An admixture in concrete is a material other than water, aggregates and Portland cement that is added to a concrete mixture immediately before or during mixing. Admixtures are used to modify a concrete mix’s properties, including its workability and reduction or acceleration of its setting time.
A variety of admixtures are available in the market that can be used for specific applications, but it is critical to understand the function and application of each type in order to make informed choices. Generally speaking, concrete admixtures are best utilized by professionals who have a detailed understanding of their interrelated effects and how to control them for maximum effect.
Accelerating admixtures speed up the initial set of concrete by increasing the rate of cement hydration. This enables shorter pouring times and early age strengths, especially in cooler temperatures. The use of accelerating admixtures is often required on construction projects with tight schedules, or where the project location is susceptible to climate variations. The most common accelerating admixtures are calcium chloride, triethenolamine and silica fume.
Water-reducing admixtures allow for the use of lower water content while retaining good workability. These admixtures are sometimes called super plasticizers and may reduce the concrete water content by up to 30%. They also increase the concrete strength, reduce its freeze-thaw susceptibility and enhance its workability and durability. Corrosion-inhibiting admixtures inhibit corrosion of reinforcing steel in the concrete and are usually used in marine structures, bridges and other projects that are exposed to high levels of chloride.