Types of Admixtures:
1. Accelerator:
- It is used when concrete is to be placed at low temperature.
- To develop early strength of concrete.
- With availability of powerful accelerator underground concreting, water proofing operations, repair works of water front structures becomes very easy.
- eg: Calcium chloride, Na2SO4, Na2CO3.
2. Retarding Admixtures:
- Retarders delay setting times by forming a thin coating on cement particle thus slowing down the reaction with water.
- It does not affect significantly the final setting time of cement.
- eg: Gypsum, Starch, etc.
3. Air entraining Admixture:
(i) Entrained air:
- It is intentionally incorporated minute spherical bubbles of size ranging from 5-80 micron distributed uniformly in entire concrete mass.
- Air entrainment improves workability of concrete.
- It also improve its durability & plasticity.
- it may have an adverse effect on strength of concrete but improvement in workability, plasticity and duability compensate for most of its loss.
- A satisfactory air entraining agent must not react chemically with cement.it lubricates the concrete mix and act like flexible ball bearing to improve the mobility of concrete mix by reducing friction between particles.
- Eg: Natural wood resin, Vinsol resin.
ii) Entraped Air:
- It is the voids present in the concrete due to insufficient compaction.
- These are the source of weakness in concrete in terms of strength and durability.
4. Water reducing admixture:
- These are the organic compounds or the combination of organic and inorganic compound added in concrete mix as a water reducing agent.
- Also known as Plasticizer. The mechanism involved in this is Dispersion.
- The action of plasticizer is mainly to fludify the mix and improve the workability of concrete mix.
- It provides concrete with low water cement ratio.
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