Lime mortar slowly turns itself back into limestone inside a wall
Bake limestone above 900 degrees Celsius and it becomes fiercely caustic quicklime. Add water and it calms into slaked lime. Spread that between bricks and, over years, it drinks carbon dioxide from the air and hardens back into the rock it came from, a loop builders call the lime cycle.
Lime is a family of materials built mainly from calcium oxides and hydroxides. The name comes from its first job, as mortar, and carries the sense of something that sticks. Its raw sources are usually limestone or chalk, though coral and sea shells will do; the industry counts any rock with at least 80 percent calcium or magnesium carbonate, marble and marl included, as limestone. Lime making reaches back to prehistory in both the Old and New Worlds, and today the material goes into steel, cement, sugar refining and wastewater treatment.
The chemistry is a round trip. Heating calcium carbonate in a kiln drives off carbon dioxide and leaves calcium oxide, known as quicklime or burnt lime. Slaking it with water yields calcium hydroxide: just enough water gives a dry powder called hydrated lime, a little extra gives a paste called lime putty. In mortar, that hydroxide reacts with carbon dioxide from air or rainwater and becomes calcium carbonate again. Because the gas must reach it, pure lime mortar will not cure under water or deep inside a thick wall.
Builders sort lime by behaviour. Pure lime is bright white and sets slowly in air; weak carbonic acid in rain gradually dissolves it, but that same solubility lets dissolved lime seep into cracks and redeposit, so the material partly heals itself. Hydraulic lime contains clay-derived silica or alumina and can set under water. The more clay, the faster it sets: feebly hydraulic lime, with 5 to 10 percent clay, takes about three weeks, while eminently hydraulic lime, with 21 to 30 percent, sets in about a day and suits damp or coastal sites. Poor lime is grey and bonds weakly.
The Romans mixed lime with volcanic ash to trigger a pozzolanic reaction, and seawater hydrating the blend let their concrete solidify in the sea, underpinning a revolution in architecture. In the United States, the usual masonry lime is Type S, a label dating from 1946 in which the S stands for special.
Source: Lime (material)
