A cement accelerator is an admixture for the use in

concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wid ...

, mortar, rendering or screeds. The addition of an accelerator speeds the setting time and thus cure time starts earlier.Justnes, H. (2000): Accelerator Blends for Portland Cement. Proceedings of Cement and Concrete Technology in the 2000s, September 6–10, 2000, Istanbul, Turkey, Vol. 1, pp. 433-442 This allows concrete to be placed in winter with reduced risk of frost damage.ACI 306R-88: Cold Weather Concreting. Concrete is damaged if it does not reach a strength of before freezing. Typical chemicals used for acceleration today are

calcium nitrate Calcium nitrate, also called ''Norgessalpeter'' (Norwegian salpeter), is an inorganic compound with the formula Ca(NO3)2(H2O)x. The anhydrous compound, which is rarely encountered, absorbs moisture from the air to give the tetrahydrate. Both anhyd ...

(Ca(NO₃)₂), calcium nitrite (Ca(NO₂)₂), calcium formate (Ca(HCOO)₂) and aluminium compounds.

Calcium chloride Calcium chloride is an inorganic compound, a salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide. Ca ...

(CaCl₂) is the most efficient and least expensive accelerator and was formerly very popular. However,

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...

anions are very corrosive for the steel of the

reinforcement bar Rebar (short for reinforcing bar), known when massed as reinforcing steel or reinforcement steel, is a steel bar used as a tension device in reinforced concrete and reinforced masonry structures to strengthen and aid the concrete under tension ...

s (rebars) so its use is no longer recommendedCement Admixtures Association: Admixture Sheet – ATS 4: Accelerating admixtures, and in many countries actually prohibited. This ''de facto'' caution comes from the fact that

anions cause severe

pitting corrosion Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic (oxidation re ...

of steel rebars. As the local corrosion of a rebar can lead to its rupture, it can compromise the tensile strength of reinforced concrete and so the structural stability of an edifice.

Thiocyanate Thiocyanate (also known as rhodanide) is the anion . It is the conjugate base of thiocyanic acid. Common derivatives include the colourless salts potassium thiocyanate and sodium thiocyanate. Mercury(II) thiocyanate was formerly used in pyr ...

compounds can also corrode reinforcing but are safe at recommended dosage levels. Sodium compounds might compromise the long term compressive strengthVolset, D. (2010). The use of antifreeze agents. http://www.mapei.com/public/NO/linedocument/the_use_of_anti-freeze_agents.pdf if used with alkali-reactive aggregates. Novel alternatives include cement based upon calcium sulphoaluminate (CSA), which sets within 20 minutes, and develops sufficient rapid strength that an airport

runway According to the International Civil Aviation Organization (ICAO), a runway is a "defined rectangular area on a land aerodrome prepared for the landing and takeoff of aircraft". Runways may be a man-made surface (often asphalt, concre ...

can be repaired in a six-hour window, and be able to withstand aircraft use at the end of that time, as well as in

tunnel A tunnel is an underground passageway, dug through surrounding soil, earth or rock, and enclosed except for the entrance and exit, commonly at each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube cons ...

s and underground, where water and time limitations require extremely fast strength and setting.

References

Concrete Concrete admixtures {{engineering-stub