Calthemite is a secondary deposit, derived from
concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...
,
lime,
mortar or other calcareous material outside the
cave
A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...
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=127][Smith, 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
speleothem
A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending o ...
s, such as
stalactite
A stalactite (, ; from the Greek 'stalaktos' ('dripping') via
''stalassein'' ('to drip') is a mineral formation that hangs from the ceiling of caves, hot springs, or man-made structures such as bridges and mines. Any material that is soluble an ...
s,
stalagmite
A stalagmite (, ; from the Greek , from , "dropping, trickling")
is a type of rock formation that rises from the floor of a cave due to the accumulation of material deposited on the floor from ceiling drippings. Stalagmites are typically ...
s, flowstone etc.
[Hill, C A and Forti, P, (1997). Cave Minerals of the World, Second Edition. untsville, 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
A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending o ...
", due to its definition (''spēlaion'' "cave" + ''théma'' "deposit" in ancient Greek) can only be used to describe secondary deposits in
cave
A cave or cavern is a natural void in the ground, specifically a space large enough for a human to enter. Caves often form by the weathering of rock and often extend deep underground. The word ''cave'' can refer to smaller openings such as sea ...
s 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
A leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed.
Leachate is a widely used term in the environmental sciences wher ...
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, 637–644][Lees, T P, (1992). "Deterioration Mechanisms". 10–36 ]hapter 2
The following is a glossary of terms used in the description of lichens, composite organisms that arise from algae or cyanobacteria living among filaments of multiple fungus species in a mutualistic relationship.
Until the end of the 18th cen ...
in Mays, G C (Ed.), Durability of Concrete Structures Investigation, repair, protection. & 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
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms wh ...
,
marble
Marble is a metamorphic rock composed of recrystallized carbonate minerals, most commonly calcite or dolomite. Marble is typically not foliated (layered), although there are exceptions. In geology, the term ''marble'' refers to metamorpho ...
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 to normal "
speleothem
A speleothem (; ) is a geological formation by mineral deposits that accumulate over time in natural caves. Speleothems most commonly form in calcareous caves due to carbonate dissolution reactions. They can take a variety of forms, depending o ...
" 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
Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is t ...
(CO
2) from the surrounding air facilitates the reactions to deposit
calcium carbonate as a secondary deposit. CO
2 is the
reactant
In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...
(diffuses into solution) as opposed to speleothem chemistry where CO
2 is the
product (degassed from solution).
It is most likely that the majority of calcium carbonate (CaCO
3) creating calthemites in shapes which, mimicking speleothems, is precipitated from solution as
calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scra ...
as opposed to the other, less stable,
polymorphs of
aragonite
Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...
and
vaterite
Vaterite is a mineral, a polymorph of calcium carbonate ( Ca C O3). It was named after the German mineralogist Heinrich Vater. It is also known as mu- calcium carbonate (μ-CaCO3). Vaterite belongs to the hexagonal crystal system, whereas calcit ...
.
Calthemites are generally composed of
calcium carbonate (CaCO
3) which is predominantly coloured white, but may be coloured
[White W.B., (1997), "Color of Speleothems", Cave Minerals of the World, (2nd Edition) Hill C. and Forti P. untsville, Alabama: National Speleological Society Inc.239–244] red, orange or yellow due to
iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of w ...
(from rusting reinforcing) being transported by the leachate and deposited along with the CaCO
3.
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 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
The Anthropocene ( ) is a proposed geological epoch dating from the commencement of significant human impact on Earth's geology and ecosystems, including, but not limited to, anthropogenic climate change.
, neither the International Commission ...
.
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
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term "'' lime''" connotes calcium-containing inorganic ...
(CaO) in cement.
Concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...
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
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime ( calcium oxide) is mixed or slaked with water. It has m ...
(Ca(OH)
2), which under the right conditions can further dissociate to form
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
(Ca
2+) and
hydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water ...
(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
A leachate is any liquid that, in the course of passing through matter, extracts soluble or suspended solids, or any other component of the material through which it has passed.
Leachate is a widely used term in the environmental sciences wher ...
solution
pH.
[Maekawa, K, Ishida, T and Kishi, T, (2009). Multi-Scale Modeling of Structural Concrete. xford, UK: Taylor and Francis.225–235.]
The chemical formula is:
Calcium hydroxide
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime ( calcium oxide) is mixed or slaked with water. It has m ...
will readily react with any free CO
2 to form
calcium carbonate (CaCO
3) [].
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
3 within the mix, until all available CO
2 in the mix has been used up. Additional CO
2 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. artfield, Herts, UK: Association of Science Education./ref>] 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 quations & 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, 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
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime ( calcium oxide) is mixed or slaked with water. It has m ...
(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 quations 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. If [] is depositing the CaCO3 to creating calthemites, their growth will be at a much slower rate than quations 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, however the solution will still not attain the Ca2+ carrying capacity of quations to
The reactions quations to could be simplified to that shown in [], however the presence of carbonic acid (H2CO3) and other Chemical species, 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
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime ( calcium oxide) is mixed or slaked with water. It has m ...
(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 quations to Calcium is also part of other hydration products in concrete, such as calcium aluminium hydrates and calcium aluminium iron hydrate. The chemical quations 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.
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.
Unusual occurrences
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 quations to 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.
CaCO3 deposition and stalactite growth
The growth rates of calthemite stalactite straws, stalagmites and flowstone etc., is very much dependent on the supply rate and continuity of the saturated leachate solution to the location of CaCO3 deposition. The concentration of atmospheric CO2 in contact with the leachate, also has a large influence on how quickly the CaCO3 can precipitate from the leachate. Evaporation of the leachate solution and ambient atmospheric temperature appears to have very minimal influence on the CaCO3 deposition rate.[Ver Steeg, K, (1932). "An unusual occurrence of stalactites and stalagmites". The Ohio Journal of Science, Vol.32(2), 69–83.]
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. One calthemite soda straw
A soda straw (or simply straw) is a speleothem in the form of a hollow mineral cylindrical tube. They are also known as tubular stalactites. Soda straws grow in places where water leaches slowly through cracks in rock, such as on the roofs of ...
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. 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. 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.
Despite both being composed of Calcium Carbonate, Calthemite straws are on average just 40% the mass per unit length of speleothem straws of equivalent external diameter. This is due to the different chemistry involved in creating the straws. The calthemite straws have a thin wall thickness and a less-dense calcium carbonate structure compared to speleothem straws.[Smith, GK, (2021). "Comparison of calthemite and speleothem straw stalactites, and environmental conditions influencing straw diameter", Cave and Karst Science, Transactions of the British Cave Research Association, Vol.48(1), 3–11]
Calthemite straws can vary in outside diameter as they grow in length. Changes in diameter can take a matter of days or weeks and are due to changes in drip rate over time. A slow dripping calthemite straw tends to be slightly larger in diameter than a fast-dripping straw.
Calcite rafts on solution drops
Calcite rafts were first observed by Allison in 1923 on solution drops attached to concrete derived straw stalactites, and later by Ver Steeg. 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). 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. 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.
Stalagmites
If the drip rate is quicker than one drop per minute, most of the CaCO3 will be carried to the ground, still in solution. 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. 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.
Rimstone or gours
Calthemite rimstone
Rimstone, also called gours, is a type of speleothem (cave formation) in the form of a stone dam. Rimstone is made up of calcite and other minerals that build up in cave pools. The formation created, which looks like stairs, often extends into fl ...
or gours can form beneath concrete
Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most ...
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. 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. 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
Popcorn (also called popped corn, popcorns or pop-corn) is a variety of corn kernel which expands and puffs up when heated; the same names also refer to the foodstuff produced by the expansion.
A popcorn kernel's strong hull contains the se ...
), 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.
See also
*
*
References
External links
Smith, G.K. (2016), "Calcite straw stalactites growing from concrete structures", Cave and Karst Science 43(1), pp 4–10
Calcite rafts can be seen spinning around a solution drop surface (YouTube video)
Small rafts have joined up to form lattice work of rafts on calthemite soda straw drop (Youtube video)
*B. Schmidkonz, "Watch a dripstone grow", J. Chem. Educ., 94 (2017) 1492–1497 {{doi, 10.1021/acs.jchemed.7b00215
Calcium minerals
Carbonate minerals
Concrete
Inorganic chemistry
Speleothems
Corrosion