Editing Concrete (section)

===Admixtures===
Admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. Admixtures are defined as additions "made as the concrete mix is being prepared".<ref name=text>{{cite book|isbn=978-0-7277-3611-6 |doi=10.1680/pc.36116.185 |chapter=Admixtures and Special Cements |title=Portland Cement: Third edition |author1=Gerry Bye |author2=Paul Livesey |author3=Leslie Struble |year=2011|doi-broken-date=1 November 2024 }}</ref> The most common admixtures are retarders and accelerators. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of batching/mixing.<ref name="FHWA Admixtures">{{cite web |author=U.S. Federal Highway Administration |author-link=Federal Highway Administration |title=Admixtures |url=http://www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.html |access-date=25 January 2007 |date=14 June 1999 |archive-url=https://web.archive.org/web/20070127132641/http://www.fhwa.dot.gov/infrastructure/materialsgrp/admixture.html |archive-date=27 January 2007 }}</ref> (See {{section link||Production}} below.) The common types of admixtures<ref>{{cite web |author=Cement Admixture Association |url=http://www.admixtures.org.uk/types.asp |title=Admixture Types |access-date=25 December 2010 |archive-url=https://web.archive.org/web/20110903081932/http://www.admixtures.org.uk/types.asp |archive-date=3 September 2011 }}</ref> are as follows:

* [[Accelerant|Accelerators]] speed up the hydration (hardening) of the concrete. Typical materials used are [[calcium chloride]], [[calcium nitrate]] and [[sodium nitrate]]. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored, even though they are less effective than the chloride salt. Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather.
* [[Air entrainment|Air entraining agents]] add and entrain tiny air bubbles in the concrete, which reduces damage during [[Weathering|freeze-thaw]] cycles, increasing [[Reinforced concrete structures durability|durability]]. However, entrained air entails a tradeoff with strength, as each 1% of air may decrease compressive strength by 5%.<ref>{{cite web |last1=Hamakareem |first1=Madeh Izat |title=Effect of Air Entrainment on Concrete Strength |url=https://theconstructor.org/concrete/effect-air-entrainment-concrete-strength/8427/ |website=The Constructor |date=14 November 2013 |access-date=13 November 2020}}</ref> If too much air becomes trapped in the concrete as a result of the mixing process, [[defoamer]]s can be used to encourage the air bubble to agglomerate, rise to the surface of the wet concrete and then disperse.
* Bonding agents are used to create a bond between old and new concrete (typically a type of polymer) with wide temperature tolerance and corrosion resistance.
* [[Corrosion inhibitor]]s are used to minimize the corrosion of steel and steel bars in concrete.
* Crystalline admixtures are typically added during batching of the concrete to lower permeability. The reaction takes place when exposed to water and un-hydrated cement particles to form insoluble needle-shaped crystals, which fill capillary pores and micro-cracks in the concrete to block pathways for water and waterborne contaminates. Concrete with crystalline admixture can expect to self-seal as constant exposure to water will continuously initiate crystallization to ensure permanent waterproof protection.
* [[Pigment]]s can be used to change the color of concrete, for aesthetics.
* [[Plasticizer]]s increase the workability of plastic, or "fresh", concrete, allowing it to be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics.
* [[Superplasticizer]]s (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Superplasticizers are used to increase compressive strength. It increases the [[#Sample analysis—workability|workability]] of the concrete and lowers the need for water content by 15–30%. 
* Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.
* [[Retarder (chemistry)|Retarders]] slow the hydration of concrete and are used in large or difficult pours where partial setting is undesirable before completion of the pour. Typical retarders include [[sugar]], [[sodium gluconate]], [[citric acid]], and [[tartaric acid]].<ref>{{Citation |last=Bensted |first=John |title=14 - Special Cements |date=1998-01-01 |work=Lea's Chemistry of Cement and Concrete (Fourth Edition) |pages=783–840 |editor-last=Hewlett |editor-first=Peter C. |url=https://linkinghub.elsevier.com/retrieve/pii/B9780750662567500266 |access-date=2024-11-03 |place=Oxford |publisher=Butterworth-Heinemann |doi=10.1016/b978-075066256-7/50026-6 |isbn=978-0-7506-6256-7}}</ref>