Cement accelerator

A cement accelerator is an admixture for the use in concrete, mortar, rendering or screeds. The addition of an accelerator speeds the setting time and thus cure time starts earlier.[1] This allows concrete to be placed in winter with reduced risk of frost damage.[2] Concrete is damaged if it does not reach a strength of 500 pounds per square inch (3.4 MPa) before freezing.[3]: 19 

Typical chemicals used for acceleration today are calcium nitrate (Ca(NO3)2), calcium nitrite (Ca(NO2)2), calcium formate (Ca(HCOO)2) and aluminium compounds. Calcium chloride (CaCl2) is the most efficient and least expensive accelerator[4] and was formerly very popular. However, chloride anions are very corrosive for the steel of the reinforcement bars (rebars) so its use is no longer recommended[5] and in many countries actually prohibited. This de facto caution comes from the fact that chloride anions cause severe pitting corrosion 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 compounds can also corrode reinforcing but are safe at recommended dosage levels.[6] Sodium compounds might compromise the long term compressive strength[7] if used with alkali-reactive aggregates.[3]: 6 

Novel alternatives include cement based upon calcium sulphoaluminate (CSA), which sets within 20 minutes, and develops sufficient rapid strength that an airport runway 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 tunnels and underground, where water and time limitations require extremely fast strength and setting.[8]

References
  1. 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
  2. ACI 306R-88: Cold Weather Concreting. "Archived copy" (PDF). Archived from the original (PDF) on 2011-07-25. Retrieved 2011-03-05.{{cite web}}: CS1 maint: archived copy as title (link)
  3. Korhonen, Charles J.; Cortez, Edel R.; Durning, Timothy A. (1997), "Antifreeze Admixtures for Concrete", Special Report 97-26, Cold Regions Research and Engineering Laboratory, ISBN 9781428913158
  4. "ASCC Position Statement #31" (PDF). Concrete International. Vol. 32, no. 2. American Concrete Institute. February 1, 2010. p. 55.
  5. Cement Admixtures Association: Admixture Sheet – ATS 4: Accelerating admixtures, "Archived copy" (PDF). Archived from the original (PDF) on 2011-09-03. Retrieved 2011-03-05.{{cite web}}: CS1 maint: archived copy as title (link)
  6. Nmai, Charles; Corbo, Jack (November 1, 1989). "Sodium thiocyanate and corrosion of steel in concrete and mortar". Concrete International. Vol. 11, no. 11. pp. 59–67.
  7. Volset, D. (2010). The use of antifreeze agents. http://www.mapei.com/public/NO/linedocument/the_use_of_anti-freeze_agents.pdf
  8. "The Use of Calcium Sulfoaluminate Rapid Setting Cement for Underground Construction". 26 November 2013.


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