WHAT ARE THE MINERAL ADMIXTURES USED IN CONCRETE?

The following 4 mineral admixtures can be used for making concrete.

1. Rice husk ash
2. Metakaolin
3. Silica fume
4. Fly as

1. Rice Husk Ash

It is obtained by burning the rice husk in a controlled manner without causing environmental pollution. This is generated in huge quantity in the rice shelters and each tone of paddy produce 40 kg rice husk ash. It consist of,

• 90% of silica
• 5% of carbon
• 2% of K₂0

The specific surface area of rice husk is 40 to 100 m²/kg. Rice husk exhibits high pozzolanic characteristics and contribute to high strength and high impermeability of concrete because the size of cement particles is about 100 microns whereas the size of rice husk ash is 25 micron.

2. Metakaolin

It is the unpurified thermally activated ordinary clay and kaolin clay. It shows high pozzolanic reactivity and reduction in Ca(OH)₂ even as early as one day. It is found that cement paste undergoes distinct densification and as a result helps to increases the strength and decreases the permeability.

3. Silica Fume

These are the byproduct of the reduction of high purity quartz with coal in electrical arc furnaces in the manufacture of Ferro silicon and Silicon metal. These are spherical in shape and size less than 0.1 micron. Surface area of silica fume is 20m2/g and it size is 100 times smaller than average cement particles. Chemical composition of silica fume is,

• 85% of silica
• 1% Al₂O₃
• 6% of Fe₂O₃
• 12% of carbon

4. Fly Ash

It is the residual form obtained from combustion of powdered coal and transported by the flue gases and collected by electrostatic precipitation. It has a surface area 300 to 700 m²/kg. Its particles are finer than cement particles. It is light grey to dark grey in colour. Its chemical composition is,

• 30 to 60% of silicon
• 15 to 30% of Aluminum oxide (Al₂O₃)
• 30% of carbon
• 1to 7% of calcium oxide and
• Small amount of MgO & SO

It is used in mass concreting in dams, retaining walls etc.