Accelerants for Curing Concrete


Accelerating admixtures, also known as accelerants, are added to concrete to shorten set time and provide timely strength to concrete compounds. Accelerants may be inorganic compounds such as soluble chlorides, silicates, fluosilicates and carbonates, or an organic compound such as triethanolamine. All accelerants reduce cure times so less time is required to protect the concrete from freezing during cold weather pours. One drawback to most accelerants is the increased shrinkage it can cause as the concrete dries.

Calcium Chloride

  • Calcium chloride is the most common accelerant used in concrete and has been used to speed up the setting and hardening process since 1885. The regular flake form contains a minimum of 77 percent calcium chloride while concentrated flake, pellet or granular calcium chloride contains a minimum of 94 percent. The most commonly available liquid calcium chloride used for commercial purposes, has a calcium chloride percentage of 29.

Drawbacks of Calcium Chloride

  • At early stages, calcium chloride added to concrete increases drying shrinkage although sodium sulfate added to the mix reduces the shrinkage somewhat. Calcium chloride has an unfortunate corrosive effect on metal embedded in concrete. This tendency has limited the use of calcium chloride accelerants where metal rebar is imbedded in the pour. Engineers have had to look for accelerants that do not contain chloride. The result has been the development of sulfates, formates and nitrates for concrete mixes.

Non- Chloride Accelerants

  • A reaction between ammonia and ethylene oxide produces an oily, water-soluble liquid called triethanolamine -- another accelerant alternative to calcium chloride. This fishy-smelling compound is usually mixed with other compounds in a mix and rarely used alone. Three other non-chloride accelerants now in use include calcium formate, calcium nitrite and calcium thiosulfate. The latter two increase the strength of concrete in its early developmental stages. Calcium formate must be used in higher concentrations to equal the effects of sodium chloride.

Further Considerations

  • All concrete mixtures altered by accelerants must be tested for chloride, ph and solids content. Infrared spectrometry should also be used to ensure that altered mixes meet the specifications of the concrete that was originally approved for a particular application. Calcium chloride should never be used as an accelerant in wet or warm weather because of its corrosive effect on re-inforcing metals. Some non-calcium chloride accelerators may be soluble salts that can also cause corrosion. Care must therefore be taken in accelerant selection for warm weather builds.

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