflorescence ,a situation whereby salts from the bricks crystallize resulting in discolouration and loss of strength.
The final compounded transformations of these physical and chemical changes lead to a significantly superior strength material compared to the raw clay.
The mechanical properties of the baked bricks are characterized by a high compressive strength and bending strength, as well as very good tolerance to the effects of moisture, temperature fluctuations and chemical agents.
This durability is why for centuries fired bricks have been employed in construction of everything from ziggurats and Roman aqueducts to houses and factories.
In conclusion, therefore, the phenomenon of baked bricks becoming stronger is may be attributed to dehydration, vitrification, compaction and chemical change that occur during firing of these products.
The reduction of porosity through leaching of water, formation of a glassy phase and creation of new mineral phase enhances the strength, durability and stress resistance of the brick. As perfected over the centuries this has made baked bricks one of the most dependable and popular construction materials in human civilization which has helped in as along as the structures they are placed on.
Studying the process of producing baked bricks extends appreciation for architectural creativity of early societies but it also helps future engineers and material scientists to better understand the foundations of construction.
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