Cement accelerator
{{Short description|Chemical used in concrete production}}
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.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. {{cite web|url=http://www.ccagc.org/pdfs/ACI_306R-88_Cold_Weather_Concreting.pdf |title=Archived copy |accessdate=2011-03-05 |url-status=dead|archiveurl=https://web.archive.org/web/20110725150450/http://www.ccagc.org/pdfs/ACI_306R-88_Cold_Weather_Concreting.pdf |archivedate=2011-07-25 }} Concrete is damaged if it does not reach a strength of {{convert|500|psi|MPa}} before freezing.{{Citation |last1=Korhonen |first1=Charles J. |last2=Cortez |first2=Edel R. |last3=Durning |first3=Timothy A. |institution=Cold Regions Research and Engineering Laboratory |year=1997 |title=Antifreeze Admixtures for Concrete |work=Special Report 97-26 |isbn=9781428913158}}{{rp|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{{cite magazine |date=February 1, 2010 |title=ASCC Position Statement #31 |url=https://www.ascconline.org/Portals/0/docs/POSITION-STATEMENTS/PS-31_Calcium%20Chloride.pdf |magazine=Concrete International |volume=32 |issue=2 |page=55 |publisher=American Concrete Institute |access-date=June 23, 2016 |archive-date=January 23, 2016 |archive-url=https://web.archive.org/web/20160123005850/http://www.ascconline.org/Portals/0/docs/POSITION-STATEMENTS/PS-31_Calcium%20Chloride.pdf |url-status=dead }} 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 recommendedCement Admixtures Association: Admixture Sheet – ATS 4: Accelerating admixtures, {{cite web|url=http://www.admixtures.org.uk/downloads/ATS%204%20Accelerating%20admixtures.pdf |title=Archived copy |accessdate=2011-03-05 |url-status=dead|archiveurl=https://web.archive.org/web/20110903081208/http://www.admixtures.org.uk/downloads/ATS%204%20Accelerating%20admixtures.pdf |archivedate=2011-09-03 }} 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.{{cite magazine |last1=Nmai |first1=Charles |last2=Corbo |first2=Jack |date=November 1, 1989 |title=Sodium thiocyanate and corrosion of steel in concrete and mortar |magazine=Concrete International |volume=11 |issue=11 |pages=59–67}} 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.{{rp|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.{{Cite web|url=https://tunnelingonline.com/use-calcium-sulfoaluminate-rapid-setting-cement-underground-construction|title = The Use of Calcium Sulfoaluminate Rapid Setting Cement for Underground Construction|date = 26 November 2013}}