ADDITIVES USED FOR BRICK MANFACTURING

Why additives used?

Additives are used to improve some properties of clay mass, such as

• Shaping characteristics
• Drying behavior
• Firing behavior

More than that, some of the additives help in conservation of agricultural land and utilization of waste materials.

And remember, additives must contain certain physical and chemical characteristics so that it does not adversely affect the performance or durability of bricks.

What are different types of additives used?

In green brick making additives are of the following types:

• Internal fuel
• Anti shrinkage material
• Structure opening material

1. INTERNAL FUEL

For firing of green bricks in an energy efficient and economical manner, addition of internal fuel is an option and under circumstances desirable. Without internal fuel content, firing of solid green bricks is not recommended. Internal fuel not only reduces the external fuel consumption but also reduces the stack emission. Addition of internal fuel also upgrades the fired bricks quality.

Internal fuel can be coal or waste material produced by the process industries, such as:

• Coal dust
• Industrial waste like boiler ash, distillary waste, sponge iron waste
• Agro waste like rice husk, saw dust, etc.

For brick making, internal fuel is mixed with the soil during mixing and pugging process. Quantity of addition depends on the quality of the soil and the calorific value of the internal fuel.

2. ANTI-SHRINKAGE MATERIAL

To transform highly plastic soils for brick making, anti-shrinkage materials are added. This is to avoid high shrinkage and resultant cracks during drying under direct sunlight. Depending upon the availability, the following types of anti-shrinkage materials can be added.

Fine river sand

Should not be bigger than releasing sand, depending on the fineness of the clay a maximum of 30% fine river sand can be added (exclude mica sand).

Note: River sand which shines should be checked for mica. Mica are flaky materials generally white or brown in colour. When heated at lower temperatures, it expands considerably. Thus its presence within a green brick during firing will result in cracks. The bigger size of the mica flakes the higher the chances of cracks in the fired bricks and dull sound.

Medium sand

<2 mm grain size, in case of plastic clays a maximum of 15% can be added.

Stone dust

<2 mm grain size, in case of plastic clays a maximum of 15% can be added.

Sandy soil

It is usually used as a releasing agent during moulding. In case of plastic clays a maximum of 30% can be added. The exact quantity of anti-shrinkage material to be added always depends on the plasticity of the clay. It is advisable to contact a professional and qualified brick making expert to determine the most appropriate amount and mixing technique of anti shrinkage material required.

3. STRUCTURE OPENING MATERIAL

During periods of very low humidity and high temperature a structure opening material could be used in green brick making to avoid rapid drying damage of green bricks, especially shrinkage cracks and warping. Structure opening material also performs anti shrinkage functions. The materials generally used for structure opening are:

• Rice husk
• Saw dust
• Mustard husk

The exact quantity of pore opening material to be added always depends on the plasticity of the clay. It is advisable to contact a professional and qualified brick making expert to determine the most appropriate amount and mixing technique of pore opening material required. In general with any kind of plastic to semi-plastic soil a maximum of 2% by weight of dry green brick, pore opening material should be added. Too much addition reduces the strength and increases the water absorption rate of the fired brick.