LIME: Everything you need to know to get started
Lime has been used as a building material for thousands of years because of its unique properties. Lime is strong, durable and flexible, which means that it can withstand construction loads without breaking down. It is also breathable, allowing moisture to evaporate, reducing the risk of dampness and mold.
Historically lime has been used as a mortar and plaster in many parts of the world. It’s unclear when exactly lime started being used in building, but it’s well documented that the Roman Empire used a lot of lime based mortars. 
Lime is produced by heating limestone (sedimentary rock composed mainly of calcium carbonate) in a kiln to produce quicklime (calcium oxide). The quicklime is then slaked (mixed) with water to produce hydrated lime (calcium hydroxide), which can be used in numerous construction applications.
Lime can be used in a variety of construction applications, including mortar, plaster and rendering. It can also be used as a soil stabilizer and binder in concrete. In recent years, lime has become popular as a sustainable building material because it is produced from natural resources and has lower energy consumption compared to other building materials. 
TYPES OF LIME
There are three main types of lime: quicklime, slaked or hydrated lime and hydraulic lime. Each type of lime has its own unique properties and applications in construction. 
QUICKLIME, also known as calcium oxide, is produced by heating limestone in a kiln at high temperatures. Quicklime is highly reactive and corrosive, it’s typically used in industrial settings and must be handled with special care. Some frequent uses of quicklime are:
- Quicklime is used in steelmaking as a flux to remove impurities and reduce the melting point of steel.
- Lime is used in the production of various chemicals such as calcium carbide and citric acid.
- Quicklime is sometimes used to stabilize soil in construction projects, for example in road and building foundations.
- Quicklime is used in industrial waste treatment to neutralize acidic or contaminated wastewater by raising its pH.
HYDRATED LIME, also known as slaked lime or calcium hydroxide, is produced by adding water to quicklime. It can come in a powder form or in a paste form called lime putty. Hydrated lime is less reactive than quicklime and has a variety of applications in construction, some of them are:
- As an ingredient in mortars to improve workability, water resistance and durability.
- Hydrated lime can be added to plasters and renders to improve their adhesion, strength and water resistance.
- It can be used to improve soil stability and strength in construction projects.
- Hydrated lime is sometimes added to asphalt to improve its properties such as durability and resistance to rutting.
HYDRAULIC LIME (HL) and NATURAL HYDRAULIC LIME (NHL) is a type of lime that stiffens and hardens when exposed to water, similar to hydraulic cement. It is produced by burning limestone containing clay (NHL) or by adding pozzolan materials either before or after burning the limestone in a lime kiln (HL), thus producing a lime with certain hydraulic properties. The difference between hydraulic and natural hydraulic lime is the production process. Natural hydraulic lime is often used in conservation work to repair historic buildings where the use of Portland cement would be too strong and could cause damage to the existing structure.
Some of the most common uses of hydraulic lime are:
- to restore or repair mortar joints in historic masonry structures.
- to create a traditional lime render finish that is compatible with historic buildings.
- in the conservation of historic buildings because its properties are similar to the original lime-based materials.
Hydraulic or hydrated lime? It’s better to use hydraulic lime in settings where it would be troublesome to provide warm and dry conditions for the hydrated lime to set.
NATURAL HYDRAULIC LIME GRADES
NHL has three different grades: NHL 2, NHL 3.5 and NHL 5. These categories have historically been associated with the terms feebly hydraulic, moderately hydraulic, and eminently hydraulic. These terms refer to the degree of hydraulicity of each class, meaning the ability to be fixed underwater without exposure to air.
NHL 2 is softer and slow setting, suitable for internal applications or where conservation is a primary concern with soft or deteriorating stones and bricks.
NHL 3.5 is for general building, suitable for moderately permeable masonry materials. Basically, if you don’t need to use NHL 2 or NHL 5 then use this.
NHL 5 is stronger and faster setting, more suitable for dense, durable or impermeable materials with severe exposure to weather or water.
When choosing lime the strongest is not always the best. It’s important that the mortar is softer than the masonry in order to allow the masonry to breathe. Nevertheless the mortar has to be strong enough to last, so the softest is not always the best.
What should you consider when working with lime?
Quicklime is more caustic than hydrated and hydraulic lime, but generally the safety measures are the same for all lime types. When working with lime it’s necessary to take into account the following safety measures:
PPE: The usage of appropriate personal protective equipment (PPE), including gloves, eye and respiratory protection.
STORAGE: Lime has to be stored in a dry place away from moisture and incompatible substances.
HANDLING: When using lime, it’s necessary to avoid the formation of dust by using appropriate handling methods such as wetting or using a dust suppression system. If spillages happen they should be cleaned up immediately using appropriate methods such as wetting the lime and then wiping it up.
VENTILATION: When working in confined or poorly ventilated spaces, it’s necessary to make sure there is sufficient ventilation and respiratory protection.
There are also different classifications of lime, depending on their purity and particle size. The most common ones are high-calcium lime, produced from pure limestone, and dolomitic lime, produced from limestone with a magnesium content. These different types and grades of lime offer a variety of possibilities for use in construction and can be used in different combinations to obtain specific properties and characteristics for building materials.
Lime is widely used in the renovation of historic buildings. It is a popular choice for restoration and conservation work on historic buildings because it is compatible with the original materials used in the construction of these buildings. Many of the historic buildings that have survived to the present day were built with lime-based mortars and plasters. Hydraulic lime is often used in restoration work because its properties are similar to the original lime-based materials. It is often used to repair or restore mortar joints in historic masonry structures and to create traditional lime render finishes.
Some uses of lime in construction are in Aerated concrete blocks, Mortar, Plaster, Lime Concrete, Limewash, Conservation/ heritage, Sand-lime bricks.
Lime rendering or limewash is a method of painting walls with a paint base that allows the masonry to breathe, thus providing both protection and aesthetic appeal. Limewash also is antibacterial and low cost finish because of these properties it’s often used in agricultural buildings.
European Lime Association (EuLA) provides information about the availability of lime in Europe, as well as the uses and benefits of lime in various industries, including construction, agriculture, and environmental protection. Also their members are lime producers and suppliers throughout the EU. You can find more information here: www.eula.eu
In Europe lime generally is an easily accessible and affordable material.
LIME AND CONCRETE
To better illustrate lime as building material lets compare it to the most popular construction material – concrete.
COMPOSITION: Lime is primarily composed of calcium oxide (quicklime) or calcium hydroxide (hydrated lime), while concrete is made from a mixture of cement, aggregates (such as sand and gravel), and water. The main component in cement that gives it the bonding properties is limestone but it has other ingredients like clay, shale, iron ore, and gypsum. Typically in the production of one tonne of cement the energy used is 4GJ. To produce one tonne of lime about 3GJ of energy is required. 
STRENGTH: Concrete is generally stronger than lime, with compressive strengths typically ranging from 20 to 40 MPa, while lime mortar typically has a compressive strength of 1 to 5 MPa. However, the strength of lime can be increased by adding other materials such as pozzolanic additives, for example NHL 5 has the compressive strength of 10 to 20 MPa.
FLEXIBILITY: Lime is more flexible than concrete, and can move and bend with the natural expansion and contraction of buildings. This makes it a good choice for historic buildings that may shift over time. Concrete, on the other hand, is more rigid and can crack under stress.
DURABILITY: Concrete is generally more durable than lime, and can withstand exposure to weather and heavy use over time. Lime can also be durable if properly maintained, but it may require more frequent repairs and maintenance than concrete.
SUSTAINABILITY: Lime is generally considered to be more environmentally friendly than concrete, as it has a lower carbon footprint and embodied energy (the total energy consumed in lifespan of a material). Lime also has the potential to be recycled or reused, while concrete is more difficult to recycle.
APPLICATION: Lime is commonly used in restoration and conservation work, as well as in some construction applications, such as soil stabilization and plastering. Concrete is used in a wide variety of construction applications, including foundations, walls, and pavements.
Lime is a softer and more flexible material than concrete and can be a good choice for historic restoration work and other projects where flexibility and breathability are important. Lime also has a smaller carbon footprint and lower embodied energy than concrete, making it a more sustainable choice in most cases. However, lime has a lower compressive strength than concrete and may not be suitable for applications where high strength is required, such as foundations or structural walls. Lime is also slower to cure than concrete and may take longer to fully harden and reach its maximum strength.
In general, lime and concrete have different strengths and weaknesses. However, in some cases, lime can be used as a substitute for concrete, but it is important to consider the specific needs of the particular project and consult a licensed engineer or architect to determine the appropriate building material. Lime is a good choice for historic restoration and conservation work, as well as plaster work, flooring and in places where there is no necessity for high compressive strength.
IS LIME A NATURAL MATERIAL?
Natural hydraulic lime is made from limestone which naturally contains some clay, artificial hydraulic lime is made by adding forms of silica or alumina such as clay to the limestone during firing, or by adding a pozzolana to pure lime.
Lime is considered a natural material because it is derived from naturally occurring limestone, a sedimentary rock composed mainly of calcium carbonate. In the lime production process, limestone is heated at high temperatures, which causes it to decompose into calcium oxide (quicklime) and carbon dioxide. This process, known as calcination, is a natural chemical reaction that has been used for thousands of years to produce lime.
Although lime is not a renewable material as it is made from limestone which is a finite resource that is formed over millions of years through natural processes. Once limestone has been extracted and processed into lime, it cannot be regenerated during a person’s lifetime. However limestone is an abundant material and the production of lime has relatively low energy consumption. It´s a recyclable material and has the unique ability to reabsorb carbon dioxide during its lifetime.
In addition, lime has many natural properties that make it an ideal building material, including its ability to absorb carbon dioxide from the air, its antimicrobial properties, and its ability to regulate humidity levels in buildings. All these properties make lime a highly desirable and sustainable natural material that can be used in construction, agriculture, and many other applications.
Lime has been used for its abilities for a long time and its usage is in a wide range of fields from construction to gardening. This natural material provides the ability to make more breathable living environments as lime allows the air to circulate through. Also lime is almost carbon neutral as in its lifetime it reabsorbs the amount of carbon dioxide that is used in its production. In construction lime is mainly used for plaster, renders or as a mortar, lime could also be mixed with cement to change cements texture.
At Critical Concrete we have used lime in the workshop flooring, making of tiles and hempcrete and woodcrete. Keep an ear out for an article about natural flooring that is coming soon!
Check out the A4 printout below for the most important information about lime!
Cover photo from: https://unsplash.com/photos/4X5ySjDNLPg