Admixtures are a mix of several chemicals or single chemical that are added to mortar, concrete or grout during mixing or in wet condition just after mixing or even after hardened mix to have a concrete of desired of properties apart from natural concrete.
There have some standard that are used to define and evaluating the quality of admixture. New European Standard EN 934 was developed in 2002 which covers almost all type of admixture. The current part 2, out of recently developed five parts, covers concrete.
Parts 1-3 of BS 5075 also deals with concrete admixture and BS 4887 covers admixture for mortars. ASTM C494 is the most precisely specified standard that are widely used in USA and many other countries of the world as a reference for defining, using and selection of admixture.
Mechanism of working admixtures follows one or more of the following actions:
• Chemical influence on the hydration process of cement, Changing the rate of stiffening either by accelerating or retarding the rate of chemical reaction in one or more cementing phase.
• Achieving better dispersion of particles producing adsorption on cement surfaces. This action results plasticizing or superplasticizing.
• Achieving increment of air entrainment by influencing the surface tension of water.
• Increasing mix cohesion or plastic viscosity influencing the water rheology.
• To introduce specific properties, like increasing corrosion protection of steel embedded with concrete or water repellence, using special chemical over the hardened surface of concrete.