WHAT ARE THE ADMIXTURE FOR MANUFACTURE OF BRICK ?

Certain admixtures such as fly ash, sandy loam, rice husk ash, basalt stone dust, etc. are often required not only to modify the shaping, drying and firing behaviour of clay mass, but also to help conserve agricultural land and utilise waste materials available in large quantities. These admixtures should, however, have a desirable level of physical and chemical characteristics so as to modify the behaviour of clay mass within the optimum range without any adverse effect on the performance and durability. Some of the basic physio-chemical requirements of conventional admixtures are as under.

Fly Ash

A waste material available in large quantities from thermal power plants can be added to alluvial, red, black, marine clays, etc. The fly ash contains amorphous glassy material, mullite, hamatite, magnetite, etc. and shows a chemical composition similar to brick earths. These silicates also help towards strength development in clay bodies on firing, when mixed in optimum proportion depending on the physio-chemical and plastic properties of soils to be used for brick making. The proportion of fly ash mixed as an admixture to the brick earth should be optimum to reduce drying shrinkage, check drying losses and to develop strength on firing without bloating or black coring in fired product. The crystallites present in the fly ash should comply with the resultant high temperature phases in the finished product.

Characteristics : 

• Texture-   fineness 200 to 300 m2/kg
• Maximum coarse material( +1 mm )- 0.5%
• Maximum unburnt carbon percentage by mass- 15%
• Maximum wate percentage by mass- 0.1%

Sandy Loam

Addition of sandy loam is often found effective in controlling the drying behaviour of highly plastic soil mass containing expanding group of clay minerals. Sandy loam should preferably have a mechanical composition as specified below. The material should, however, meet the other requirement as well.

Clay (less than 2 micron)    8-10%

Silt (2-20 micron)               30-50%                                                Sand(more than 2 micron)  40-60%

Rice Husk Ash

The ash should preferably have unburnt carbon content in the range of 3–5% and should be free from extraneous material. It can be used with plastic black red soils showing excessive shrinkage.

Basalt Stone Dust

Basalt stone occurs underneath the black cotton soil and its dust is a waste product available in large quantity from basalt stone crushing units. The finer fraction from basalt stone units is mixed with soil mass to modify the shaping, drying and firing behaviour of bricks. The dust recommended for use as an additive with brick earth should be fine (passing 1 mm sieve), free from coarse materials or mica flakes and should be of non-calcitic or dolomitic origin.

Note:- less than 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.