Lime is an ancient building material that continues to be used today, because it has many benefits. It is porous and permeable, which creates a breathable building with superior moisture controlling capabilities, as long as the correct materials have been used throughout. It is flexible, which is well suited to timber buildings and those structures likely to move with moisture and heat. They are long lasting and fire proof. Mechanical properties are enhanced by mixing in fibres, such as hemp, wool and horse hair (although now more likely to be synthetic).

Before the appearance of Portland cement around 1800s, mortars for binding stones in construction were formulated with lime. And both lime and gypsum have been used since ancient times to render and plaster buildings. Cement and gypsum now dominate due to their low cost, easy of use, availability, rapid set time and rock-like hardness. However, since lime offers some sustainable advantages, as well as being better suited to restoration projects, it is once again growing in popularity.

There are two main forms: natural hydraulic lime (NHL) and hydrated lime.

Hydraulic lime is made by heating limestone with impurities, such as silica (SiO2) and alumina (Al2O3), to 1,000-1,250 degC. The resulting material sets through hydrolysis caused by a reaction with water in the atmosphere. This forms a material with greater strength than hydrated lime and in a shorter time. It is also capable of hardening under water and in damp conditions, depending on the grade.

Hydrated lime, produced by mixing quicklime with water at a ratio of around 1:3, hardens through a process of carbonisation, whereby it takes in CO2 from the atmosphere. While it is slower to harden than hydraulic lime, the end result is more flexible and permeable, making it better suited to breathable houses and restoration projects. As well as its use as a construction material, hydrated lime is used in food preparation (E526).

The carbon footprint of lime is relatively high. Production requires high temperatures and CO2 is driven off as the limestone is turned into quicklime. But this value does not include the CO2 absorbed by the lime over its lifetime, which can render it virtually carbon neutral. Old lime can be recycled and used again. It tends to be softer than the materials it is stuck to, such as brick and stone, and this makes it relatively easy to remove without damaging the masonry. It is broken up into a powder and ‘knocked up’ with water and some new lime and ready to use like new.