SPONSORED
Chemical Admixtures in Concrete Mix : Plasticizers
Plasticizers or water reducers, and superplasticizer or high range water reducers, are chemical admixtures that can be added to concrete mixtures to improve workability. Unless the mix is "starved" of water, the strength of concrete is inversely proportional to the amount of water added or water-cement (w/c) ratio. In order to produce stronger concrete, less water is added, which makes the concrete mixture less workable and difficult to mix, necessitating the use of plasticizers, water reducers, superplasticizers or dispersants.
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 workability.
Lubrication
As these agents are organic by nature, thus they lubricate the mix reducing the friction and increasing the workability.
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.
