Editing Stalactite (section)

===Concrete stalactites===
[[Image:Mostar interchange concrete stalactites.jpg|right|thumb|Concrete stalactites]]
[[File:Calthemite straw stalactites - air movement.jpg|thumb|left|upright|[[Calthemite]] soda straw stalactites under a concrete slab]]
{{Main|Calthemite}}

Stalactites can also form on [[concrete]], and on plumbing where there is a slow leak and where there are [[Water hardness|calcium, magnesium or other ions]] in the water supply, although they form much more rapidly there than in the natural cave environment.  These secondary deposits, such as stalactites, stalagmites, flowstone and others, which are derived from the lime, mortar or other calcareous material in [[concrete]], outside of the "cave" environment, can not be classified as "[[speleothem]]s" due to the definition of the term.<ref name=Hill&Forti1986 /> The term "[[calthemite]]" is used to encompass these secondary deposits which mimic the shapes and forms of speleothems outside the cave environment.<ref name=SmithGK2016>Smith, G K. (2016). "Calcite straw stalactites growing from concrete structures". Cave and Karst Science 43(1), pp4-10.</ref>

The way stalactites form on concrete is due to different chemistry than those that form naturally in limestone caves and is due to the presence of [[calcium oxide]] in cement.  Concrete is made from aggregate, sand and cement. When water is added to the mix, the calcium oxide in the cement reacts with water to form [[calcium hydroxide]] (Ca(OH)<sub>2</sub>). The [[chemical formula]] for this is:<ref name="Classroom">{{Citation | last1 =Braund | first1 =Martin  | last2 =Reiss  | first2 =Jonathan | publication-date =2004 | title =Learning Science Outside the Classroom | pages=155–156| publisher =Routledge | isbn =0-415-32116-6 | year =2004
}}</ref>

::{{chem|Ca||O||(s)}} + {{chem|H|2|O||(l)}} → {{chem|Ca||(OH)|2||(aq)}}

Over time, any rainwater that penetrates cracks in set (hard) concrete will carry any free [[calcium hydroxide]] in [[Solution (chemistry)|solution]] to the edge of the concrete. Stalactites can form when the solution emerges on the underside of the concrete structure where it is suspended in the air, for example, on a ceiling or a beam.  When the solution comes into contact with [[air]] on the underside of the concrete structure, another [[chemical reaction]] takes place.  The solution reacts with [[carbon dioxide]] in the air and [[precipitate]]s [[calcium carbonate]].<ref name="Classroom"/>

::{{chem|Ca||(OH)|2||(aq)}} + {{chem|C||O|2||(g)}} → {{chem|Ca||C||O|3||(s)}} + {{chem|H|2|O||(l)}}

When this solution drops down it leaves behind particles of calcium carbonate and over time these form into a stalactite.  They are normally a few centimeters long and with a [[diameter]] of approximately {{convert|4 to 5|mm|abbr=in}}.<ref name="Classroom"/>  The growth rate of stalactites is significantly influenced by supply continuity of {{chem|Ca||2+}} saturated solution and the drip rate. A straw shaped stalactite which has formed under a concrete structure can grow as much as 2&nbsp;mm per day in length, when the drip rate is approximately 11 minutes between drops.<ref name=SmithGK2016 /> Changes in [[leachate]] solution [[pH]] can facilitate additional chemical reactions, which may also influence [[calthemite]] stalactite growth rates.<ref name=SmithGK2016 />