Degrading concrete has been the focus of many studies and the most obvious sign is calcium-rich leachate seeping from a concrete structure.
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. 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 (CO) from the surrounding air facilitates the reactions to deposit calcium carbonate as a secondary deposit. CO is the reactant (diffuses into solution) as opposed to speleothem chemistry where CO is the product (degassed from solution).
It is most likely that the majority of calcium carbonate (CaCO) 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 (CaCO) which is predominantly coloured white, but may be coloured red, orange or yellow due to iron oxide (from rusting reinforcing) being transported by the leachate and deposited along with the CaCO. 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.