Calthemite is a secondary deposit, derived from concrete, lime, mortar or other calcareous material outside the cave environment.Smith, G.K. (2016). "Calcite straw stalactites growing from concrete structures", Cave and Karst Science 43(1), 4–10. http://bcra.org.uk/pub/candks/index.html?j=127Smith, G K., (2015). "Calcite Straw Stalactites Growing From Concrete Structures". Proceedings of the 30th 'Australian Speleological Federation' conference, Exmouth, Western Australia, edited by Moulds, T. pp 93 -108 Calthemites grow on or under man-made structures and mimic the shapes and forms of cave speleothems, such as stalactites, stalagmites, flowstone etc.Hill, C A and Forti, P, (1997). Cave Minerals of the World, Second Edition. [Huntsville, Alabama: National Speleological Society Inc.] Calthemite is derived from the Latin calx (genitive calcis) "lime" + Latin < Greek théma, "deposit" meaning ‘something laid down’, (also Mediaeval Latin thema, "deposit") and the Latin –ita < Greek -itēs – used as a suffix indicating a mineral or rock. The term "speleothem",Moore, G. W. (1952). "Speleothems – a new cave term". National Speleological Society News, Vol.10(6), p.2. due to its definition (spēlaion "cave" + théma "deposit" in ancient Greek) can only be used to describe secondary deposits in caves and does not include secondary deposits outside the cave environment.
Origin and composition Degrading concrete has been the focus of many studies and the most obvious sign is calcium-rich leachate seeping from a concrete structure.Macleod, G, Hall, A J and Fallick, A E, (1990). "An applied mineralogical investigation of concrete degradation in a major concrete road bridge". Mineralogical Magazine, Vol.54, 637–644Lees, T P, (1992). "Deterioration Mechanisms". 10–36 [Chapter 2] in Mays, G C (Ed.), Durability of Concrete Structures Investigation, repair, protection. [E & FN Spon Press.] Print Ekström, T, (2001). "Leaching of concrete: Experiments and modelling". (Report TVBM-3090). Lund Institute of Technology Division of Building Materials. https://portal.research.lu.se/ws/files/4827018/1766469.pdf.
Calthemite stalactites can form on concrete structures and "artificial caves" lined with concrete (e.g. mines and tunnels) significantly faster than those in limestone, marble or dolomite caves.Sefton, M, (1988). "Manmade" speleothems. South African Speleological Association Bulletin, Vol.28, 5–7. This is because the majority of calthemites are created by chemical reactions which are different from normal "speleothem" chemistry.
Calthemites are usually the result of hyperalkaline solution (pH 9–14) seeping through a calcareous man-made structure until it comes into contact with the atmosphere on the underside of the structure, where carbon dioxide (CO2) from the surrounding air facilitates the reactions to deposit calcium carbonate as a secondary deposit. CO2 is the reactant (diffuses into solution) as opposed to speleothem chemistry where CO2 is the product (degassed from solution). It is most likely that the majority of calcium carbonate (CaCO3) creating calthemites in shapes which, mimicking speleothems, is precipitated from solution as calcite as opposed to the other, less stable, polymorphs of aragonite and vaterite.
Calthemites are generally composed of calcium carbonate (CaCO3) which is predominantly coloured white, but may be colouredWhite W.B., (1997), "Color of Speleothems", Cave Minerals of the World, (2nd Edition) Hill C. and Forti P. [Huntsville, Alabama: National Speleological Society Inc.] 239–244 red, orange or yellow due to iron oxide (from rusting reinforcing) being transported by the leachate and deposited along with the CaCO3. Copper oxide from copper pipes may cause calthemites to be coloured green or blue. Calthemites may also contain minerals such as gypsum.
The definition of calthemites also includes secondary deposits which may occur in manmade mines and tunnels with no concrete lining, where the secondary deposit is derived from limestone, dolomite or other calcareous natural rock into which the cavity has been hollowed out. In this instance the chemistry is the same as that which creates speleothems in natural limestone caves (equations 5 to 8) below. It has been suggested the deposition of calthemite formations are one example of a natural process which has not previously occurred prior to the human modification of the Earth's surface, and therefore represents a unique process of the Anthropocene.
Chemistry and pH 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)2), which under the right conditions can further dissociate to form calcium (Ca2+) 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.Maekawa, K, Ishida, T and Kishi, T, (2009). Multi-Scale Modeling of Structural Concrete. [Oxford, UK: Taylor and Francis.] 225–235.
The chemical formula is:
Calcium hydroxide will readily react with any free CO2 to form calcium carbonate (CaCO3) [].Ho, D W S and Lewis, R K, (1987). "Carbonation of concrete and its prediction". Cement and Concrete Research, Vol.17, 489–504. 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 CaCO3 within the mix, until all available CO2 in the mix has been used up. Additional CO2 from the atmosphere will continue to react, typically penetrating just a few millimetres from the concrete surface.Borrows, P, (2006a). Chemistry Outdoors. School Science Review – Outdoor Science, Vol.87(320), 24–25. [Hartfield, Herts, UK: Association of Science Education.] Because the atmospheric CO2 cannot penetrate very far into the concrete, there remains free Ca(OH)2 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)2 in solution to the underside of the structure. When the Ca(OH)2 solution comes in contact with the atmosphere, CO2 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 CO32− and the leachate becomes saturated with Ca2+.Liu, Z and He, D, (1998). Special speleothems in cement-grouting tunnels and their implications of the atmospheric CO2 sink. Environmental Geology, Vol.35(4), 258–262 The following chemical formulae [Equations & ] will most likely be occurring, and [] responsible for the deposition of CaCO3 to create stalactites under concrete structures.Ishida, T and Maekawa, K, (2000). "Modeling of pH profile in pore water based on mass transport and chemical equilibrium theory". Translation from Proceedings of Japan Society of Civil Engineers (JSCE), No.648/Vol.47.Newton, K, Fairchild, I and Gunn, J, (2015). "Rates of calcite precipitation from hyperalkaline waters, Poole's Cavern, Derbyshire". Cave and Karst Science. Vol.42(3), 116–124, and "Corrigenda" Vol.43(1), 48
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,Pourbaix, M, (1974). "Atlas of electrochemical equilibria in aqueous solutions". 2nd English edition. [Houston, TX: National Association of Corrosion Engineers.] 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)2) 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, CO2 rich groundwater or rainwater would form carbonic acid (H2CO3) (≈pH 7.5 – 8.5)Hartland, A, Fairchild, I J, Lead, J R, Dominguez-Villar, D, Baker, A, Gunn, J, Baalousha, M and Ju-Nam, Y, (2010). "The dripwaters and speleothems of Poole's Cavern: a review of recent and ongoing research", Cave and Karst Science, Vol.36(2), 37–46. and leach Ca2+ 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.Hagelia, P, (2011). "Deterioration Mechanisms and Durability of Sprayed Concrete for Rock Support in Tunnels". Doctoral thesis presented at Technische Universiteit Delft, Netherlands. If [] is depositing the CaCO3 to creating calthemites, their growth will be at a much slower rate than [Equations and ], as the weak alkaline leachate has a lower Ca2+ carrying capacity compared to hyperalkaline solution. CO2 is degassed from solution as CaCO3 is deposited to create the calthemite stalactites. An increased CO2 partial pressure (PCO2) and a lower temperature can increase the HCO3− concentration in solution and result in a higher Ca2+ carrying capacity of the leachate,Herman, J S, (2005). "Water Chemistry in Caves", Encyclopedia of Caves, (1st edition) edited by Culver D., White W., 609- 614 however the solution will still not attain the Ca2+ carrying capacity of [Equations to ]
The reactions [Equations to ] could be simplified to that shown in [], however the presence of carbonic acid (H2CO3) 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 (CaCO3) previously precipitated (deposited) in the old concrete and degassing CO2 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)2) 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 (H2CO3, HCO3− and CO32−) 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.