What, Where and How?
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 Singleton Birch, it is mainly chalk quarried in Lincolnshire.
To convert the raw material into lime, heating to temperatures between 800 oC - 1000 oC is required. At these temperatures the chalk 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 below. It is calcium hydroxide which, when mixed with sand to make a mortar, builders have used over the millennia. The setting process is the re-absorption of carbon dioxide from the atmosphere called carbonation.
Calcium hydroxide can be used by Builders in two forms, firstly, if it is hydrated with an excess of water, lime putty is produced or if the water quantity is reduced a powder is formed which can be bagged. 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.
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.
Singleton Birch have worked to develop a range of NHL products by sourcing the appropriate chalk or limestone for burning, based on its' geology and mineralogy
Lime Terminology
calcium carbonate
also known as: chalk, limestone, agricultural lime
chemical formula: CaCO3
calcium oxide
also known as: quicklime, burnt lime
chemical formula: CaO
calcium hydroxide
also known as: hydrated lime, slaked lime
chemical formula: Ca(OH)2
Building Lime Terminology
Air Lime
Air lime, or high calcium lime does not have any hydraulic component. It can be quicklime for slaking or hydrated lime. It gains strength slowly, by combining with atmospheric carbon dioxide to form calcium carbonate (as per the lime cycle). Several grades of air lime are identified in EN459 the European standard for Building Lime.
Hydraulic Lime
Lime with hydraulic or cementitious properties which will set when exposed to moisture. Several grades of hydraulic lime are identified in EN459 the European standard for Building Lime.
Natural Hydraulic Lime
Hydraulic lime which does not contain any performance enhancing additives. Its properties are as a result of the mineralogy of the calcium carbonate stone which is quarried for burning.
Hydrated Lime
Hydrated lime is NOT Hydraulic lime and will not set in contact with water.
CL90 & CL90 S
Grades of air lime for building as described in EN459 the European standard for Building Lime. CL90 is the purest grade of building quicklime and CL90 S is the purest grade of hydrated lime for building. Several grades of air lime are identified in EN459 the European standard for Building Lime.
Other Chemical & Miscellanous Terminology
Carbon dioxide
chemical formula: CO2
Water
chemical formula: H20
Silica
chemical formula: Si02
Alumina
chemical formula: Al203
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