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Lime is made from calcium carbonate. This can be found in a variety of forms for example chalk, limestone or sea shells. In the British Isles sources are almost invariably chalk or limestone. In the case of Eden Hot Lime Mortar, the limestone comes from Shap in Cumbria.
To convert the raw material into lime, heating to temperatures between 800°C- 1000°C is required. At these temperatures the limestone breaks down by giving off carbon dioxide leaving calcium oxide which is known as quicklime.
Quicklime is unstable and reacts, often very rapidly, with water to form calcium hydroxide. This process produces heat and is known as hydration. When exposed to the atmosphere this calcium hydroxide can react again by absorbing carbon dioxide from the atmosphere to once again become calcium carbonate.
This is known as the lime cycle and is shown in the diagram. It is calcium hydroxide which, when mixed with water and sand make a hot mortar, builders have used over the millennia. The setting process is the re-absorption of carbon dioxide from the atmosphere called carbonation
The Lime Cycle is one of nature`s best known examples of chemistry. This classic series of chemical reactions is the basis for numerous applications, many of which affect our lives every day.
The image illustrates how limestone (calcium carbonate) turns into quicklime after heating, then into hydrated lime (slaked lime or calcium hydroxide) after adding water and finally back into chalk by combining with carbon dioxide from the air.
The reactions involved in lime manufacture are:
Calcium hydroxide can be used by Builders in two forms, firstly, if it is hydrated with a mixture of sharp sand and water, hot lime mortar is produced, or with an excess of water, lime putty is produced. All sources of pure calcium carbonate produce a similar result, however the Romans discovered that the calcium hydroxide would also react with volcanic ash, essentially a mixture of silica and alumina. The resulting mixes set very much harder and for the first time enabled structures of high strength to be built. By varying the quantity of silica and alumina, mortars (and lime concretes) of different strengths could be produced. These reactions are referred to as pozzolanic – from the town of Pozzuoli in Italy from where the original material came. Eden Hot Lime mortar sells Argical-M 1000 pozzolan.
The processes described so far use chalk or limestone that is relatively pure containing over 95% calcium carbonate. However some limestones are less pure and can contain quantities of silica and alumina which when heated produce very similar results to the Roman mixes of pure lime with volcanic ash. These impure limestones if they contain quantities of silica and alumina are referred to as Natural Hydraulic Limes (NHL).
Hydraulic in this context refers to the ability of the material to set under water as opposed to pure air limes which can only set when exposed to atmospheric carbon dioxide. As with addition of pozzolanic silica and alumina, the strength of NHL's depends upon the quality of these materials in the limestone. As the quantities increase so does the strength but also the free lime (calcium hydroxide) in the mortar reduces so that the set depends more on chemical reaction and the process of carbonation reduces.