Lime

Lime was the main binder for buildings for thousands of years before Portland cement, and it remains valuable for conservation work, plasters, soil stabilisation and some mortars. Lime starts as limestone (calcium carbonate). Burning it at about 900 °C drives off carbon dioxide, leaving quicklime (calcium oxide). Adding water, called slaking, produces hydrated lime (calcium hydroxide) with a lot of heat. In use, hydrated lime slowly reacts with carbon dioxide from the air and turns back into calcium carbonate, which is how lime mortar hardens. This process, carbonation, is slow and works from the surface inward.

Fat lime (high-calcium lime) slakes vigorously, expands a lot and hardens only by carbonation; it is used for plaster and whitewash. Hydraulic lime contains clay impurities that form silicates and aluminates, so it can set under water and gains more strength, closer to a weak cement. Poor lime has many impurities and is of little use. Pozzolanas such as surkhi (burnt clay dust), fly ash and volcanic ash react with lime to form hydraulic compounds, the basis of Roman concrete.

Lime mortars are softer and more flexible than cement mortars, breathe moisture and allow masonry to move without cracking, which is why heritage structures should be repaired with lime rather than cement. Lime stabilisation of clay soils reduces plasticity, swelling and moisture sensitivity, improving subgrades for roads and railways. Lime is also used in water treatment for softening and pH correction. It is caustic and must be handled with eye and skin protection.