building
project intro three main categories: Pure, Type S and Hydraulic difference: pure lime has high % of calcium carbonate, and fewest impurities. type S contains compounds (typically oxides) which increase plasticity and speed up the set. hydraulic lime contains natural or added clay which allows it to set under water. pure lime includes: quicklime, hydrated lime, lime putty, air lime and fat lime. Type S includes: type S hydrate and dolomitic lime. hydraulic lime includes: hydraulic quicklime, hydraulic lime, natural hydraulic lime, NHL-2, 3.5, and 5. raw material: limestone (calcium carbonate). composition varies depending on where it is mined. processing: limestone is crushed and heated to about 900 - 1000 degrees C. CO2 is driven off leaving calcium oxide or 'quicklime'. a dry powder or lumpy residue. hydration: quicklime is added to a controlled amount of water to produce calcium hydroxide. this is hydrated lime (dry) or lime putty (wet). hydraulic lime is hydrated in the same way, but should not be made into a wet putty until shortly before use as it will start to set. application: hydrated lime is combined with water, sand, aggregates and fibre such as straw or horse hair, to produce lime mortar or plaster (also called render and stucco). the ratios depend on the particular use. lime putty does not require added water. best to allow lime putty to age for several months or years before use to ensure thorough hydration. too much water causes shrinkage and cracking. lime must be allowed to dry slowly. which to choose: pure lime is better suited to walls that do not require great structural strength. hydraulic lime, due to its higher impermeability and strength, works well for load-bearing walls and wetter environments. type S and hydraulic limes are more appropriate in situations where a faster set is required. setting: lime sets slowly, gradually increasing in strength and hardness over several years. it reabsorbs CO2, turns back into limestone and neutralises much of the CO2 the emissions given off during processing. |
Lime offers us a much more environmentally-friendly alternative to Portland cement. It also has properties, such as a degree of flexibility and the ability to 'heal' small cracks, that make it more effective in many building applications. Its production and use date back several thousands of years.
Lime production uses about 20% less energy than the production of ordinary Portland cement (OPC). For every ton of OPC, almost a ton of CO2 is released into the atmosphere making it one of the worst greenhouse gas (GHG) producers of any manufactured products, slightly behind iron and steel!
Only transportation and energy generation produce more GHG emissions.
The term 'Lime' is used to describe a range of materials that are produced from limestone. Understanding the various types, characteristics and uses of lime can be a little confusing at first, and there is quite an array of names you may come across, including:
Pretty confusing, but it doesn't need to be.
For a glossary of the various names and terms, click here.
The more common terms in use are the ones in bold above. They cover all the different types of lime that you might encounter and I will use these in the rest of this description.
For the purposes of building and plastering all these different types can be divided into three categories: Pure, Type S and Hydraulic limes. The principle difference between all of these limes is in the purity and composition of the source material. All are derived from limestone which is composed of calcium carbonate (CaCO3), a sedimentary rock made up of the shells of millions of tiny sea creatures.
Pure lime, as the name implies, has the fewest impurities and is usually about 95% calcium carbonate.
Type S lime contains various other compounds - usually magnesium or aluminum oxide - which impart different properties to the material, most notably its workability and the speed at which it sets. It has only recently become popular in North America because of improvements in the slaking process which allows the oxides to be effectively hydrated.
Before these advances came about it formed a weaker material unfavourable for mortar or plaster. Consequently it never gained wide-spread acceptance in Europe, where it is typically was only used as an additive in cement to improve its plasticity.
Hydraulic lime contains varying amounts of clay. This produces its unique 'hydraulic' property which means it will set under water. The percentage of clay is proportional to the strength and hardness of the material, its flexibility, and the speed at which it sets.
stage 1 - calcination
The first stage in the production of lime involves crushing and heating limestone until a chemical reaction occurs which produces calcium oxide (CaO) and drives off carbon dioxide (CO2). This process is called Calcination. Calcination occurs at about 900 - 1000 degrees C and the resultant powder or lumpy material is called Quicklime.
stage 2 - hydration
The next stage is to turn quicklime into either hydrated lime or lime putty by combining it with water. When quicklime is combined with a specific ratio of water (about 3:1) a chemical reaction occurs which results in another dry powdered material - calcium hydroxide (Ca(OH)2) or hydrated lime. This process is called 'slaking'. Hydraulic lime also is hydrated in the same manner. If more water is added it becomes lime putty which usually has a consistency similar to stiff pottery clay. Properly hydrated lime and lime putty are not volatile like quicklime because enough water has been added to satisfy their 'thirst'.
Warning: Quicklime is a volatile material and can react quite violently with water - even the moisture in your skin - giving off sufficient heat to cause burns. It must be stored and handled with appropriate care and precautions.
A pound of quicklime can produce as much as 490 Btu's when hydrated. This is sufficient energy to raise the temperature of a pint of water by 490 degrees F or over 250 degrees C.
When hydrating quicklime it is very important to add the quicklime to an ample quantity of water, and NOT the other way round so that the water can absorb the heat without causing a violent reaction.
Pure lime can be hydrated to lime putty and kept almost indefinitely before use as long as it is protected from drying out. In fact, lime putty improves with age and it is beneficial to keep it for several months or even years. This ensures that most if not all of the CaO turns into Ca(OH)2, which improves the performance of the material. It is said that the Romans kept their lime putty for up to three years before using it.
Modern quicklime is usually hydrated using large commercial equipment that carefully controls the quantities of water and lime. However, in olden times it was not uncommon for the quicklime to be buried in a pit at the proposed building site and left for a year or more. During this time it would slowly absorb moisture from the ground and turn into perfect lime putty!
In contrast to lime putty, dry hydrated lime and hydraulic lime should not be kept for long periods. Unless they are well protected from moisture, they will start to set and gradually turn back into limestone. Hydraulic lime cannot be kept as lime putty since it will set in the presence of water.
stage 3 - mortar, render, plaster, stucco and wash
Which type of lime to chose depends on where and how it will be used. If compressive strength is needed, as in foundations or the stucco of a load-bearing straw-bale wall, then it would be appropriate to use the strongest and least permeable material, either NHL 3.5 or 5. If you are plastering the walls of a post and beam structure where strength is less important than flexibility and permeability, then using pure lime or Type S might be a better choice.
In most cases the lime is combined with 2 - 3 parts sand or other aggregates. When mixed together, the lime fills the spaces between the aggregate particles, therefore too much aggregate will result in unfilled spaces and a weaker bond.
The correct ratio can be determined by doing a Void Test (see the glossary for a description). Once the appropriate ratio has been established, it should be maintained carefully throughout the mixing and plastering process. This is called 'batching' and it plays an important part in the integrity of the final product.
Depending on the application, chopped straw or horse hair can be added to provide tensile strength. The addition of brick dust, 'fly ash' from coal furnaces, pummice or other such 'pozzolans' can also increase the strength and reduce the setting time of lime. (see the glossary for a definition of pozzolans).
The courseness of the sand depends on whether it is being used for exterior or interior walls, and whether it is a final coat. All aggregates used with lime should be 'sharp' in texture (with jagged surfaces) as this ensures a better bond and a mixture of different particle sizes is recommended.
With the exception of washes, when using any type of lime on walls it is a good idea to keep the mixture stiff. The actual preparation of mature lime putty into mortar or plaster can be laborious, as the putty must be kneaded and worked until it is quite plastic. It should stick to the underside of a trowel. Any temptation to add more water should be resisted. The less water it contains, the less shrinkage and cracking will occur as it dries. This is particularly important with exterior walls.
It also is important to wet the surface onto which the lime will be applied. For example, if applying a top coat onto existing plaster, the wall should be soaked down the night before until water in running off it, and then wetted again immediately before applying the new plaster. This prevents the moisture from being 'sucked' out by the underlying layer and causing premature shrinkage and cracking.
Lime washes are usually mixed with sufficient water to create a milky solution. To this can be added pigments and other additives such as caseine which provide both colour and protection for the walls.
stage 4 - carbonation
Finally, the lime starts to set and turn back into calcium carbonate, or limestone. This is a slow and gradual process during which carbon dioxide is absorbed from the atmosphere and water is released. After the lime has been used it should be protected from exposure to the sun or wind to prevent it from drying too fast.
As the outer surface of the lime hardens, it impedes the penetration of CO2 into the plaster and thus significantly slows the rate at which the remaining lime sets and strengthens. It is therefore important to cover the walls with sheets and periodically 'mist' them to slow down the process. The use of sheets also protects the walls from rain which could wash away the lime before it is set. Depending on the conditions, this dampening should continue for between four and seven days.
Lime will become hard to the touch within a few days but the actual process of carbonation continues for decades. Its strength and hardness continue to increase with age. Consequently, the conventional system of testing lime mortars and plasters when they are about 28 days old - the time by which cement and concrete is usually completely set - provides an unrealistically low measure of their eventual strength.
Although lime will set 'as hard as rock' it remains slightly 'plastic' or flexible due to its molecular structure. This characteristic allows it to absorb some movement caused by settling over time without cracking. Cement, although stronger, cannot absorb sustained movement and will eventually crack.
Furthermore, if lime develops small cracks during the carbonation process, water entering the crack will dissolve any 'free lime' on either side of the crack. As the water evaporates the dissolved lime crystalises in the crack, effectively sealing it.
By the time the lime has completely set it will have reabsorbed virtually all of the CO2 that was released during its chemical conversion to quicklime, thereby neutralising a large part of its carbon footprint. Cement does not reabsorb CO2.
variations
Earlier in this document I mentioned 'feebly', 'moderately' and 'eminently' hydraulic. These terms were used in the 19th C to differentiate between types of hydraulic lime which set slower or faster according to their clay content. The current system classifies hydraulic lime as NHL 2, 3.5 and 5. These figures represent its relative permeability, flexibility, strength and the speed at which it sets. NHL 5 is the least permeable, least flexible, strongest and quickest setting. The three 19th C terms are not interchangeable with the modern classification, since 'moderately hydraulic' is probably closest to NHL 2 in character.
An excellent article on lime is contained in The Lime Spectrum written by Ian Brocklebank and published by the Institute of Historic Building Conservation.
For more good information on lime and how to use it, visit:
www.oikos.com/library/naturalbuilding/lime.html/
www.buildinglimesforum.org.uk
Lime production uses about 20% less energy than the production of ordinary Portland cement (OPC). For every ton of OPC, almost a ton of CO2 is released into the atmosphere making it one of the worst greenhouse gas (GHG) producers of any manufactured products, slightly behind iron and steel!
Only transportation and energy generation produce more GHG emissions.
The term 'Lime' is used to describe a range of materials that are produced from limestone. Understanding the various types, characteristics and uses of lime can be a little confusing at first, and there is quite an array of names you may come across, including:
- Pure Lime also know as Quicklime or Non-hydrated lime;
- Hydraulic Quicklime;
- Hydrated Lime also called Dry Hydrate;
- Type S, type N, and Dolomitic lime;
- Lime Putty also known as Air Lime, Fat Lime or non-hydraulic Lime;
- Natural Hydraulic Lime (NHL) which comes in different types: NHL-2, 3.5 and 5, and;
- Hydraulic Lime (HL) which is a man-made version of NHL.
Pretty confusing, but it doesn't need to be.
For a glossary of the various names and terms, click here.
The more common terms in use are the ones in bold above. They cover all the different types of lime that you might encounter and I will use these in the rest of this description.
For the purposes of building and plastering all these different types can be divided into three categories: Pure, Type S and Hydraulic limes. The principle difference between all of these limes is in the purity and composition of the source material. All are derived from limestone which is composed of calcium carbonate (CaCO3), a sedimentary rock made up of the shells of millions of tiny sea creatures.
Pure lime, as the name implies, has the fewest impurities and is usually about 95% calcium carbonate.
Type S lime contains various other compounds - usually magnesium or aluminum oxide - which impart different properties to the material, most notably its workability and the speed at which it sets. It has only recently become popular in North America because of improvements in the slaking process which allows the oxides to be effectively hydrated.
Before these advances came about it formed a weaker material unfavourable for mortar or plaster. Consequently it never gained wide-spread acceptance in Europe, where it is typically was only used as an additive in cement to improve its plasticity.
Hydraulic lime contains varying amounts of clay. This produces its unique 'hydraulic' property which means it will set under water. The percentage of clay is proportional to the strength and hardness of the material, its flexibility, and the speed at which it sets.
Reproduced with kind permission of the Institute of Historic Building Conservation (IHBC). Diagram by Ian Brocklebank and taken from context, the journal of the IHBC.
The first stage in the production of lime involves crushing and heating limestone until a chemical reaction occurs which produces calcium oxide (CaO) and drives off carbon dioxide (CO2). This process is called Calcination. Calcination occurs at about 900 - 1000 degrees C and the resultant powder or lumpy material is called Quicklime.
The next stage is to turn quicklime into either hydrated lime or lime putty by combining it with water. When quicklime is combined with a specific ratio of water (about 3:1) a chemical reaction occurs which results in another dry powdered material - calcium hydroxide (Ca(OH)2) or hydrated lime. This process is called 'slaking'. Hydraulic lime also is hydrated in the same manner. If more water is added it becomes lime putty which usually has a consistency similar to stiff pottery clay. Properly hydrated lime and lime putty are not volatile like quicklime because enough water has been added to satisfy their 'thirst'.
Warning: Quicklime is a volatile material and can react quite violently with water - even the moisture in your skin - giving off sufficient heat to cause burns. It must be stored and handled with appropriate care and precautions.
A pound of quicklime can produce as much as 490 Btu's when hydrated. This is sufficient energy to raise the temperature of a pint of water by 490 degrees F or over 250 degrees C.
When hydrating quicklime it is very important to add the quicklime to an ample quantity of water, and NOT the other way round so that the water can absorb the heat without causing a violent reaction.
Pure lime can be hydrated to lime putty and kept almost indefinitely before use as long as it is protected from drying out. In fact, lime putty improves with age and it is beneficial to keep it for several months or even years. This ensures that most if not all of the CaO turns into Ca(OH)2, which improves the performance of the material. It is said that the Romans kept their lime putty for up to three years before using it.
Modern quicklime is usually hydrated using large commercial equipment that carefully controls the quantities of water and lime. However, in olden times it was not uncommon for the quicklime to be buried in a pit at the proposed building site and left for a year or more. During this time it would slowly absorb moisture from the ground and turn into perfect lime putty!
In contrast to lime putty, dry hydrated lime and hydraulic lime should not be kept for long periods. Unless they are well protected from moisture, they will start to set and gradually turn back into limestone. Hydraulic lime cannot be kept as lime putty since it will set in the presence of water.
Which type of lime to chose depends on where and how it will be used. If compressive strength is needed, as in foundations or the stucco of a load-bearing straw-bale wall, then it would be appropriate to use the strongest and least permeable material, either NHL 3.5 or 5. If you are plastering the walls of a post and beam structure where strength is less important than flexibility and permeability, then using pure lime or Type S might be a better choice.
In most cases the lime is combined with 2 - 3 parts sand or other aggregates. When mixed together, the lime fills the spaces between the aggregate particles, therefore too much aggregate will result in unfilled spaces and a weaker bond.
The correct ratio can be determined by doing a Void Test (see the glossary for a description). Once the appropriate ratio has been established, it should be maintained carefully throughout the mixing and plastering process. This is called 'batching' and it plays an important part in the integrity of the final product.
Depending on the application, chopped straw or horse hair can be added to provide tensile strength. The addition of brick dust, 'fly ash' from coal furnaces, pummice or other such 'pozzolans' can also increase the strength and reduce the setting time of lime. (see the glossary for a definition of pozzolans).
The courseness of the sand depends on whether it is being used for exterior or interior walls, and whether it is a final coat. All aggregates used with lime should be 'sharp' in texture (with jagged surfaces) as this ensures a better bond and a mixture of different particle sizes is recommended.
With the exception of washes, when using any type of lime on walls it is a good idea to keep the mixture stiff. The actual preparation of mature lime putty into mortar or plaster can be laborious, as the putty must be kneaded and worked until it is quite plastic. It should stick to the underside of a trowel. Any temptation to add more water should be resisted. The less water it contains, the less shrinkage and cracking will occur as it dries. This is particularly important with exterior walls.
It also is important to wet the surface onto which the lime will be applied. For example, if applying a top coat onto existing plaster, the wall should be soaked down the night before until water in running off it, and then wetted again immediately before applying the new plaster. This prevents the moisture from being 'sucked' out by the underlying layer and causing premature shrinkage and cracking.
Lime washes are usually mixed with sufficient water to create a milky solution. To this can be added pigments and other additives such as caseine which provide both colour and protection for the walls.
Finally, the lime starts to set and turn back into calcium carbonate, or limestone. This is a slow and gradual process during which carbon dioxide is absorbed from the atmosphere and water is released. After the lime has been used it should be protected from exposure to the sun or wind to prevent it from drying too fast.
As the outer surface of the lime hardens, it impedes the penetration of CO2 into the plaster and thus significantly slows the rate at which the remaining lime sets and strengthens. It is therefore important to cover the walls with sheets and periodically 'mist' them to slow down the process. The use of sheets also protects the walls from rain which could wash away the lime before it is set. Depending on the conditions, this dampening should continue for between four and seven days.
Lime will become hard to the touch within a few days but the actual process of carbonation continues for decades. Its strength and hardness continue to increase with age. Consequently, the conventional system of testing lime mortars and plasters when they are about 28 days old - the time by which cement and concrete is usually completely set - provides an unrealistically low measure of their eventual strength.
Although lime will set 'as hard as rock' it remains slightly 'plastic' or flexible due to its molecular structure. This characteristic allows it to absorb some movement caused by settling over time without cracking. Cement, although stronger, cannot absorb sustained movement and will eventually crack.
Furthermore, if lime develops small cracks during the carbonation process, water entering the crack will dissolve any 'free lime' on either side of the crack. As the water evaporates the dissolved lime crystalises in the crack, effectively sealing it.
By the time the lime has completely set it will have reabsorbed virtually all of the CO2 that was released during its chemical conversion to quicklime, thereby neutralising a large part of its carbon footprint. Cement does not reabsorb CO2.
Earlier in this document I mentioned 'feebly', 'moderately' and 'eminently' hydraulic. These terms were used in the 19th C to differentiate between types of hydraulic lime which set slower or faster according to their clay content. The current system classifies hydraulic lime as NHL 2, 3.5 and 5. These figures represent its relative permeability, flexibility, strength and the speed at which it sets. NHL 5 is the least permeable, least flexible, strongest and quickest setting. The three 19th C terms are not interchangeable with the modern classification, since 'moderately hydraulic' is probably closest to NHL 2 in character.
An excellent article on lime is contained in The Lime Spectrum written by Ian Brocklebank and published by the Institute of Historic Building Conservation.
For more good information on lime and how to use it, visit:
www.oikos.com/library/naturalbuilding/lime.html/
www.buildinglimesforum.org.uk
Links... (hover over links for info)
The Living Building Challenge
Fotoprint
John Gower - Gower design group
Chelsey Braham - C D B Design
Earth Futures
BCSEA
SIPDistribution.ca
Chemicallime.com
Perlite.com
Strawbale.com
HomesteadHouse.ca
Eco-Sense.ca
Building Limes Forum
Straw bale fire test movie
Straw bale earthquake test movie
Susanne Dannenberg - visual artist
Goodshepherdwool.com
Elke Cole