What is Mineral Admixture Function, types and Uses in concrete

What is admixture

There are essentially four ingredients in concrete i.e., the coarse aggregates, fine aggregates cement, and water. However, in order to improve certain properties of concrete both in plastic and hardened stage, it is necessary to add the fifth ingredient in the concrete mix. The fifth ingredient can be a mineral admixture or chemical admixture.

If the concrete mix is properly designed, most concrete does not need any admixture. However. there may be a need to produce concrete with a particular result and using admixture could be the most convenient way. It is very important to note that any type of admixture should be used only when there is a valid reason to use it. An admixture should be employed only after appropriate evaluation of its effects on the particular concrete under the conditions in which concrete is intended to be used. 

It is important to conduct tests on the representative samples of the materials for a particular job under simulated job conditions in order to obtain reliable information on the properties of concrete containing admixtures. 

The admixtures ranging from the addition of minerals, chemicals, and waste materials have been used to modify the properties of concrete in plastic and hardened stages. Admixtures are generally added in a relatively small quantity to change the properties of the mix.

Functions of mineral admixtures

Some of the important functions for which the admixtures could be used are given below following the points.

1. It also to retard the setting and hardening of concrete.
2. It improves the workability (by air-entraining agents) without increasing water content.
3. It increases the durability of concrete.
4. It increases the impermeability.
5. It produces lightweight concrete (i.e., to reduce the weight of concrete per mo).
6. It reduces the chances of bleeding.
7. It reduces the chances of segregation.
8. It accelerates or retards the process of hydration of cement.
9. It increases the bond between the old and new concrete surfaces.
10. It increases the anti-skid property of concrete.

Types Of Mineral Admixtures

Sometimes minerals are added in the concrete mix to modify or improve the properties in the green stage and hardened stage. These minerals can be naturally obtained or a by-product (or a waste product). 

These minerals have high pozzolanic characteristics and hence are helpful in achieving improved properties in a concrete mix. Some of the mineral admixtures are following the given below.

1. Fly ash 
2. Blast furnace slag 
3. Rice husk ash 
4. Silica fumes.

1. Fly ash 

The fly ash or pulverized fuel ash (PFA) is the residue from the combustion of pulverized coal collected by the mechanical or electrostatic separators from the fuel gases of thermal power plants. About 85% to 98% of the ash from the fuel gases is retrieved in the form of fly ash. Fly ash accounts for 70% to 80% of total coal ash. The Physical Properties of Fly ash following below.

• Fly ash consists mainly of spherical glassy particles ranging from 1 mm to 150 mm in diameter. 
• The colour of fly ash is light grey or dark grey. 
• The specific gravity of fly ash usually varies between 3500 to 5000 cm-/gm i.e., it is than Portland cement. 

Like portland cement, fly ash contains oxides of calcium, aluminium and silicon amount of calcium oxide is considerably less. The main constituents of fly ash are following given below.

• Silicon dioxide - (SiO2)  -           (30% to 60%) 
• Aluminium oxide – Al2O3 -       (15% to 30%) 
• Unburnt fuel (Carbon)  -            (up to 30%) 
• Calcium oxide (Cao)   -               (1% to 7%) 
• Magnesium oxide (MgO) -        (Small amount) 
• Sulphur trioxide (SO3)  -           (Small amount) 

The carbon content in fly ash should be as low as possible, whereas the silica should be as high as possible, whereas the silica content should be as high as possible. 

The fly ash may be used in concrete either as an admixture or in-part replacement of cement With the use of fly ash, at an early age there is a decrease in the compressive strength but disappears at the ages of 3 months to one year. The composition of fly ash varies with the type of fuel burnt, load on the boiler and type of separator, etc. The advantages of using fly ash as an admixture are following below.

• It is used as a partial replacement of cement.
• It is used as a partial replacement of fine aggregate.
• It reduces the heat of hydration.
• It reduces the permeability of concrete.
• It improves the resistance to sulphate attack.

Uses - Following below the uses of using Fly Ash as an admixture material.

• It is a waste by-product and hence can be utilized for effective environmental pollution control.
• It can be used as an admixture in concrete mix for mass concrete works such as dams, retaining walls etc.

2. Blast furnace slag 

It is a by-product obtained during the production of iron. The oxide composition of granulated blast furnace slag is similar to portland cement so far as the oxides of calcium, aluminium and silicon are concerned, but it contains less calcium oxide. 

The granulated slag can be used for the production of blast furnace slag cement. To produce this cement the slag is mixed with portland cement clinkers and finely ground gypsum. The use of blast furnace slag as an admixture Following are the physical properties of Fly Ash given below. 

• It produces a lower rate of heat evolution and is helpful in mass concrete works.
• It improves the resistance to sulphate attack.
• Improvement in workability is probably equivalent to about a 5% increase in water content, which is insignificant. 
• Effect of blast furnace slag on the workability is much less than those of fly ash due to ser specific surface area of 3250 cm/gm. 

3. Rice husk  ash 

It is generally generated in the rice mills when paddy grains are separated. 200 kg of husk is used from 1 tonne of paddy. It is obtained by burning rice husk in a controlled her without causing environmental pollution. Rice husk ash is available in abundance 1-tonne paddy produces 40 kg of rice husk ash. The specific surface area of rice husk ash is 40 - 100 2om. The particle size of the rice husk ash is 25 microns whereas the average size of the cement article is 100 microns. Rice husk ash has pozzolanic properties, therefore, it can be used as an admixture. The principal constituents of RHA are given below following the points.

• Silica (SiO2) - 80 to 90% (Major Content) 
• K 0-2%
• Unburnt Carbon 

Uses of RHA as an admixture are following given below the points.

• It provides high strength to concrete.
• It improves the impermeability of concrete.
• It increases the resistance of concrete in acidic environments.

4. Silica fume

Silica fume is a by-product obtained from the reduction of high purity quartz with coal in electric arc furnace in the manufacturing of silicon metal. The fumes are condensed and collected by filtering the gases from the furnace. The main properties of Silica Fumes are given below following the points.

• The SiO2 content varies from 85 - 98%.
• Particle size is less than 0.001 mm i.e., 100 times smaller than the average particle size of cement. 
• They are spherical in shape. 

The principal constituents of silica fume are given below.

• SiO2 -      85% Minimum (and Maximum up to 98%)
• Al2O3 -   0.1% 3. Fe2O3 - 0.6%
• Carbon -  12% 

Uses of silica fumes as an admixture are given below.

• It helps in improving the modulus of elasticity of concrete.
• It helps in reducing the bleeding in the plastic stage.
• It improves the permeability of the concrete mix.
• It helps in improving the chemical resistance of concrete.
• It improves freezing and thawing resistance.

Also read - What is Chemical Admixture Function, types and Uses in concrete