An accelerator admixture speeds up the set time of concrete by accelerating the cement hydration process. This increases the early strength gain of concrete but does not affect the final strength. This admixture is especially useful when cold weather concreting conditions hinder normal concrete setting times, or if the project requires rapid start-up of concrete placement.

Accelerators are typically liquid and can be added to the mix immediately before or during mixing. Unlike powdered concrete admixtures, they can be dispensed in a controlled manner into the mixer to ensure that a uniform mix is achieved. This can be done using a metered batch system that takes the admixture from a bulk tank directly into the mixer, or in smaller quantities through a metering tube or glass dispensing bottle.

There are several different types of concrete accelerating chemical admixtures available, all with different effects and properties on the microstructure and behavior of the concrete. Generally, they are classified into four categories: soluble inorganic salts, soluble organic compounds, quick-setting admixtures and miscellaneous materials. Manufacturers may also combine raw materials to create custom blends of concrete accelerating chemicals.

One of the most common accelerating admixtures is calcium chloride, which was used widely in the past for its low cost and rapid hydration. However, it has been found to cause problems such as excessive water reduction and corrosion of reinforcing steel in long-term applications. Non-chloride accelerating admixtures such as DARASET 400 are now available to achieve the same quick setting and high early strength of calcium chloride, without the potential for corrosion of embedded or stress-relieved metal.