What's the purpose of using the water-reducing admixture and plasticizers ?

What s the purpose of using the water-reducing admixture and plasticizers ?
 

Admixture       
                                                                       
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.

Types 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

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:

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 work-ability.

Lubrication:

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

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

Super Plasticizers:

These are more recent and more effective type of water reducing admixtures also known as high range water reducer. The main benefits of super plasticizers can be summarized as follows:

Increased fluidity:

  • Flowing

  • Self-leveling

  • Self-compacting concrete

  • Penetration and compaction round dense reinforcement

Reduced W/C ratio:

  • Very high early strength, >200% at 24 hours or earlier

  • Very high later age strengths, >100 MPa or 15000 psi.

  • Reduced shrinkage, especially if combined with reduced cement content.

  • Improved durability by removing water to reduce permeability and diffusion.


The commonly used Super Plasticizers are as follows:

Sulphonated melamine formaldehyde condensates (SMF)

Give 16–25%+ water reduction. SMF gives little or no retardation, which makes them very effective at low temperatures or where early strength is most critical. However, at higher temperatures, they lose workability relatively quickly. SMF generally give a good finish and are colorless, giving no staining in white concrete. They are therefore often used where appearance is important.

Sulphonated naphthalene formaldehyde condensates (SNF)

Typically give 16–25%+ water reduction. They tend to increase the entrapment of larger, unstable air bubbles. This can improve cohesion but may lead to more surface defects. Retardation is more than with SMF but will still not normally exceed 90 minutes. SNF is a very cost-effective.

Polycarboxylate ether superplasticizers (PCE)

Typically give 20–35%+ water reduction. They are relatively expensive per liter but are very powerful so a lower dose (or more dilute solution) is normally used.
In general the dosage levels are usually higher than with conventional water reducers, and the possible undesirable side effects are reduced because they do not markedly lower the surface tension of the water.
 
 
 

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