Limescale

The colour varies from off-white through a range of greys and pink or reddish browns, depending on the other minerals present.

In addition to being unsightly and hard to clean, limescale can seriously damage or impair the operation of various plumbing and heating components.

The type found deposited on the heating elements of water heaters consists mainly of calcium carbonate (CaCO3).

When heating hard water on the stove, these gas bubbles form on the surface of the pan prior to boiling.

By the Middle Ages the limestone-like limescale accretions from the inside of the aqueduct were particularly desirable as a building material, called "Eifel marble" in an area with little natural stone.

[2] Trade to the west took it to England as a high-status export material in the 11th and 12th centuries, where it was made into columns for a number of Norman English Cathedrals.

These large cathedral cloisters needed several hundred such columns around an open quadrangle, which must have been supplied by a well-organized extraction and transport operation.

Limescale build-up inside a pipe reduces both liquid flow through the pipe and thermal conduction from the liquid to the outer pipe shell. Both effects will reduce the pipe's overall thermal efficiency when used as a heat exchanger .
This column in the Bad Münstereifel church in Germany is made from the calcium carbonate deposits that built up in the Roman Eifel Aqueduct over several centuries of use.