Admixtures

ADMIXTURES  are the chemical that added into the concrete before or during mixing.
The admixtures can considered as 2 types, which are chemical admixtures and mineral admixtures.
There are 5 types of admixtures:
Plasticisers, Superplasticisers, Accelerators, Retarders, Air Entraining Agents


Chemical Admixtures:

Plasticiser

Plasticisers (it also known as water-reducers.)


Plasticisers are ploymers, they can absorbed onto the surface of cement grains with an ionic group pointing outwards. Beside that, it increased the workability of concrete without increasing water content.
Example: Carbohydrates and organic acids was material used of water-reducing.
With the lower of water/cement ratio which will increases the strength of the concrete with the same amount of cement.
There are few effects of Plasticisers which is:
- delaying (retarding) the set, make the concrete mixture difficult to mix and less workable
- decreasing the early strength gain of the concrete
- entrain air in the form of air bubble
- contain impurities affecting strength of hardened concrete
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Superplasticisers  (it also known as high-range water reducers.)


Superplasticisers was added into concrete mixtures which to improve the
fluidity and workability. The effect of Superplasticisers is greater than
obtainable with Plasticisers.
Normally only 1 or 2% of Superplasticisers was used of cement which is sufficient enough.
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Accelerators are mostly used in
 winter season countries.

Accelerators which is used to increase rate of setting to get the high speed of the heat revolution and it was suggested to be used in cold weather.
Example: Calcium Chloride (CaCl2) was the commonly thing which used in the cement to accelerators both the initial and final setting stage due to it was effective and readily available.
CaCl2 not recommended to be used in reinforced concrete (rc) due to it lead to corrosion of embedded steel reinforcement.
Beside, there are few number of alternative chloride-free accelerators which now are used based on to calcium formate, sodium aluminate or triethanolamine.
There are used to increase the hardening of cement paste, thereby it also:
- enhancing the early stage of strength gain
- reducing the curing time which the concrete was placed in cold weather
- reducing the setting time
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Retarders is used to delay the time of cement mix.

Retarders was uses to :
- counteracting the accelerating effect of hot weather
- controlling the set in large pours which the concreting may take few several hours
- when the concrete has to be transported over long distance it used to delaying the set
To retard the setting of cement admixture based on sugars (sucrose), starches, citric acid, zinc oxide or boric oxide.
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Air in the cement

Air Entraining Agents (AEAs) is used to entrain a controlled quantity of air in the form of microscopic, discontinuous and uniformly distributed bubbles in the cement paste.
The primary effect of entrain air causes disruption of the continuity of capillary pores and reduce the internal stress caused by expansion of pore water during freezing of the cement.
There are 2 effects of Air Entrainment :
- improvement in workability of mix.
- increase in porosity result in drop in strength, improvement in workabiltiy means the loss can be partially offset by reducing the water/cement ratio.
Example: Vinsol Resins, Alkysulfonates and Alkylsufates can be achieved as Air Entrainment.
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Damp-proofing Admixtures is use to prevent water movement by capillary action.

These are some videos about Admixture:

Retarders

Air Entraining Agents

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Mineral Admixtures:

Admixtures that having cementitious properties, which react mainly with the calcium hydroxide liberated by the hydration process.

Fly Ash (pozzoloans)

This admixture is to reduce the permeability, voids, expansion, heat of hydration, bleeding and segregation.

Besides, it increases compressive strength, workability, durability, flexual strength of concrete, protect the steel reinforcement from corrosion, lower the water requirement.

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Silica Fume (also known as mircosilica)

It is added to the concrete mixing to improve concrete properties, reduces the permeability of concrete to the chloride ions to protect the steel reinforcement from corrosion which usually located in the chloride-rich areas (sea).