Admixture is defined as a material, other than cement, water and aggregates, that is used as an ingredient of concrete and is added to the batch immediately before or during mixing to modify one or more properties of concrete in plastic or hardened state.
Function of Admixtures:
- To increase the strength, durability and workability of concrete.
- To increase the resistance to chemical attack, freezing and thawing effects.
- To accelerate or retard the rate of hydration of cement.
- To reduce the permeability of concrete.
- To reduce the segregation and bleeding of concrete.
- To reduce the shrinkage during setting of concrete.
- To control the alkali-aggregate reactions.
Classifications of Admixtures:
- Chemical Admixtures
- Mineral Admixtures
CHEMICAL ADMIXTURES:
- Accelerators
- Retarders
- Plasticizers
- Super Plasticizers
- Colouring Admixture
- Air entraining Admixture
- Water Proofing Admixture
Accelerators:
Accelerating admixtures are added to concrete to increase the rate of early strength
development in concrete to
- permit earlier removal of formwork;
- reduce the required period of curing;
- advance the time that a structure can be placed in service
- partially compensate for the retarding effect of low temperature during cold weather
concreting - in the emergency repair work.
Calcium chlorides, caustic soda, caustic potash, etc are commonly used accelerators.
Retarders:
A retarder is an admixture that slows down the chemical process of hydration so that concrete remains plastic and workable for a longer time than concrete without the retarder. Calcium sulphate (gypsum), calcium borate, glucose etc are commonly used retarders.
Plasticizers (Water Reducers):
The organic substances or combinations of organic and inorganic substances, which allow a reduction in water content for the given workability, or give a higher workability at the same water content, are termed as plasticizing admixtures. The basic products constituting plasticizers are as follows:
- Anionic surfactants such as lignosulphonates and their modifications and derivatives, salts of sulphonates hydrocarbons.
- Nonionic surfactants, such as polyglycol esters, acid of hydroxylated carboxylic acids and their modifications and derivatives.
- Other products, such as carbohydrates etc.
Lignosulphonic acid is most commonly used plasticizers.
When plasticizers are used, they get adsorbed on the cement particles. The adsorption of charged polymer on the particles of cement creates particle-to-particle repulsive forces which overcome the attractive forces. This repulsive force is called Zeta Potential, which depends on the base, solid content, quantity of plasticizer used. The overall result is that the cement particles are deflocculated and dispersed. When cement particles are deflocculated, the water trapped inside the flocs gets released and now available to fluidify the mix.
Plasticizers are of two types:
- Accelerating Plasticizers
- Retarding Plasticizers
Superplasticizers:
Superplasticizers, also known as High Range Water Reducers, are the improved version of plasticizer used for production of flowing, self levelling, self
compacting and for the production of high strength and high performance concrete which permit the reduction of water to the extent upto 30 percent without reducing workability in contrast to the possible reduction up to 15 percent in case of plasticizers..Superplasticizers can produce:
- at the same w/c ratio much more workable concrete than the plain ones,
- for the same workability, it permits the use of lower w/c ratio,
- as a consequence of increased strength with lower w/c ratio, it also permits a reduction of cement content.
- The superplasticizers also produce a homogeneous, cohesive concrete generally without any tendency for segregation and bleeding.
Carboxylic acrylic ester (CAE) and multicarboxylatether (MCE) are generally used as superplasticizers.
Air Entraining Admixtures:
Air entrainment will effect directly the following three properties of concrete:
- Increased resistance to freezing and thawing.
- Improvement in workability.
- Reduction in strength.
The common air entraining agents are Vinsol resin, Darex, N Tair, Airalon, Orvus, Teepol, Petrosan and Cheecol.
Water Proofing Admixtures:
It consists of water repellent materials such as calcium chlorides, zinc sulphates, aluminium sulphates etc. and is generally used in hydraulic structures.
POZZOLONIC OR MINERAL ADMIXTURE:
Pozzolanic materials are siliceous or siliceous and aluminous materials, which in themselves possess little or no cementitious value, but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide liberated on hydration, at ordinary temperature, to form compounds, possessing cementitious properties. Pozzolanic materials can be divided into two groups: natural pozzolana and artificial pozzolana.
Natural Pozzolans
- Clay and Shales
- Opalinc Cherts
- Diatomaceous Earth
- Volcanic Tuffs and Pumicites.
Artificial Pozzolans
- Fly ash
- Blast Furnace Slag
- Silica Fume
- Rice Husk ash
- Metakaoline
- Surkhi.
Fly Ash:
Fly ash is finely divided pozzolanic material resulting from the combustion of powdered coal and transported by the flue gases and collected by electrostatic precipitator. Use of right quality fly ash, results in reduction of water demand for desired slump. With the reduction of unit water content, bleeding and drying shrinkage will also be reduced.
Silica Fume:
Silica fume, also referred to as microsilica or condensed silica fume, is another material that is used as an artificial pozzolanic admixture. It is a product resulting from reduction of high purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy. It is reported that fresh concrete containing microsilica is vulnerable to plastic shrinkage cracking and, therefore, sheet or mat curing should be considered. However, Hardened Concrete containing microsilica showed outstanding characteristics in the development of strength and durability.
Rice Husk Ash:
Rice husk ash, is obtained by burning rice husk in a controlled manner without causing environmental pollution. When properly burnt it has high `SiO_2` content and can be used as a concrete admixture. Rice husk ash exhibits high pozzolanic characteristics and contributes to high strength and high impermeability of concrete.
Surkhi:
Surkhi is an artificial pozzolana made by powdering bricks or burnt clay balls. It is used as a water proofing agent. It is not commonly used nowadays.
Metakaolin:
Unpurified materials like activated ordinary clay and kaolintic clay are terned as Metakaolin. Highly reactive metakaolin is made by water processing to remove unreactive impurities to make 100% reactive pozzolan. Such a product, white or cream in colour, purified, thermally activated is called High Reactive Metakaolin (HRM). The high reactive metakaolin is having the potential to compete with silica fume.
Local Materials as Admixtures in Nepal:
Present possibilities:
- Rice husk ash
- Calcined clay pozzolona (Brick dust / Burned clay dust)
- Stone dust
Future possibilities with industrial development:
- Silica fume
- Fly ash
- Blast furnace slag
- Various chemical admixtures.
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