brick is one of the oldest and most widely used building materials and it is often used to build walls and facades of buildings. Bricks have different types in terms of ingredients; But here we introduce two types clay brick and calcareous sand brick that are mostly used as building materials to build walls and facade. (Many bricks have industrial applications such as refractory bricks)
1- Calcareous sand bricks
2- Clay bricks
Calcium silicate bricks are also called calcareous sand bricks. In making this type of facade brick, lime and silica are used instead of clay. To produce these bricks, lime and silica are used in a certain ratio and with mineral pigments. In the production process, sand and silica or quartz powder are first mixed with lime and then the lime is allowed to fully hydrate. The mixture is then compressed inside a mold and transferred to an autoclave. The autoclave is left at a temperature of about 200 ° C for several hours until the chemical reaction between lime and silica is complete and the brick hardens. Due to the possibility of using mineral pigments, lime sand bricks can be produced in different colors such as white, black, ocher, etc.
These types of bricks, as their name suggests, are made of clay.
Clay in terms of chemical and constituents can include minerals such as feldspar, illite and kaolin and some other minerals such as calcite – quartz, mica and alumina, etc. The main constituents of these minerals are often silicon, aluminum, oxygen, alkali metals, and basic metals such as iron and manganese. The percentage of elements and constituents of clay depends on the type of mother rock and the length of the transported route, climatic conditions and topography of each region.
Soils suitable for brick production are low refractory soils that are baked at a temperature of less than 1250 ° C. The temperature required to bake bricks depends on the type of soil and the number of mineral elements in the soil. Suitable temperature is between 900 and 1200 degrees. Substances such as iron oxide, lime, sodium and potassium salts reduce the melting point of the soil. Clay, if it is high in silica and alumina, is considered as a refractory soil and is baked at a temperature above 1250 degrees.
The color of the soil changes according to the conditions of the oven and the cooking temperature, as well as the ingredients in it after the cooking operation. Clay containing 10% manganese oxide turns yellow, dark brown, purple and black after firing and acts as a strong smelter. Minerals such as hematite and limonite (iron compounds) give the soil a red or light brown yellow color. The use of this type of soil in facade bricks depends on the firing temperature, producing red, brown, burnt and black bricks. Most clay soils contain the remains of animal and plant organisms that give the soil a gray or almost black color. In brick production, the burning of this organic matter during firing can sometimes be effective in saving fuel, but the quality of the physical and mechanical properties of the brick will probably be reduced due to the space left by the burning of organic matter.
According to the standard, the appropriate amount of silicon oxide is between 40 and 60% of the soil. Silica grains, on the other hand, constantly increase in volume as the temperature rises during the firing of the brick, which is not consistent with the uniform accumulation of clay during firing, and hair cracks form in the bricks around the silica grains. Increasing the amount of silica makes the brick thicker and heavier and increases its firing temperature.
The acceptable range of aluminum oxide is 9 to 21%. Aluminum oxide is a component of refractory compounds and its increase in soil increases the melting point of bricks and bricks at higher temperatures. Also, its high level in the soil causes the raw clay to crack when it dries and the bricks to break during baking.
Iron oxide melts in bricks at temperatures above 1000 ° C (reduces the firing temperature of bricks). Clay with 5 to 8% iron oxide produces red brick when baked. Iron oxide clay depends on the percentage of iron oxide, the percentage of alkaline compounds and the temperature and conditions of the furnace (oxide or reduction reduction) after firing to produce bricks of different colors. At temperatures below 1000 ° C and under oxidation conditions, the brick is red, and at 1100 ° C and in the furnace under oxidation conditions, the brick production turns light reddish brown. When the kiln is in the regenerative state, the condition of the brick will be gray or black. If the clay contains 3 to 4 percent alkaline compounds such as sodium and potassium or is baked at a high temperature, the color of the brick will be more purple.
A small amount of calcium carbonate in powder form does not damage the brick and causes the brick to whiten, but a large amount of it acts as a melt and reduces the degree of melting of the clay, and in the heat of the kiln, clay is formed. Be a dough and melt
Also, if limestone is used as large grains inside the brick. When baked in a kiln, it turns into quicklime (calcium oxide) and, if used, absorbs water from the mortar and swells, causing the bricks to crack, so-called shedding.
To prevent the brick from disintegrating due to lime blossoming (spilled phenomenon), if lime is present in the soil as a coarse grain, the brick soil should be sieved.
Calcium, sodium, potassium and magnesium sulfate are more or less present in rice. Large amounts of these materials cause the bricks to turn white after consumption. The compounds of calcium, sodium, potassium and magnesium in the clay reduce the melting of the brick, so the brick is baked at a low temperature.
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