Within set concrete there remains some free "calcium hydroxide" (Ca(OH)), which can further dissociate to form Ca and hydroxide (OH) ions". Any water which finds a seepage path through micro cracks and air voids present in concrete, will readily carry the (Ca(OH)) and Ca (depending on solution pH and chemical reaction at the time) to the underside of the structure where leachate solution contacts the atmosphere. Carbon dioxide (CO) from the atmosphere readily diffuses into the leachate and causes a chemical reaction, which precipitates (deposits) calcium carbonate (CaCO) on the outside of the concrete structure. Consisting primarily of CaCO this secondary deposit derived from concrete is known as "calthemite" and can mimic the shapes and forms of cave "speleothems", such as stalactites, stalagmites, flowstone etc. Other trace elements such as iron from rusting reinforcing steel bars may be transported and deposited by the leachate at the same time as the CaCO. This may colour the calthemites orange or red.
The chemistry involving the leaching of calcium hydroxide from concrete can facilitate the growth of calthemites up to ≈200 times faster than cave speleothems due to the different chemical reactions involved. The sight of calthemite is a visual sign that calcium is being leached from the concrete structure and the concrete is gradually degrading.
In very old concrete where the calcium hydroxide has been leached from the leachate seepage path, the chemistry may revert to that similar to "speleothem" chemistry in limestone cave. This is where carbon dioxide enriched rain or seepage water forms a weak carbonic acid, which leaches calcium carbonate (CaCO) from within the concrete structure and carries it to the underside of the structure. When it contacts the atmosphere, carbon dioxide degasses and calcium carbonate is precipitated to create calthemite deposits, which mimic the shapes and forms of speleothems. This degassing chemistry is not common in concrete structures as the leachate can often find new paths through the concrete to access free calcium hydroxide and this reverts the chemistry to that previously mentioned where CO is the reactant.