Calthemite

[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] 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.

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.

[14] 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.

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,[7] the predominant carbon species is CO32− and the leachate becomes saturated with Ca2+.

The leachate solution pH influences which dominant carbonate species (ions) are present,[11][16][18] so at any one time there may be one or more different chemical reactions occurring within a concrete structure.

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 chemical [Equations 1 to 4] are responsible for creating the majority of calthemite stalactites, stalagmites, flowstone etc., found on manmade concrete structures.

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.

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.

[26] Calcite rafts were first observed by Allison in 1923[27] on solution drops attached to concrete derived straw stalactites, and later by Ver Steeg.

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.

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.

[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.

[1] The leachate that does reach 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.