The purpose of this information sheet is to provide a general introduction to using lime-based mortars and emphasizing its use as a workable and viable binder. It hopes to demonstrate the possibility of use of lime outside the monument. It is aimed at young designers and builders who may have little knowledge of lime-based mortars but who want to apply best practice when they use these materials in new build construction projects as well as restoration projects.
Using lime does not require rare and complicated skills, but needs an amount of care and knowledge. Good workmanship is vital to the success of lime renders. It is essential for professionals to understand their materials while working on site. Modern plasters used to work with cement and gypsum expect their material to set within hours.
Why use lime mortars?
In the 21st Century when considerable expertise has been developed in the field of cement based mortars why are we discussing the use of lime mortars?
1. Lime is "Sustainable"
· Lime has less embodied energy than cement, i.e. the energy required to make lime is much less than cement
· Free lime absorbs carbon dioxide in the setting process of carbonation.
· It is possible to produce lime on a smaller scale.
· The gentle binding properties of lime enable full recycling and reuse of materials. Bricks laid with lime mortar can easily be separated. eg In the restoration of the Ram Lal Khemka Haveli, Kashmere Gate, we were able to source old bricks from the BirlaMills which were being demolished.
2. Lime is "Breathable"
Traditional buildings rely for their weatherproofing on their ability to hold and evaporate water. Lime mortars are vapour and water permeable and allow buildings to breathe as these absorb a certain amount of water and subsequently allow it to evaporate from the building. This reduces the risk of trapped moisture and consequent damage to the building fabric.In simple terms, the greater the extent of pure lime and permeability the better this is for the building. Though this needs to be balanced with durability.
3. Lime is "Flexible"
The nature of ground conditions and the elements are such that all buildings are subject to varying degrees of movement over time. Buildings built with lime have an inherent flexibility and are more tolerant to movement, and are more likely to develop many fine cracks than the individual large cracks which occur in stiffer cement-bound buildings. Hairline cracks in lime mortars chemically seal themselves, and are often described as being ‘self healing’Lime built structures are flexible and can adjust to seasonal and structural movement without damage. Movement within masonry walls built with lime-based mortars can be accommodated in the individual bed joints between the masonry units, reducing the need for vertical movement joints.
4. Lime Contributes To A Healthy Environment
Lime is caustic and has been extensively used for its disinfectant qualities. Lime is also used for water purification. Lime mortars, plasters, renders and limewash have been used to create hygienic surfaces and improve comfort conditions within buildings for thousands of years.
5. Disfiguring By Cement Can Be Avoided By The Use Of Lime
On site temptation to use quick and easy solutions for short term gain can lead to long-term problems. The attraction of using excess cement to be ‘safe’ is understandable if not desirable. The fact that it is plentiful, inexpensive and readily available adds to the problem. There is a high probability that over-strong and dense mixes that are not fit for purpose will be used in excess. The physical damage and unsightly aesthetic that results from this can be avoided by the use of lime.
When to use lime mortars?
It is possible to use lime-based mortars and lime-based renders in many applications but it is important that the decision is made early in the design and planning process so all involved in the design and construction can make adjustments to the design and to the programme of work.
1. New build
Lime-based mortars can be used in all aspects of new build construction but have gained most acceptance in the construction of masonry walls. Lime is also used in the laying of lime concrete floor slabs, internal and external plasters.
2 Repair, Conservation and refurbishment
This is an area of construction with the longest history of lime use as it includes the conservation and restoration of historic buildings. Some aspects of refurbishment can be considered as conservation, for example the matching of repair and replacement mortars to those already in the building, but they may also provide opportunities for using lime-based mortars in the construction of extensions or rebuilding of structures as part of the refurbishment.
When considering the use of lime-based mortars, it is very important to be realistic and accept that there are some constraints, as well as areas where the properties of limebased mortars are perceived as being limiting, and to address these issues.
· Speed of construction: The build rate with lime-based mortar is perceived as being very slow compared with cement-based mortars or thin bed mortars. Lime-based mortars do develop strength more slowly.
· Rate of gain of strength: Compressive strength develops more slowly in lime-based mortars than in cement-based mortars. As a result, the compressive strengths of lime-based mortars are usually quoted at 91 days rather than 28 days. However lime-based mortar will typically have reached half its 91-day strength by 28 days.
What is lime?
Lime is calcium oxide or calcium hydroxide. The word "lime" originates with its earliest use as building mortar and has the sense of "sticking or adhering". All types of building lime are produced by burning limestone, or other chemically similar material.
Lime and lime products have been used for long in the building construction for more than 6000 years. From the Greek & Roman to the Mesopotamian,Cretan civilization has shown traces of lime mortar in construction of large lime concrete domes to external plastering and rendering. Vitrivus, known as the father of architecture,has written in detail about lime in his of the Ten Books of Architecture.In India the lime technology was known from ancient times, evident from the lime gypsum base found in the drains of Mohenjodaro, though its application was limited as mud plaster and mortar were preferred for brick construction or it was dry stone construction with large stones blocks held together by gravity and iron clamps. Lime plaster has also been as base of paintings in the caves of Ajanta and Kaushambhi. It was with advent of Islamic architecture that lime was used extensively in construction.
There was loss of interest in lime after the discovery of Portland Cement in 19th century and it slowly waned completely after the First World War and the beginning of what we could call modernity in engineering and material sciences. As the use of lime-based mortars declined during the 20th century, architectural styles and construction processes adapted to changing market demands and the availability of materials. In particular, buildings were designed with taller, thinner masonry walls, which were built much more quickly. This favoured the use of Portland cement-based mortars. Traditional lime-based mortars were more suited to the slower construction of thick masonry walls, which were required to resist only very low vertical stresses, and where the resistance to lateral loading depended more on the interlocking of the masonry units and gravity than on the adhesion between the units.
The rise in awareness of the need for sustainability in construction in recent years has led practitioners to re-consider the use of lime-based mortars. For guidance they turned to experience in building restoration and conservation, where limebased mortars had continued to be used. Revival in the use of lime-based mortars requires the re-assessment of the construction of walls built of stone, brick or block to take into account all the qualities of a modern limebased mortar, allowing it to become competitive in the new build construction sector.
The Lime Cycle
Lime does not occur naturally but must be manufactured. The material passes through several stages:
All types of building lime are produced by burning limestone, or other chemically similar material. Calcium Carbonate (which may be limestone,marble, chalk or sea shells) gives up carbon dioxide when heated at 900°C and converts to Calcium Oxide (also known as quicklime).
Quicklime is a caustic, unstable material that reacts vigorously with water to form calcium hydroxide (hydrated lime or slaked lime or putty lime).
Lime is generally used for building purposes in the form of calcium hydroxide (slaked lime), it is mixed with sand and water to form various kinds of mortar and render for building purposes. When the masonry has been laid, the slaked lime in the mortar slowly begins to react with carbon dioxide to form calcium carbonate (limestone) according to the reaction:
Ca(OH)2 + CO2 → CaCO3 + H2O.
This process by which limestone is converted to quicklime by heating, then to slaked lime by hydration, and reverted to limestone by carbonation is known as the Lime Cycle.
Types of Lime
Non Hydraulic Lime
Limestone (or chalk etc) which contain a high percentage of calcium carbonate produce pure lime. It relies for its hardening on drying and reconversion of calcium hydroxide back to calcium carbonate. This process takes place slowly and requires the correct balance of moisture and temperature.
Limestone which contain a proportion of clay minerals will produce lime with a positive set an ability to set in wet conditions and a denser harder nature than pure lime. Hydraulic lime is generally available in the form of dry hydrate
What are lime mortars?
Mortars are a mixture of an aggregate, a binder, and water. In a lime-based mortar, the binder is lime; whereas in a cement-based mortar, the binder is cement. Over the past 100 years, mortars with cement as the binder have largely replaced lime based mortars.
How to make lime mortars?
To make usable products such as mortar, plaster and limewash, quicklime is converted into hydrated lime by chemical combination with water. This is known as slaking.
|Limestone is always added to water as explained in the lime cycle above, it will release heat as an exothermic reaction as seen in the picture above, this process is known as slaking|
|Demonstrating the reverse if water is poured on limestone it creates an explosive reaction and is not recommended.|
Traditionally slaking could be carried out
1. By adding quicklime to water
2. By mixing quicklime and damp sand
The basic ingredients of traditional lime mortars are sand and lime. Good lime mortars contain aggregate and lime in correct proportions well beaten together and generally matured before use.
1. Choosing the right aggregate is very important, the quantity and nature of the aggregate in a mortar is critical to its workability and performance. Sand should be coarse containing some large sharp angular pieces to get a firm mix, soft fine sand gives a weak mortar.
2. Thorough beating (knocking up) or compressing and good maturing of the basic mortar mix are critical for achieving good performance. Large quantities of mortar are normally made in a roll pan mixer and small quantities may be made by hand. It is the beating process that influences the final quality and performance of the mortar. Stirring together lime putty, or hydraulic lime powder, sand and water will not produce an effective mortar. Proportions of lime and aggregate should be such that lime fills the spaces between the sand grains. A ratio of 1 lime to 3 aggregate is normal but can be varied on a case to case basis.
3. For use with non-hydraulic limes where more durability or a more rapid set is required "pozzolanic materials" are sometimes added. The commonly used pozzolans are volcanic materials (ash), ground fired clay (bricks/tiles), waste from steel making, PFA (waste from coal fired power station) added to increase setting. In India, pozzolans are brick dust (surkhi), egg white, urad dal, methi dana, gud, guggal (molasses) etcWhen surkhi is added for higher strength the proportion of sand is normally reduced.
4. Lime putty and sand can be measured by volume.
The various techniques of using quicklime and sand-
1. Hot mixing using quicklime and sand
By mixing quicklime and damp sand the moisture in the mix will react with quicklime converting it to hydrated lime in the form of a paste. Hot lime mortars involves mixing crushed quicklime and sand with sufficient water to slake the lime and achieve a plastic mortar mix. The mortar is normally allowed to mature for 8 to 12 weeks.
2. Mixing slaked lime putty and sand
This method involves first slaking the quicklime to a putty and later mixing putty and sand together for non hydraulic limes. To make lime putty fresh quicklime is added to water and stirred to produce a thick creamy consistency. Never add water to quicklime. The lime putty is normally matured for 3 months before use.
Methods of Mixing
The most effective method of mixing is in a roller or pan mill (mortar mill).The materials should be by beating them well together, normally without the addition of water, until a plastic mix is achieved. This may take at least 20 minutes in a roll pan mixer. Small quantities of mortar can be mixed by hand thorough mixing and beating is essential. Maturing times for non-hydraulic mortars are generally around 12 weeks. Longer maturing times will improve quality of the mortar.
The basic rules of using lime mortars include:
· Use well prepared materials, which are appropriate for the particular job.
· Use well prepared materials, which are appropriate for the particular job.
· Prepare joints and surfaces well.
· Avoid over working the surface of the mortar.
· Leave with an open textured surface; protect and cure the new mortar
· All types of lime mortar, including hydraulic limes, must be adequately cured and must achieve carbonation in order to attain their potential performance and durability.
· The process of carbonation requires slow drying in conditions which include a degree of moisture or humidity, a reasonable temperature. Some fluctuation in moisture and temperature levels appears to be beneficial.
· Over-rapid drying will result in a weak, crumbly mortar because the lime will revert to unstable calcium hydroxide rather than converting to calcium carbonate.· In rapid drying conditions, due to wind as well as sun, extra care is needed with protection, and light mist spraying may be needed to prevent the surface drying before the depth of the material.
|Matured lime putty is mixed with damp sand to form a good mortar mix|
|Slaked and matured lime putty should be of a cheese like consistency and is sieved before being used for mortar process to ensure there are no impurities|
1. Gurmeet S Rai & Paromita De Sarkar (2006) , " What are Lime Mortars" Conservation Brief" INTACH
2. Sangeeta Bias (2006), "Why use Lime" Conservation Brief INTACH
3.The technology and use of hydraulic lime by John Ashurst
(available at http://www.buildingconservation.com/articles/hydraulic/hydraulic.htm)