Mineral admixtures are mostly derived from other substances and not chemically manufactured. Mineral admixtures make concrete mixtures more cohesive, economical, reduce permeability, increase strength, and influence other properties of concrete.
Mineral admixture impacts the nature of the hardened concrete through hydraulic activity or pozzolanic activity. Pozzolanas are cementitious materials and include natural Pozzolanas (such as the volcanic ash used in Roman concrete), flyash, and silica fume.
Mineral admixtures are included fly ash, silica fume, ground granulated blast furnace slag (GGBS), metakaolin, rice husk ash, Alccofine which process certain characteristics through which they influence the properties of concrete differently.
Pulverized coal is combusted in thermal power for electricity generation. A by-product of the pulverized coal combustion reaction is fly ash. The electrostatic precipitators utilized inside chimneys of the thermal power plants remove fly ash before ejecting out the combustion gases into the atmosphere. Fly ash is a very fine molecule-like residue, which has pozzolanic properties. Hence it is often blended with cement.
Fly debris is a fine molecule like buildup, which has pozzolanic properties. Thus it is regularly mixed with concrete.
Fly ash consists of silica, alumina and calcium oxide as its major components. Fly ash can be of two types namely,
- C type
- F type
C type fly ash is rich in calcium and possesses both properties cementitious & pozzolanic whereas F type fly ash is low in calcium content and possesses only pozzolanic properties.
Effects of Fly Ash on Concrete
- Fly ash particles are commonly spherical & enhance the workability of concrete.
- The setting time of concrete is also enhanced by mixing fly ash with it. Increased setting time of concrete allows better hardening and finally, a better strength is obtained.
- Fly ash added to the concrete mix minimizes the segregation and bleeding of concrete. Segregation of concrete is a situation in which particles of different sizes tend to segregate out. Whereas bleeding is a situation in which water comes out to the surface of the concrete. Both segregation and bleeding are unwanted.
- The temperature of fresh concrete increased above normal and when it cools down, cracks may develop. Replacing a certain quality of cement with fly ash helps to reduce this temperature rise, hence avoiding the chances of cracking of fresh concrete.
- Creep and shrinkage are generally more in fly ash added concrete mix because of the increased amount of paste in the concrete.
- Sulfate resistance is enhanced.
- Alkali aggregate reaction is inhibited.
Advantages of addition
- Improves strength and durability of concrete.
- Increased resistance towards chemical attacks.
- Better workability
Ground Granulated Blast Furnace Slag (GGBS)
Ground granulated blast furnace slag is a by-product of iron extraction from iron ore. Amongst all the mineral admixtures, ground granulated blast furnace slag (GGBS) has the highest specific gravity (2.8 to 3.0). Typically, the ground granulated blast furnace slag fineness is slightly more than that of the cement.
There are different types of slag available like air-cooled slag, expanded or foamed slag, granulated slag, and pelletized slag. From all of these only the granulated slag is normally utilized as a mineral admixture. It is a highly reactive form of slag and is commonly quenched to form a hardened matter which is then ground into small particles of fineness almost the same as that of cement. Because of that the name, ground granulated blast furnace slag (GGBS).
Ground granulated blast furnace slag (GGBS) possesses both cementitious and pozzolanic properties. An activator is needed to hydrate the slag.
Various Effects of Ground Granulated Blast Furnace Slag on Concrete
- Ground granulated blast furnace slag (GGBS) increases the initial setting time of the concrete. But, it doesn’t alter the workability of the concrete much because its fineness is almost the same as that of the cement.
- The strength gain of concrete is diminished by replacing cement in the concrete with ground granulated blast furnace slag (GGBS).
- The ultimate strength obtain can be improved by slag replacement and also the durability of the concrete is increased.
- Concrete uses in marine purposes are highly prone to chemical attack and corrosion. Ground granulated blast furnace slag is a very good admixture in this regard because it increases resistance to these attacks.
- However, concrete with ground granulated blast furnace slag is reported to have higher carbonation rates than normal Portland cement concrete.
Advantages of Addition
- Improves durability
- Increases setting time
- Strength gain continues for a long duration of time
- Reduced risk of damages by alkali-silica reaction
- Resistant to chloride and sulfate
Silica fume is a very fine particle of amorphous silica. It is produced as a by-product in an electric arc furnace in the production of elemental silica or other silicon-based compounds.
Silica fume is highly pozzolanic in nature.
Effect of Silica Fume on Concrete
- Being of a very fine nature, silica fume enhances the water demand of concrete and hence a superplasticizer is almost always used with it.
- Silica fume makes the concrete mix stickier & more cohesive. Usually, slump loss problems arise because of the addition of silica fume to the concrete mix.
- Plastic shrinkage may take place in dry regions where the evaporation rate exceeds the rate at which concrete sets.
- The permeability of concrete is reduced. Silica fume acts both as a Pozzolana and filler and due to consequent reactions, the transition zone between the aggregates and cement paste is strengthened. Chloride permeability is reduced significantly.
- The compressive strength and flexural strength of concrete are enhanced. The elastic modulus of concrete is also increased by about 15% compared to normal Portland cement concrete. Increased elastic modulus indicates that the stiffness of the concrete is increased.
- Creep and shrinkage are also increased at higher replacement levels of 10%-15%. Although, the resistance to creep & shrinkage deformation is also increased because of the increase in stiffness.
- Silica fume concrete generally shows good resistance to chemical attacks because of the reduced permeability.
- Silica fume concrete is ideal for industrial flooring because it gives excellent resistance to abrasion and erosion.
- The fire resistance of silica fume concrete isn’t impressive. It does not allow the entrapped water to vaporize out because of its low permeability. Hence, due to high pressure developing inside, the concrete tends to crack.
- Carbonation depth is usually lowered.
Advantages of Addition
- Better compressive strength and resistance to abrasion
- The reduced permeability of chloride ions
- Improves workability
- Reduces bleeding
Rice Husk Ash
During milling of the paddy coming from fields, plenty of rice husk is manufactured. This rice husk is generally utilized as a fuel. Rice husk ash is manufactured by burning the rice husk. The rice husk ash is big damage to the environment where is dumped.
Rice husk ash can be produced by field burning – produces poor quality ash, bed furnace burning (fluidized), and industrial furnace.
Rice husk ash contains a high content of silica.
Effect of Rice Husk Ash on Concrete
- Rice husk ash gives strength to the concrete.
- It also decreases permeability because it is much smaller in particle size compared to cement particles.
- It decreases the heat of hydration of concrete.
- Rice husk ash also enhances the concrete’s resistance to chloride & sulfate attacks.
Advantages of Addition
- The heat of hydration of concrete is reduces
- Permeability of concrete is reduces
- Improves resistance to chloride and sulfate attacks
Ordinary clay and kaolin clay when thermally activated, it called metakaolin, in the non-purified form. The particle size of metakaolin is much smaller than particles of cement. Metakaolin isn’t an industrial by-product like the other admixtures.
Effect of Metakaolin on Concrete
- Metakaolin provides strength to the concrete.
- It reduces the permeability of the concrete.
- It helps the concrete resist chemical attacks.
- It makes the concrete more durable.
- It helps in the development of early strength in concrete.
- Bleeding of concrete is considerably decreased upon metakaolin addition. Metakaolin is also used in fiber-cement and Ferro-cement products. It is also used in art sculptures.
Advantages of Addition
- Increases compressive strength and flexural strength of concrete
- Reduces permeability
- More resistant to chemical attacks
- Durability is increases
Comparison of Various Mineral Admixtures Based on Different Properties
|–||Fly Ash||GGBS||Silica Fume||Rice Husk Ash||Metakaolin|
|Physical characteristics||Grayish, lightweight & fine particles||Off white powdery substance heavier than fly ash||Amorphous, very fine particles, heavier than fly ash||Amorphous, very lightweight. Fine particles||Whitish in color & fine particles|
|Chemical composition||20%-60% silica 5%-35% Al2O3 1%-12% Cao Traces of MgO||28%-38% silica 8%-24% Al2O3 1%-18% Cao 1%-18% MgO||85% silica 1% Al2O3 6% Fe2O3 |
|90% silica |
|Dehydroxilated form of kaolinite clay|
|Source||By-product in thermal power plants||By-product in iron producing blast furnaces||By-product in silicon producing electric arc furnace||From rice husk||From kaolinite clay|
|% addition in concrete||Up to 30% cement replacement||25%-70% cement replacement||3-4 parts of cement replacement per 1 part of silica fume||Around 20% cement replacement||8%-10% cement replacement|
|Availability||Produced in abundance but availability is poor||Sufficient amount of GGBS is available||Sufficient quantity of silica fume is available||Abundant||Abundant|