Chemical admixtures are natural or manufactured chemicals that are added to concrete along with portland cement, aggregate, and water. They can be a liquid or powder and are added to the concrete immediately before or during the mixing process. The chemical admixtures in a mix will account for less than 5% of the total concrete.
The purpose of chemical admixtures is to give the concrete additional properties to handle circumstances such as pumping requirements or when placing concrete during extreme weather conditions. According to the Portland Cement Association, “Admixtures are classed according to function. There are five distinct classes of chemical admixtures: air-entraining, water-reducing, retarding, accelerating, and plasticizers (superplasticizers).”
The National Ready Mix Concrete Association (NRMCA) says that the most common chemical admixtures used are air-entraining agents, water-reducers, and water-reducing retarders and accelerators. They state that admixtures are added to concrete to increase “durability workability or strength characteristics of a given concrete mixture.”
Accelerators are used to increase the early strength of concrete by causing it to harden quicker and are used to protect concrete from extreme cold or when there is a tight project deadline. Chemical accelerators are chloride based or non-chloride based and calcium chloride is one type commonly used as it is inexpensive.
Retardants are used to slow down or delay the hardening process such as in situations where the job demands a slower pour due to the scope of the project or when in very hot temperatures. It provides a crew additional time to place and finish the concrete. Common retardants are lignin, sugars, and citric and tartaric acids.
Air-entraining is the process of deliberately adding microscopic air bubbles into the concrete. The purpose of doing this is to increase the concrete’s durability when faced with cycles of freezing and thawing conditions and to improve the concrete’s workability. Furthermore segregation and bleeding is diminished or completely eliminated. The only negative is that the concrete experiences a 5% reduction in strength for each 1% of air added.
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