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Calthemite is a secondary deposit, derived from concrete, lime, mortar or other calcareous material outside the cave environment.[1][2] Calthemites grow on or under, man-made structures and mimic the shapes and forms of cave speleothems, such as stalactites, stalagmites, flowstone etc.[3] 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.[1][2] The term "speleothem",[4] 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.[3]

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.[5][6][7]

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.[3][8] This is because the majority of calthemites are created by chemical reactions which are different to 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).[3] 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.[1][3]

Calthemite flowstone coloured orange by Iron Oxide (from rusting steel reinforcing) deposited along with calcium carbonate (${\displaystyle {\ce {CaCO3}}}$).
leachate seeping from a concrete structure.[5][6][7]

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.[3][8] This is because the majority of calthemites are created by chemical reactions which are different to 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).[3] 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.[1][3]

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.[3][8] This is because the majority of calthemites are created by chemical reactions which are different to 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).[3] 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.[1][3]

Calthemites are generally composed of calcium carbonate (CaCO3) which is predominantly coloured white, but may be coloured[9] 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.[1] Calthemites may also contain minerals such as gypsum.[1][3]

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 created. 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.[10]

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 [Equation 1]. 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.[11]

The chemical formula is:

(Equation 9)

<

(Equation 9)

If the leachate finds a new path through micro cracks in old concrete, this cou

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 [Equation 2]. The chemistry of concrete degradation is quite complex and only the chemistry relating to calcium carbonate deposition is considered in [Equations 1 to 9]. Calcium is also part of other hydration products in concrete, such as calcium aluminium hydrates and calcium aluminium iron hydrate. The chemical [Equations 1 to 4] are responsible for creating the majority of calthemite stalactites, stalagmites, flowstone etc., found on manmade concrete structures.[1]

Maekawa et al., (2009)[11] p. 230, provides an excellent graph showing the relationship between equilibrium of carbonic acids (H2CO3, HCO3 and CO32−) and pH in solution.[11] 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.[22][23][24]

There are a few unusual circumstances where speleothems have been created in caves as a result of hyperalkaline leachate, with the same chemistry as occurs in [Equations 1 to 4].[17][19] This chemistry can occur when there is a source of concrete, lime, mortar or other manmade calcareous material located above a cave system and the associated hyperalkaline leachate can penetrate into the cave below. An example can be found in the Peak District – Derbyshire, England where pollution from 19th century industrial lime production has leached into the cave system below (e.g. Poole's Cavern) and created speleothems, such as stalactites and stalagmites.[17][19]

CaCO3 deposition and stalactite growth

[1][25]

Calthemite straw stalactites precipitated (deposited) from hyperalkaline leachate have the potential to grow up to ≈200 times faster than normal cave speleothems precipitated from near neutral pH solution.[1][8] One calthemite soda straw has been recorded as growing 2 mm per day over several consecutive days, when the leachate drip rate was a constant 11 minutes between drips.[1] When the drip rate is more frequent than one drop per minute, there is no discernible deposition of CaCO3 at the tip of the stalactite (hence no growth) and the leachate solution falls to the ground where the CaCO3 is deposited to create a calthemite stalagmite. If the leachate supply to the stalactite straw's tip reduces to a level where the drip rate is greater than approximately 25 to 30 minutes between drops, there is a chance that the straw tip will calcify over and block up.[1] New straw stalactites can often form next to a previously active, but now dry (dormant) straw, because the leachate has simply found an easier path through t

Calthemite straw stalactites precipitated (deposited) from hyperalkaline leachate have the potential to grow up to ≈200 times faster than normal cave speleothems precipitated from near neutral pH solution.[1][8] One calthemite soda straw has been recorded as growing 2 mm per day over several consecutive days, when the leachate drip rate was a constant 11 minutes between drips.[1] When the drip rate is more frequent than one drop per minute, there is no discernible deposition of CaCO3 at the tip of the stalactite (hence no growth) and the leachate solution falls to the ground where the CaCO3 is deposited to create a calthemite stalagmite. If the leachate supply to the stalactite straw's tip reduces to a level where the drip rate is greater than approximately 25 to 30 minutes between drops, there is a chance that the straw tip will calcify over and block up.[1] New straw stalactites can often form next to a previously active, but now dry (dormant) straw, because the leachate has simply found an easier path through the micro cracks and voids in the concrete structure.

Calcite rafts were first observed by Allison in 1923[26] on solution drops attached to concrete derived straw stalactites, and later by Ver Steeg.[25] When the drip rate is ≥5 minutes between drops, calcium carbonate will have precipitated on the solution drop surface (at the end of a stalactite) to form calcite rafts visible to the naked eye (up to 0.5 mm across).[1] If the drip rate is greater than ≈12 minutes between drops, and there is very little air movement, these rafts may join up and become a latticework of calcite rafts covering the drop surface.[1] Significant air movement will cause the rafts to become scattered and spin turbulently around the drop's surface. This turbulent movement of calcite rafts can cause some to shear off the drop's surface tension and be pushed onto the outside of the straw stalactite, thus increasing the outside diameter and creating minute irregularities.[1]

Stalagmites

Calthemite stalagmite on concrete floor
[1] The leachate solution then has a chance to absorb CO2 from the atmosphere (or degas CO2 depending on reaction) and deposit the CaCO3 on the ground as a stalagmite.

In most locations within manmade concrete structures, calthemite stalagmites only grow to a maximum of a few centimetres high, and look like low rounded lumps.[27] This is because of the limited supply of CaCO3 from the leachate seepage path through the concrete and the amount which reaches the ground. Their location may also inhibit their growth due to abrasion from vehicle tyres and pedestrian traffic.[2]

Rimstone or gours

Calthemite rimstone or gours can form beneath concrete structures on a floor with a gradual sloping surface or on the side of rounded stalagmites. When the leachate drip rate is more frequent than 1 drop per minute, most of the calcium carbonate is carried by the leachate from the underside of the concrete structure to the ground, where stalagmites, flowstone and gours are created.[1] The leachate which does reaching the ground, usually evaporates quickly due to air movement beneath the concrete structure, hence micro-gours are more common than larger gours.[citation needed] In locations where the deposition site is subject to abrasion by vehicle tyres or pedestrians traffic, the chance of micro-gours forming is greatly reduced.

Coralloids

Calthemite coralloids (also known as popcorn), can form on the underside of concrete structures and look very similar to those which occurs in caves. Coralloids can form by a number of different methods in caves, however on concrete the most common form is created when hyperalkaline solution seeps from fine cracks in concrete. Due to solution evaporation, deposition of calcium carbonate occurs before any drop can form. The resulting coralloids are small and chalky with a cauliflower appearance.[citation needed]

References

1. Smith, G.K. (2016). "Calcite straw stalactites growing from concrete structures", Cave and Karst Science 43(1), 4–10. [27] This is because of the limited supply of CaCO3 from the leachate seepage path through the concrete and the amount which reaches the ground. Their location may also inhibit their growth due to abrasion from vehicle tyres and pedestrian traffic.[2]

Calthemite rimstone or gours can form beneath concrete structures on a floor with a gradual sloping surface or on the side of rounded stalagmites. When the leachate drip rate is more frequent than 1 drop per minute, most of the calcium carbonate is carried by the leachate from the underside of the concrete structure to the ground, where stalagmites, flowstone and gours are created.[1] The leachate which does reaching the ground, usually evaporates quickly due to air movement beneath the concrete structure, hence micro-gours are more common than larger gours.[citation needed] In locations where the deposition site is subject to abrasion by vehicle tyres or pedestrians traffic, the chance of micro-gours forming is greatly reduced.

Coralloids

Calthemite Calthemite coralloids (also known as popcorn), can form on the underside of concrete structures and look very similar to those which occurs in caves. Coralloids can form by a number of different methods in caves, however on concrete the most common form is created when hyperalkaline solution seeps from fine cracks in concrete. Due to solution evaporation, deposition of calcium carbonate occurs before any drop can form. The resulting coralloids are small and chalky with a cauliflower appearance.[citation needed]