The lime cycle describes how lime is made, used and how it "sets" to provide a bond. First, calcium carbonate in the form of limestone, chalk or seashells is heated in a lime kiln. This causes the carbon to disassociate from the material as carbon dioxide, leaving calcium oxide – commonly known as quicklime and a highly reactive material. When added to water (a process known as slaking), it boils and spits, producing lots of heat and calcium hydroxide.
Calcium hydroxide is combined with aggregates and water and exposed to the air, whereupon it takes up carbon dioxide to form calcium carbonate again, bonding the aggregates together and completing the lime cycle. Calcium hydroxide is available in two forms: a powder called dry hydrate of lime or lime putty, a material kept in water to prevent contact with air. Powdered lime must also be protected from contact with air, or it too will start to "set".
Lime putty matures over time. It takes on more and more water, which increases the plasticity of the putty. This means it forms a better and better bond between the mortar and the masonry and makes the finished job stronger.
A third type of lime, hydraulic lime, was used for the hemp houses. This is made from "impure" limestones that contain additional minerals such as aluminia and silica. These cause the lime to behave more like cement, so it sets faster and is more durable than non-hydraulic lime. Its use on the hemp houses enabled the shuttering to be taken down after only 24 hours. However, hydraulic lime has to be either slaked on site before use or supplied as a powder in air- and water-tight bags.
Lime is seen as more environment-friendly than other mortars because the lime cycle can be repeated and lime mortars recycled. Bricks bound with lime mortars are much more easily recycled, too. Lime also contains less embodied energy because the temperatures used to manufacture calcium oxide are about 30% lower than those used to make cement.
Lime should be used to repair buildings constructed from lime materials, as lime mortars are more flexible than Portland cement and allow a building to "breathe".
Lime is also used in some modern buildings because its greater flexibility means expansion joints in brickwork are unnecessary. The architect Quinlan Terry used lime mortar at the Richmond Riverside Development, as did Michael Hopkins and Partners at Glyndebourne.
For further information, see the Society for the Protection of Ancient Buildings' Information Sheet Nine or contact Hydraulic Lias Limes, 01458-223179.