Lime in construction

USING LIME IN CONSTRUCTION

Although experts do not agree on when humans began using lime in construction, we do know—thanks to the many examples of lime-built architecture that are still preserved today—that the use of this building material dates back at least more than 3,000 years.

Notable examples include the Pyramids of Giza in Egypt, which were coated and tinted with lime; the civilizations of Ancient Greece, which used lime as masonry mortar; and the Great Wall of China, where lime was used as a soil stabilizer and as a binder for stone.

Until the 20th century, lime was the main building material used for binding mortars and renders. However, the emergence of Portland cement in England in the late 19th century triggered a revolution in the construction sector. [1]

Because of its novel properties—considered revolutionary at the time—Portland cement became widespread in the early 20th century. Its rapid setting, ability to harden under water, and high strength made it extremely popular, pushing lime into the background.

Although today the use of Portland cement as a binder has largely replaced lime, in recent years there has been growing opposition to Portland cement, whose impact is twice that of lime due to the need to use fossil fuels during its production process.

The Portland cement industry is in a race against time to reduce its emissions to zero by 2050, and although most sector associations have already published their decarbonization roadmaps, in practice it is still unclear whether such a reduction is feasible with the technology and knowledge available today.

While we wait for highly polluting, fossil-fuel-dependent construction-material manufacturing industries to find a miracle solution to reduce their emissions to zero by 2050, it is essential to develop, research, and revalue materials that have remained in use for thousands of years in the sector—materials with a low ecological footprint, rooted in building tradition, in craftspeople and skilled trades, locally produced, and harmless to the environment and to people.

For this reason, this article focuses on revaluing lime: a low-ecological-footprint material, produced in Spain, with exceptional qualities, which in recent years has begun to expand thanks to numerous companies that not only manufacture it, but also research and promote its use based on ancestral knowledge.

In the climate emergency we are facing, it is urgent to recover materials that have been part of human history and with which we have built healthy, sustainable living spaces with zero ecological impact. [2]

THE LIME CYCLE

Producing lime for construction involves a series of chemical transformations known as the lime cycle.

The lime cycle defines the different processes limestone—the raw material—goes through until it becomes lime suitable for use, ultimately recovering its stone-like properties.

This cycle involves the following phases:

1. CALCINATION
2. HYDRATION
3. CARBONATION

1. CALCINATION

Once the limestone has been extracted from the quarry and it has been ensured that the stone is free of impurities, it is crushed into smaller pieces and graded by size to ensure an even distribution of material during the calcination process.

The lime cycle begins with calcining the stone at around 900°C. The crushed stone is placed in a lime kiln, which is heated to high temperatures—without the need to use fossil fuels—to trigger a chemical reaction in the limestone known as thermal decomposition or calcination.

During calcination, the calcium carbonate present in the limestone decomposes into calcium oxide, known as quicklime, and carbon dioxide CO₂.

This firing process releases the CO₂ present in the raw material, resulting in calcium oxide, also known as quicklime.

CaCO3 + HEAT → CaO (calcium oxide or quicklime) + CO2↑ (as a gas)

Quicklime is characterized by a fine powder appearance, with the texture of small, irregular granules that are white and bright.

2. HYDRATION

After calcination, the resulting product is slaked with water. Quicklime is submerged in water, producing a hydrated material known as slaked lime.

During this hydration process, an exothermic reaction occurs in which calcium oxide reacts with water to form calcium hydroxide, or slaked lime. The hydration reaction can generate excess heat, so it is advisable to control the temperature during the process.

CaO + H2O (water) → Ca(OH)2 (slaked lime or calcium hydroxide) + heat

Compared to quicklime, slaked lime has the appearance of a finer, softer powder, with a slightly grayish white color. It is characterized by its alkaline nature, so handling it requires protective equipment.

In construction, it is often used as a stabilizing agent in the production of lime concrete and as a base for lime mortars in coatings. It is also commonly used as a water-purifying additive.

3. CARBONATION

Slaked lime is applied as mortar, base coat, or finish coat on surfaces. Once the slaked-lime mix—combined with aggregates and water to form lime mortar—is spread, it comes into contact with the air in the atmosphere, leading to a final chemical transformation: carbonation.

This process involves the reaction of calcium hydroxide with the carbon dioxide present in the air to form calcium carbonate, or carbonated lime.

Ca(OH)2 + CO2 (carbon dioxide) → CaCO3 (calcium carbonate) + H2O (water)

Calcium carbonate gradually forms a layer on the surface that restores the properties of the original limestone. Calcium carbonate provides the solid, crystalline structure characteristic of stone, also providing stability and strength to the material.

The use of lime in construction is a manufacturing process with almost zero environmental impact. It is a closed, circular cycle, both in energy consumption and in raw material—meaning that the energy used during the different processes is offset by carbonation, which involves the absorption of CO₂ by the final product over a short period of time.

TYPES OF LIME

Broadly speaking, two types of lime are distinguished in construction:

1. AIR LIME type CL90 or CL90-S
2. HYDRAULIC LIME type NHL or HL

1. AIR LIMES

Air limes are obtained from pure limestone rocks whose content of impurities and clay materials is as low as possible. The purer the primary limestone, the better the plasticity properties of the air lime. [3]

Air lime forms once the calcination process of limestone in lime kilns at around 700°C has been completed. The resulting product is a fine white powder, to which water is added to saturation to obtain lime putty.

To achieve the hardening of lime and obtain a final stone-like finish, it is necessary to go through a carbonation process—the process by which the air-lime mix sets by absorbing carbon dioxide present in the air.

Current regulations governing lime in construction require that air lime, in paste form and suitable for commercialization, must be at least 90% pure. For this reason, air lime nomenclature is presented under the initials CL90 (Calcitic Lime).

Another option to CL90 lime is CL90-S, which consists of applying unsaturated water—that is, the minimum amount of water that, even if it does not become a paste and remains in powder form, allows the mix to be workable in the same way. [4]

PROPERTIES AND APPLICATIONS OF AIR LIME

As a highly plastic and workable material, air lime has relatively low mechanical strength. Its composition results in a porous structure that promotes the breathability of the surfaces to be covered.

For this reason, it is often used as a coating and finish, especially in the restoration of historic buildings, due to its compatibility with older materials.

Air lime is commonly used in masonry or stonework mortars, plasters or gypsum finishes for both interior and exterior coatings, or in lime paints.

2. HYDRAULIC LIMES

Hydraulic lime comes from firing limestone mixed with clay at 1100°C in a specific proportion—around 80% pure lime and 20% clay composition (in addition to clay, it also contains ferrites and aluminates). This clay, when fired, produces calcium polysilicates that give lime hydraulic properties.

The hardening process of hydraulic lime is not only related to air, as with air limes, but also to water. When hydrated, the composition of this type of lime forms insoluble and very stable substances.

In addition, hydraulic lime hardens faster than air lime, allowing it to set in all kinds of environments, both dry and humid. For this reason, the setting process can occur naturally through contact with humid environments, or by adding water to the mix in abundant quantities to ensure and accelerate the process.

Two types of hydraulic lime are distinguished. On the one hand, NHL (Natural Hydraulic Lime), which is naturally hydraulic due to the quarry itself. Its extraction is uncommon because there are few deposits of this type of lime. For this reason, HL (Hydraulic Lime) is usually used, which imitates NHL by making the mix hydraulic afterwards, adding components that allow the mix to be made hydraulic manually.

PROPERTIES AND APPLICATIONS OF HYDRAULIC LIME

Hydraulic lime offers better strength performance than air lime. This allows it to be used for both coatings and elements with structural loads.

Hydraulic lime is used to produce lime mortars and lime concretes that require additional strength. Depending on the ratio chosen between the different parts that make up the mix, the lime mass will be able to withstand greater or lesser stresses.

It can be applied in the construction of roads, platforms, embankments, and foundations where greater load-bearing capacity is required, along with improved soil strength and compaction.

AGGREGATES

Aggregates are understood to be granular materials, such as sand—fine-grained—or gravel—coarse-grained—incorporated into lime mortars or lime plasters to provide volume and strength to the mix. Aggregates act as a skeleton, providing cohesion and solidity to the whole.

The selection of aggregates will depend on the type of lime-mortar application and the desired characteristics of the final finish. Aspects such as grading and composition influence their physical and mechanical properties.

Including aggregates in the lime mix also supports the carbonation process by increasing porosity. This reaction allows air—and therefore carbon dioxide—to access the interior of the mass more easily.

On the other hand, aggregates make lime mortar more economical, allowing the amount of mixed lime to be reduced by up to half.

SAND – FINE AGGREGATE

Sand, as a fine-grained material, improves workability, making the mortar easier to handle and apply.

By providing a larger contact surface between the components of the mix, it results in improved final adhesion.

Mixes with a low sand content, known as fat limes—that is, with excess lime—tend to crack due to shrinkage. Sand therefore helps control this shrinkage characteristic of the mortar setting and drying process, minimizing potential later fissures or cracks.

GRAVEL – COARSE AGGREGATE

Gravel, by contrast, provides strength—as a coarse aggregate, it increases the overall mechanical strength, improving compressive performance.

It promotes the permeability and porosity of the mortar, improving internal water drainage, which is especially useful in exterior applications.

In terms of shape, it is preferable to avoid rounded river gravel gradings and opt for polyhedral shapes with a larger contact surface, which improve mortar adhesion. The best-performing coarse aggregates are those from quartz, siliceous, limestone, or granite rocks.

ADDITIVES

To apply lime in construction, it is necessary to add other elements to the mix that allow it to move from an initial consistency to the desired final one—for example, from powder to paste.

Additives or protective agents, in addition to their main objective of obtaining a lime paste, can also affect other aspects such as color, texture, setting speed, strength, etc.

Lime in construction offers a sustainable, minimal-impact alternative for the construction sector, so it is essential to ensure that any additives added also comply with these principles. To do so, these additives must have a natural-origin composition and a positive ecological footprint. [5]

PIGMENTS

Lime-based products—among them mortars, renders, finish coats, etc.—already have a tone inherent to the material itself, capable of giving the finish a warm and pleasant appearance.

For this reason, the decision to add pigments to the mix is not essential; they can be omitted to reveal the natural look of exposed lime.

If a specific finish is desired and pigments are to be added to the mix, there are many options of natural origin or made from recycled products.

Lime mortar can be pigmented with minimal-impact materials:
– of mineral origin, with iron oxide and the crystallization of minerals such as chalk, slate, graphite, or spinel, among others.
– of plant origin, with resin or the roots of certain trees.

OTHER ADDITIVES

Lime in construction can incorporate many types of additives, all of natural origin, which will provide one property or another to the biomaterial depending on the needs.

ROSIN

Rosin is a 100% natural consolidant derived from pine resin. It is a substance with adhesive and hardening properties that improves adhesion between the components of the mix, increasing its mechanical strength.

The presence of rosin in lime mortar helps minimize water penetration into the structure and therefore reduces the risk of moisture-related damage.

It is a low-cost, easy-to-apply protective additive, recommended for indoor use, as outdoors it takes on a slight, undesirable yellowish tone characteristic of the resin.

POTASSIUM SOAP OR COCONUT SOAP

Neutral, or potassium, soap acts as a protective additive for porous surfaces, making the substrate water-repellent while maintaining breathability and providing a soft, matte feel.

Potassium soap is made in an aqueous solution from plant-based fatty acids. There are also coconut-soap additives, made from coconut fatty acids, following the same sequence. These are applied in white stuccos, while potassium soap is used for colored stuccos.

BEESWAX

Beeswax provides extra waterproofing protection to the surface and a smooth, glossy finish, helping highlight the natural tones and veining of the substrate.

Adding beeswax to lime mortar can help improve the workability of the mix; it acts as a friction-reducing agent, making the mortar easier to handle and apply.

Its most common use is in fire-polished stuccos. Because it provides a permanent wet glossy finish, it is not commonly used in residential interiors, which tend to seek matte or satin finishes.

APPLICATIONS OF LIME IN CONSTRUCTION

Lime is a highly versatile biomaterial that offers numerous possibilities in construction. When mixed with other materials, its properties vary and, therefore, so do its uses and applications.

1. LIMEWASHES

Limewashes, also known as limewash in English, are a type of lime-based paint that is highly diluted, like watercolor.

They are most commonly used outdoors. The main function is to cover the surface with an additional protective layer while preserving the substrate’s porosity and breathability. This additional layer can also tint the substrate in white or reddish tones, among others.

2. LIME PAINTS

Lime paints are generally made from air lime, due to its breathability, flexibility, and aesthetic finish. They are distinguished by their velvety effect and their naturally bright, intense white color.

In addition to air lime, the composition of lime paints includes natural resins, powdered mineral pigments, water, and other additives that improve the behavior of the mix. [6]

Unlike limewash, lime paint, due to its composition, tends to be used more often indoors than outdoors. Its most common exterior use is in interior courtyards finished with an ETICS-type façade.

3. LIME STUCCO

One of the most common uses of lime in construction is in coatings such as stucco. The composition of lime stucco fundamentally contains air lime, due to its plasticity, flexibility, permeability, and other adapted properties.

The main application of lime stucco is in heritage restoration or rehabilitation works, both indoors and outdoors.

Another interesting application of lime stucco is in bathrooms. As it is a fully waterproof type of covering, it performs well in environments with high humidity levels.

4. LIME MORTARS

Lime mortar is a type of mortar generally composed of hydraulic lime—although air-lime mortars also exist—aggregates, and natural additives. It is the main application of lime in construction, with a long historical track record in the sector that attests to its excellent performance.

Its versatility makes it a highly competitive sustainable alternative, capable of matching today’s most widely marketed mortar: Portland cement. [7]

For this reason, although lime mortars in construction tend to be associated with rehabilitation and heritage works—which is one of their most common applications, using air lime—their use has many other applications as well.

It is used as a surface coating both indoors and outdoors—in base coats, plasters, or stuccos—as a binder for ETICS-type façades, masonry walls, brickwork, or even as a soil stabilizer for foundations in unstable clay soils. [8]

5. LIME CONCRETE

The revaluation of lime in construction, driven by today’s ecological awareness, also involves recovering ancient building systems with the intention of giving them a second life through a more contemporary lens. This is the case of argamasa, a structural building system used for centuries, with lime as the main building material. The updated version of argamasa could be considered lime concrete.

Lime concrete is presented as an alternative to Portland-cement concrete, which is so widespread today. It is composed of lime, sand, aggregates of varying gradings, and water.

Thanks to its many properties and performance characteristics, it is a building system capable of replacing cement-concrete systems in many applications. It offers excellent plasticity and workability, lower shrinkage, water-vapor permeability thanks to its porosity, water and fire resistance, among others.

Despite being presented as an innovative and competitive alternative, lime concrete is not yet regulated by the CTE (Spanish Building Technical Code). This is because it is a system with a long research path ahead, in order to adequately meet the requirements for strength, durability, safety, etc., set by current building regulations. [9]

MORE ABOUT LIME CONCRETE

It cannot be stated with certainty whether, in the future, lime concrete will be suitable for carrying large structural loads, as has been done with reinforced cement concrete over the last century. This doubt goes beyond questioning lime’s structural capacity; it is about rethinking the concrete structural system itself.

Portland-cement concrete, as with lime cement, shrinks once it has carbonated. Therefore, there is a general concern about reinforcement expansion that affects not only lime concrete but also Portland-cement concrete.

On the other hand, despite common beliefs, lime does not chemically react with reinforcing steel; that is, it is not corrosive to iron. The reason lime’s adaptability to reinforced concrete systems is questioned is due to lime’s porous nature. As a breathable biomaterial, lime promotes oxygen exchange, and this could influence early oxidation of the reinforcement.

However, the negative environmental impact of using steel reinforcement in reinforced concrete is reason enough to question this structural system once again. Removing reinforcement has become an objective for minimal-impact architecture. Instead, alternatives such as fiberglass, cellulose fibers, recycled animal fibers such as sheep’s wool, or even bamboo can be used.

To date, the main focus of lime concrete in construction is on slabs or floors. So far, slabs have been successfully built on all types of ground, both in new builds and in rehabilitation projects.

One of the final challenges for lime concrete is to become a substitute for asphalt—that is, to apply it in urban exterior pavements as a draining lime-concrete surface capable of combating the urban heat island effect.

ADVANTAGES OF USING LIME IN CONSTRUCTION

Five main advantages of using lime in construction can be listed:

1. BREATHABILITY

Lime in construction acts as a material that allows the building system to remain dry, preventing rising damp or condensation-related moisture.

The low breathability of Portland cement, unlike lime, causes salts to appear, generating a visible pathology that affects the health of the building.

2. HYGROSCOPICITY

It absorbs excess moisture from the indoor environment and releases it when conditions are dry. For this reason, lime in construction has humidity-regulating properties.

3. GOOD PERFORMANCE AGAINST WEATHERING

It performs well against weather phenomena, including moisture. Hydraulic lime, for example, does not lose strength in the presence of water; on the contrary, it hardens and improves its performance.

This resistance results in high durability. Proof of this is the many historic elements that have remained for centuries with minimal deterioration.

4. VOC-FREE

The use of lime in construction supports the creation of healthy environments free of volatile pollutants that are harmful to health.

As a natural-mineral biomaterial, it does not emit hydrocarbons into the air and therefore does not form toxic ozone, characteristic of synthetic paints.

Lime paints have a VOC-free composition. This not only prevents unpleasant odors, but also positively influences indoor air quality and therefore the comfort of people in the home.

5. POSITIVE ECOLOGICAL FOOTPRINT

As it is natural, ecological, and based on a closed cycle, lime represents a minimal-impact biomaterial, responsible toward the environment and our immediate surroundings.

In addition, lime is one of the biomaterials that emits the least carbon dioxide into the atmosphere. For this reason, its use has become one of the most recurring and competitive in minimal-impact architecture.

Another low-CO₂-emission material is clay. However, because it does not carbonate—in other words, it does not become stone-like—clay performs worse against moisture than lime.