The Traditional Lime Company

glossary of

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Hydraulic Lime: Forms of lime which are supplied as dry powder which sets when water is added (The 'hydraulic' refers to the ability of this type of lime to set under water without air). Unlike lime putty, hydraulic lime is suitable for damp or wet conditions and, because it sets hard and quickly, it is better able to withstand exposed conditions.

It is made by burning naturally occurring deposits of limestone (calcium carbonate) which contains impurities of clay: first carbon dioxide is given off and the calcium carbonate converts to quicklime (calcium oxide) then at temperatures of between 950°C and 1250°C some constituents of the clay combine with the quicklime to form silicates and aluminates of calcium, the reactive components that give hydraulic lime its chemical setting properties.

It is then slaked with just enough water to convert the quicklime to calcium hydroxide without causing a set, and it is then dried and ground. Setting and hardening occurs primarily by crystallisation of the silicates on the addition of water.

Lime putty: A mixture of lime (calcium hydroxide) in water which is used for the production of lime plasters, renders, mortars, grouts and limewash. Lime putty is made by burning limestone or chalk (both forms of calcium carbonate) at a temperature of about 900°C to make quicklime (calcium oxide) which is then 'slaked' with water. It is best matured for several months by storing it covered with a thin film of water.

Mortar: The material which fills the gaps between the stones or bricks in a masonry wall and binds them together. Its principal ingredients are usually aggregate (eg gritty sand) and a binder (eg lime putty) in the ratio of three to one, with small proportions of other additives (eg brick dust) as required.

Pozzolan: A fine powdered material which is added to non-hydraulic lime mortars to accelerate the set. The material possesses little or no cementitious value, but in a finely divided form it will react with calcium hydroxide (lime putty) in the presence of moisture to provide a chemical set. The term derives from the Italian region of Pozzuoli, near Naples, where the effects of the local volcanic ash on lime mortars was first noticed by the Romans. Other pozzolanic materials in this category include finely crushed brick or clay tile dust, pulverised fuel ash (PFA) with a low sulfate content and 'high temperature insulation' (HTI).

Render, rendering: A durable coating of lime and aggregate, sometimes reinforced with animal hair, which provides a protective covering to the walls of a building. Common substrates include rubble stonework, cob, or in the case of a timber-framed building, wattle and daub.

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