Types of Admixtures For Concrete
Admixtures For Concrete are normally categorized according to their effect
(a) Plasticizers (water-reducing agents)
(b) Superplasticizers (high-range water reducers)
(c) Air entrainers
Many admixtures provide combinations of properties such as plasticizer/retarders or plasticizer/air entrainers. Each admixture type is discussed in the following sections.
(a) Plasticizers (Water-reducers)
Chemicals to improve the plasticity of fresh concrete that is improving workability for a given w/c ratio and maintain workability with a reduced amount of water.
The main types of plasticizers are lignosulphonic acids and their salts. (e.g. Ca, Na, NH, salts) hydroxylated carboxylic acids and their salts polyglycol esters and carbohydrates.
The typical dosage of a plasticizer varies from 200 ml to 450 ml per 100 kg of cementitious material plasticizers are used in the amount of 0.1% to 0.4% by weight of cement.
When plasticizers are added they get absorbed in the cement particles.
The adsorption of charged polymer on the cement particles creates repulsive forces between particles. This repulsive force is called zeta potential which depends upon the quantity of plasticizer used.
Hence structure of cement changes from flocculated to dispersed and thus water trapped inside the flocs gets released and fludify the mix.
The adsorbed plasticizer on the surface of cement inhibits the surface hydration of cement as long as sufficient plasticizers are available.
Plasticizer decreases as the polymers get entrapped in hydration products.
Plasticizers usually increase the slump of concrete with given water content.
Plasticizers can reduce the water requirement of a concrete mix for given workability as a rule of thumb by about 10%.
The addition of a plasticizer makes it possible to achieve a given strength with lower cement
Plasticizers may improve pump ability.
These admixtures are chemically distinct from normal plasticizers and although their action if basically the same, it is more marked.
When they are used to produce flowing concrete a rapid loss of workability can be expected and therefore they should be added just prior to placing.
Among the cement constituents, C₂A exerts a major influence on the properties of super-plasticizer. Finer the cement higher the super-plasticizer does.
Examples are sulphonated melamine formaldehyde condensates, naphthalene sulphonates formaldehyde condensates, modified lignosulphonate (MLS) and a mixture of saccharate and acid amines.
The higher the molecular mass higher is the efficiency of the super-plasticizer.
It is capable of reducing water requirements by 20 to 40%.
An air-entraining agent introduces air in the form of minute bubbles distributed uniformly throughout the cement paste. The main types include salts of wood resin, animal or vegetable fats and oils and sulphonated hydrocarbons.
Following are some examples of air-entraining agents:
1. Natural wood resins and their soaps, of which vinsol resin is the best.
2. Animal or vegetable fats and oils such as tallow, and olive oil and their fatty acids such as stearlic acids and oleic acids and their soaps.
3. Wetting agents such as alkali salts or sulphated and sulphonated organic compounds.
Air entrainment reduces the strength of concrete and overdosing can cause major loss of strength.
1% air may cause a strength loss of 5%.
The use of ground blast furnace slag and fly ash tends to reduce the amount of air entrained.
- Where improved resistance of hardened concrete to damage from freezing and thawing is required.
- For improved workability, especially in harsh or lean mixes.
- To reduce bleeding and segregation.
The effect of air entrainment on the properties of concrete:
(a) Reduces the tendencies of segregation.
(b) Reduces the bleeding.
(c) Decreases the permeability.
(d) increases the resistance to chemical attack.
(e) Permits reduction sand content.
(f) Improves placeability and early finishing
(g) Reduces the cement content cost and heat of hydration.
(h) Reduces the unit weight.
(i) Permits reduction in water content
(j) Reduces the alkali-aggregate reaction.
(k) Reduces the modulus of elasticity.
These admixtures (notably, calcium chloride) speed up the chemical reaction of the cement and water and so accelerate the rate of setting and/or early gain in strength of concrete.
The less commonly used accelerators consist of NaCl, Na₂SO3, NaOH, Na₂CO3, K₂SO4 and KOH.
Where rapid setting and high early strengths are required (e.g. in shaft sinking.)
Where the rapid turnover of moulds or formwork is required.
Concreting takes place under very cold conditions.
All chloride-based accelerators promote corrosion of reinforcing steel and should not be used in
(i) reinforced concrete
(ii) water-retaining structures
(iii) Accelerators work more effectively at lower ambient temperatures.
These admixtures slow the chemical reaction of the cement and water leading a longer setting times and slower initial strength gain.
The most common retarder is calcium sulphate. Other examples include hydroxylated carboxylic acids, lignins, sugar, cellulose products and some phosphates.
When placing concrete in hot weather, particularly when the concrete is pumped
To prevent cold joints due to duration of placing.
In concrete which has to be transported for a long time.
If a mix overdoses beyond the limit recommended by the supplier, retardation can last for days.
Retarders often increase plastic shrinkage and plastic settlement cracking.
Delayed addition of retarders can result in extended retardation.
Cement mortar or concrete should be impervious to water under pressure and also should have sufficient resistance to absorption of water.
Examples of water repellent materials such as soda and potash soaps are chemically active, whereas calcium soaps, resin, vegetable oils fats, waxes and coal tar residue are chemically inactive.
To stop bleeding, paraffin wax at about 0.2-0.75 per cent by mass of cement or air entrainment is used. Air entrainment latter is more effective but requires a high degree of control.
- These are siliceous materials which are themselves inactive but react in the presence of water with
time to form compounds having cementitious properties.
- Pozzolanas react with free lime in cement and improve the durability of concrete, and reduce the rate of hardening of concrete, which is the principal objection to its use.
- Examples of pozzolana are lime, fly ash, burnt clay and blast furnace slag.
NOTE: The use of fly ash as an admixture in concrete reduces the segregation and increases the workability of concrete.
- The main constituents of fly ash are:
2. Aluminium oxide
3. Ferrous oxide
- Fly ash is a pozzolana. Pozzolana is a siliceous or aluminosiliceous material that, in finely divided form and in the presence of mixture, chemically reacts with the calcium hydroxide released by the hydration of Portland cement to form additional calcium silicate hydrate and other cementitious compounds.
|Example – Which of the following admixture is not a chemically inactive water proofer?
(a) Soda soaps
Example – Consider the following statements:
1. improve workability
3. reduce segregation during placement
Which of the above statement(s) is/are correct?