Admixtures of Concrete - CE

Admixtures

A material other than water, aggregates, or cement that is used as an ingredient of concrete or mortar to control setting and early hardening, workability, or to provide additional cementing properties.

Why is admixture used?

Over decades, attempts have been made to obtain concrete with certain desired characteristics such as high compressive strength, high workability, and high performance and durability parameters to meet the requirement of complexity of modern structures.

The properties commonly modified are the heat of hydration, accelerate or retard setting time, workability, water reduction, dispersion and air-entrainment, impermeability and durability factors.

Type of Admixtures

Chemical admixtures - Accelerators, Retarders, Water-reducing agents, Super plasticizers, Air entraining agents etc.

Mineral admixtures - Fly-ash Blast-furnace slag, Silica fume and Rice husk Ash etc

Chemical admixtures

1. Water-reducing admixture / Plasticizers:

These admixtures are used for following purposes:

1.    To achieve a higher strength by decreasing the water cement ratio at the same workability as an admixture free mix.

2.    To achieve the same workability by decreasing the cement content so as to reduce the heat of hydration in mass concrete.

3.    To increase the workability so as to ease placing in accessible locations

4.    Water reduction more than 5% but less than 12%

5.    The commonly used admixtures are Ligno-sulphonates and hydrocarbolic acid salts.

6.    Plasticizers are usually based on lignosulphonate, which is a natural polymer, derived from wood processing in the paper industry.

Actions involved:

 The following are the actions involved, 

1.    Dispersion:

Surface active agents alter the physic chemical forces at the interface. They are adsorbed on the cement particles, giving them a negative charge which leads to repulsion between the particles. Electrostatic forces are developed causing disintegration and the free water become available for workability.

2.    Lubrication:

As these agents are organic by nature, thus they lubricate the mix reducing the friction and increasing the workability.

3.    Retardation:

A thin layer is formed over the cement particles protecting them from hydration and increasing the setting time. Most normal plasticizers give some retardation, 30–90 minutes

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