Admixtures Used In Concrete and its Mechanism for action
Admixture mechanism of action
Admixtures work by one or more of the following actions:
- Chemical interaction with the cement hydration process, typically causing acceleration or retardation of the rate of reaction of one or more of the cement phases.
- Adsorption onto cement surfaces, typically causing better particle dispersion (plasticizing or super plasticizing action).
- Affecting the surface tension of the water, typically resulting in increased air entrainment.
- Affecting the rheology of the water, usually resulting in increased plastic viscosity or mix cohesion.
- Introducing special chemicals into the body of the hardened concrete that can affect specific properties such as corrosion susceptibility of embedded steel or water repellence. The benefits obtained from using the admixture are often in hardened properties.
However, with the possible exception of the last bullet point, these actions all affect the properties of the wet concrete between the time of mixing and hardening in one or more of the following ways to Admixtures Used In Concrete :
Affect water demand Plasticizing or water reducing
- Change the stiffening rate Accelerating/retarding Change the air content Increase (or decrease) entrained air
- Change the plastic viscosity Cohesion or resistance to bleed and segregation of the mix
One of these effects will usually be the primary property, the property for which the admixture is being used. However, the admixture can also affect one or more of the other wet properties. These are called secondary effects and it is often these which are key to admixture selection within an admixture type.
For example, a water-reducing admixture may be required to give low water-cement ratio for durability but if the concrete mix lacks cohesion and is prone to bleed, a water-reducing admixture which also increases cohesion would be appropriate to prevent the bleed. The primary function is as a water reducer; the secondary function is to improve cohesion. Incorrect admixture selection could make the bleed worse. Because of these secondary effects, an admixture that works well on one concrete plant with one source of cement and aggregates could be quite unsuitable on another plant a few miles away from where a different aggregate source is being used or a different binder combination is employed.