What is admixture?
The Admixture is defined as a material other than cement, aggregates, and water used as an ingredient of concrete, added to the immediately batch before or during mixing, to modify the properties of ordinary concrete to make it more suitable for the control situation.
Type of admixture
- Water Reducing
- Air entraining concrete
- Corrosion Inhibiting
- Fungicidal, Germicidal, Insecticidal
- Air detraining Admixture
- Alkali Aggregate Expansion Inhibiting
- Gas forming
Why is admixture used?
- The ingredients added to the concrete, which are not Portland cement, aggregates, or water, are known as an admixture. While it is not mandatory, sometimes selectively used to improve the concrete mix.
- The admixture is used in particular to alter its properties in several ways. Some common uses include improving workability, increase or decrease the curing time, and increase the strength of concrete.
- The additives can also be used for aesthetic reasons, to change the color of the cement. Whenever possible, the workability of concrete, water tightness, and resistance to being achieved by selecting suitable types of aggregate, Portland cement, and maintaining good water-cement ratio.
- When this is not possible or under special circumstances such as cold weather, high temperatures, greater wear, or prolonged exposure to deicing salts or other chemicals, additives may be useful.
A different function of admixture
- To delay the initial and final setting times.
- To modify the amount.
- To reduce the settlement.
- To reduce the rate of loss of settlement.
- To increase strength.
Admixture can be broadly classified into two types: Physical characteristics and Chemical characteristics. A mineral is basically derived from other substances and is not chemically manufactured. Fly ash, blast furnace slag, silica fume are popular examples of mineral.
- Fly ash - Greyish, lightweight & fine particles
- GGBFS - Off white powdery substance heavier than fly ash
- Silica fume - Amorphous, very fine particles, heavier than fly ash
- Rice husk ash - Amorphous, very lightweight, fine particles. (Threat to the environment.)
- Metakaolin - Whitish in color & fine particles.
- Fly ash -20 – 60 % silica ,5 – 35 % Al2O3,1 – 12 % CaO, Traces of MgO
- GGBFS -,28 – 38 % silica,8 – 24 % Al2O3,30 – 50 % CaO,1 – 18 % MgO
- Silica fume - 85 % silica,1 % Al2O3,6 % Fe2O3,12 % carbon
- Rice husk ash - 90 % silica,5 % carbon,3 % K2O
- Metakaolin - Dehydroxilated form of kaolinite clay
- Fly ash - By-product in thermal power plants
- GGBFS - By-product in the iron-producing blast furnaces
- Silica fume - By-product in silicon producing electric arc furnaces
- Rice husk ash - From rice husk
- Metakaolin - From kaolinite clay
% addition in concrete
- Fly ash -Upto 30 % cement replacement
- GGBFS - 25 – 70 % cement replacement
- Silica fume - 3 – 4 parts of cement replacement per 1 part of silica fume
- Rice husk ash - Around 20 % cement replacement
- Metakaolin - 8 – 10 % cement replacement
- Fly ash - Produced in abundance but availability is poor
- GGBFS -Sufficient amount of GGBFS is available
- Silica fume -Sufficient quantity of silica fume is available
- Rice husk ash - Abundant
- Metakaolin -Abundant
Advantages of admixture
- Fly ash - Improves the strength and durability of concrete, Increased resistance towards chemical attacks, Better workability.
- GGBFS - Improved durability, Increased setting time, Strength gain continues for a long period of time, Reduced risk of damages by the alkali-silica reaction, Resistant to chloride, and sulphate attacks.
- Silica fume - Better compressive strength and resistance to abrasion, Reduced permeability of chloride ions, Improved workability, Reduces bleeding.
- Rice husk ash - Heat of hydration of concrete is reduced, Permeability of concrete is reduced, Improved resistance to chloride and sulphate attacks.
- Metakaolin - Increased compressive & flexural strengths, Reduced permeability, More resistant to chemical attacks, Durability is increased.
Disadvantage of Admixture
- Drying shrinkage has increased.
- It offers reduced resistance to sulfate attack.
- CaCl2 high risk of steel corrosion - not allowed in reinforced concrete.
- It is more expensive and less effective.