Self-Healing of Lime Mortars

Lime mortars have been a staple in construction for millennia, admired for their durability, flexibility, and ecological benefits. One of their remarkable properties stands out for its almost magical quality: self-healing. This phenomenon allows lime mortars to repair their own cracks and fissures, maintaining structural integrity and prolonging the lifespan of buildings. But how exactly does this process of self-healing lime mortars work? What is the science behind the phenomenon?

How does self-healing work?

Self-healing in lime mortars involves a three-step process: dissolution, transport, and re-precipitation of calcium compounds. Here’s a closer look at each stage:

Dissolution

When cracks and fissures form in lime mortars, they create pathways for water to enter. This water interacts with the calcium hydroxide (a key component of lime mortar), leading to its dissolution. This means that calcium ions (Ca²⁺) and hydroxide ions (OH⁻) are released into the water, making it a medium for transporting these dissolved compounds.

Transport

Water carries the dissolved calcium compounds through the mortar’s network of cracks and fissures. This transportation phase is crucial as it spreads the healing agents throughout the damaged areas.

Re-precipitation

Once the water carrying the dissolved calcium compounds reaches the exposed surfaces within the cracks, the final step of the process begins. Carbon dioxide from the air dissolves in the water, forming carbonic acid. This acid reacts with the calcium ions to form calcium carbonate. Calcium carbonate (CaCO₃), also known as calcite, is a solid that precipitates out of the solution and fills the cracks, effectively sealing them and restoring the mortar’s integrity.

‘A self-healing process can be defined as a spontaneous (or with small external trigger) in-situ repair of micro-damage occurring to materials as consequence of weathering or overloading processes. The self-healing process in lime mortar can be summarized as follows: (rain) water allows the dissolution of calcium bearing compounds and transport them, from a zone rich in binder, to voids and cracks eventually present in the mortar. In this way small cracks may be filled with re-crystallized calcium compounds, in a natural self-healing process. Self-healing of historical lime mortar is therefore a natural and spontaneous process, and not the result of special design features.

For the sake of clarity, it is necessary to distinguish the process of dissolution and re-precipitation of calcium compounds from the process of dissolution and re-crystallization of soluble salts. The first process interests little soluble calcium compounds and occurs therefore in materials with high moisture content, where water percolates in fissures and cracks; its effect is either self-healing, if re-precipitation occurs in the voids and crack in the material, or formation of encrustations, if re-precipitation occurs at the surface. The second process interests soluble salts: these can be dissolved even in the presence of a relatively low moisture content in the material and transported in solution through the pore system of the material mainly by capillary forces. The effect is the formation of efflorescences, if crystallization occurs at the surface, or crypto-florescences, if crystallization occurs in the pores of the material. Where efflorescences cause only an aesthetical damage, crypto-florescences may lead to high crystallization pressures in pores and possibly damage the material.’

Lubelli, Barbara & Nijland, T. & Hees, R.P.J.. (2011). Self-healing of lime based mortars: Microscopy observations on case studies. Heron. 56. (accessed at https://www.researchgate.net/publication/241886678_Self-healing_of_lime_based_mortars_Microscopy_observations_on_case_studies)

What are the benefits of self-healing lime mortars?

Lime mortars’ self-healing ability offers several significant advantages, especially in the context of modern construction and heritage conservation.

Buildings constructed with lime mortars can withstand the test of time much better than those using conventional cement-based mortars. The self-healing process ensures that minor cracks and fissures are automatically repaired, reducing the need for frequent maintenance and extending the structure’s lifespan.

Lime mortars are inherently more sustainable than their cement counterparts. The self-healing process reduces the need for repairs and replacements, leading to less material consumption over time. Additionally, lime mortars produce less CO₂ than cement, aligning with environmentally friendly building practices.

Many historical buildings were originally constructed using lime mortars. Preserving these structures while maintaining their authenticity is a significant challenge. The self-healing properties of lime mortars help conserve these heritage buildings by naturally repairing the damage caused by time and environmental factors, thereby preserving historical integrity.

EWI Pro’s Heritage Lime Range

The EWI Pro Lime Range is an ever-growing range of products which focuses on heritage restoration and sustainability. The latest product to launch into the range is a fine finish Lime Plaster.

Lime Plaster

EWI-749 Fine Finish Lime Plaster is a versatile product suitable for internal and external applications. It comes in a 20kg bag and is designed to provide a smooth, fine finish to walls and ceilings. Lime Plaster is perfect for refurbishing historical buildings where maintaining authenticity is crucial. It’s also suitable for modern constructions that aim to combine traditional materials with contemporary design.

• Breathability: Lime plaster is highly breathable, allowing moisture to evaporate naturally, which helps in preventing dampness and mould growth.
• Flexibility: Its natural elasticity makes it resistant to cracking, accommodating minor movements in the building structure.
• Eco-Friendly: Made from natural lime, this plaster has a lower carbon footprint than traditional cement-based plasters.
• Aesthetic Appeal: The fine finish provides an aesthetically pleasing surface, ideal for heritage restoration projects and modern builds.

Lime Render

EWI-747 Lime Render NHL 3.5 is a natural hydraulic lime render supplied in a 23kg bag. EWI-747 Lime Render is ideal for external walls, particularly in conservation areas and listed buildings where traditional aesthetics and materials are required. It also benefits new constructions that need durable and breathable exterior finishes.

• Durability: NHL 3.5 lime renders are known for their strength and longevity, making them ideal for external applications.
• Weather Resistance: This render offers excellent resistance to rain and frost, helping to protect the underlying structure.
• Permeability: It allows buildings to breathe, which is essential for the health of the building and its occupants.
• Compatibility: Suitable for use on many substrates, including stone, brick, and cob.

Lime Basecoat

EWI-748 Lime Basecoat NHL 5 is a high-strength basecoat available in a 21kg bag. It provides a solid foundation for subsequent layers of plaster or render and works as an adhesive for insulation boards. It suits renovation projects and new builds, ensuring a stable and long-lasting foundation. For Mineral Wool insulation purposes, we often advise a thin parge layer of Lime Basecoat to smooth out the fluffy surface of mineral wool boards.

• Strength: NHL 5 lime basecoats are extremely robust, providing excellent adhesion and a strong base for finishes.
• Breathability: Like other lime-based products, it ensures the structure can breathe, preventing moisture build-up.
• Compatibility: It can be used on various substrates, making it a versatile choice for different construction needs.
• Flexibility: It accommodates slight movements in the building, reducing the risk of cracking in the finish layers.