text: The way stalactites form on concrete is due to different chemistry than those that form naturally in limestone caves and is the result of the presence of calcium oxide (CaO) 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)), which under the right conditions can further dissociate to form calcium (Ca) and hydroxide (OH) ions []. All of the following chemical reactions are reversible and several may occur simultaneously at a specific location within a concrete structure, influenced by leachate solution pH.
  The chemical formula is:
  Calcium hydroxide will readily react with any free CO to form calcium carbonate (CaCO) []. The solution is typically pH 9 – 10.3, however this will depend on what other chemical reactions are also occurring at the same time within the concrete.
  This reaction occurs in newly poured concrete as it sets, to precipitate CaCO within the mix, until all available CO in the mix has been used up. Additional CO from the atmosphere will continue to react, typically penetrating just a few millimetres from the concrete surface. Because the atmospheric CO cannot penetrate very far into the concrete, there remains free Ca(OH) within the set (hard) concrete structure.
  Any external water source (e.g. rain or seepage) which can penetrate the micro cracks and air voids in set concrete will readily carry the free Ca(OH) in solution to the underside of the structure. When the Ca(OH) solution comes in contact with the atmosphere, CO diffuses into the solution drops and over time the reaction [] deposits calcium carbonate to create straw shaped stalactites similar to those in caves.
  This is where the chemistry becomes a bit complicated, due to the presence of soluble potassium and sodium hydroxides in new concrete, which supports a higher solution alkalinity of about pH 13.2 – 13.4, the predominant carbon species is CO and the leachate becomes saturated with Ca. The following chemical formulae [Equations  & ] will most likely be occurring, and [] responsible for the deposition of CaCO to create stalactites under concrete structures.
  As the soluble potassium and sodium hydroxides, are leached out of the concrete along the seepage path, the solution pH will fall to pH ≤12.5.  Below about pH 10.3, the more dominant chemical reaction will become []. The leachate solution pH, influences which dominant carbonate species (ions) are present, so at any one time there may be one or more different chemical reactions occurring within a concrete structure.
  In very old lime, mortar or concrete structures, possibly tens or hundreds of years old, the calcium hydroxide (Ca(OH)) may have been leached from all the solution seepage paths and the pH could fall below pH 9. This could allow a similar process to that which creates speleothems in limestone caves [Equations  to ] to occur. Hence, CO rich groundwater or rainwater would form carbonic acid (HCO) (≈pH 7.5 – 8.5) and leach Ca from the structure as the solution seeps through the old cracks []. This is more likely to occur in thin layered concrete such as that sprayed inside vehicle or railway tunnels to stabilise loose material. If [] is depositing the CaCO to creating calthemites, their growth will be at a much slower rate than [Equations  and ], as the weak alkaline leachate has a lower Ca carrying capacity compared to hyperalkaline solution. CO is degassed from solution as  CaCO is deposited to create the calthemite stalactites. An increased CO partial pressure (P concentration in solution and result in a higher Ca carrying capacity of the leachate, however the solution will still not attain the Ca carrying capacity of [Equations  to ]
  The reactions [Equations  to ] could be simplified to that shown in [], however the presence of carbonic acid (HCO) and other species are omitted. The chemical formula [] is usually quoted as creating "speleothems" in limestone caves, however in this instance the weak carbonic acid is leaching calcium carbonate (CaCO) previously precipitated (deposited) in the old concrete and degassing CO to create calthemites.
  If the leachate finds a new path through micro cracks in old concrete, this could provide a new source of calcium hydroxide (Ca(OH)) which can change the dominant reaction back to []. The chemistry of concrete degradation is quite complex and only the chemistry relating to calcium carbonate deposition is considered in [Equations  to ]. Calcium is also part of other hydration products in concrete, such as calcium aluminium hydrates and calcium aluminium iron hydrate. The chemical [Equations  to ] are responsible for creating the majority of calthemite stalactites, stalagmites, flowstone etc., found on manmade concrete structures.
  Maekawa et al., (2009) p. 230, provides an excellent graph showing the relationship between equilibrium of carbonic acids (HCO, HCO and CO) and pH in solution. Carbonic acid includes both carbonates and bicarbonates. The graph provides a good visual aid to understanding how more than one chemical reaction may be occurring at the same time within concrete at a specific pH.
  Leachate solutions creating calthemites can typically attain a pH between 10–14, which is considered a strong alkaline solution with the potential to cause chemical burns to eyes and skin – dependent on concentration and contact duration.