Masonry Pore Size: The Impact on Capillary Action & Vapour Diffusion

In masonry construction and restoration, the choice between lime putty, natural hydraulic lime (NHL), and cement mortar significantly impacts moisture management and structural stability. Pore size is a critical factor that influences capillary action, vapour diffusion, and the overall performance of these materials. Additionally, understanding how water droplets in liquid form and water vapour interact with different pore sizes is essential for ensuring long-term building health. This article explores the scientific differences in pore structure between lime mortar, NHL, and cement, including their effects on water behaviour and structural integrity.

The Role of Pore Size in Masonry Mortars
Pore size refers to the tiny voids within mortar that allow water and water vapour to pass through. These pores can range from large, interconnected voids to small, tightly packed capillaries. The size and distribution of these pores influence how moisture behaves in the material:

Capillary Action is the movement of liquid water through a porous material due to surface tension and adhesive forces, mechanised by hydrogen bonds. Smaller pores enhance capillary action, allowing liquid water to rise more quickly through the material.

Vapour Diffusion is the movement of water vapour through the material, mechanised by vapour pressure and the syphon effect – both of which are influenced by evaporation and wind drying. Large networks of interconnected pores facilitate easier vapour diffusion, enabling moisture to escape from the masonry.

Water Droplet Size vs. Vapour Size: Liquid water droplets are much larger than water vapour molecules. A typical water droplet is about 0.1 to 1 millimetre (mm) in diameter, while water vapour molecules are around 0.0001 mm (0.1 micrometre) in size. This difference in size means that when mortar pores are small enough, they can block liquid water while still allowing water vapour to pass through.

Lime Mortar: High Vapour Permeability and Flexibility
Air lime mortars (non-hydraulic lime mortars) are made from lime putty and sand and cure through carbonation, forming calcium carbonate. Lime mortars are well-known for their high vapour permeability, slower capillary action, and flexibility, making them a preferred choice for historic structures.

Pore Size in Lime Mortar: Lime mortars have relatively large and well-connected pores, typically ranging from 0.1 to 10 micrometres (μm). The average pore size is around 1 to 5 μm. These larger pores allow for significant vapour permeability while still managing the movement of liquid water.

Capillary Action: Due to their larger pores, lime mortars exhibit slower capillary action, meaning that water rises more slowly through the material. This slower movement allows for more evaporation and reduces the risk of moisture accumulation.

Vapour Diffusion: The large, interconnected pores in lime mortars allow for excellent vapour diffusion, enabling water vapour to escape easily from the masonry. This breathability helps prevent moisture build-up and the associated issues of dampness and decay.

Interaction with Water Droplets: When lime mortar pores are small enough, they can block liquid water droplets while still allowing water vapour to pass through. This selective permeability helps manage moisture effectively without trapping it inside the wall.

Flexibility: Lime mortars are highly ‘flexible’, allowing them to accommodate natural building movements. In truth, the flexibility of lime is a misconception. It actually microfractures and reseals itself.

NHL Lime: Enhanced Vapour Diffusion and Reduced Capillary Action

Natural Hydraulic Lime (NHL) mortars offer a middle ground between air lime mortars and cement, providing strength through both carbonation and a hydraulic reaction. NHL mortars are particularly valued for their improved vapour diffusion and reduced capillary action compared to both air lime and cement mortars.

Pore Size in NHL Lime: NHL mortars have smaller pores than air lime mortars but still maintain a significant degree of permeability. Pore sizes in NHL mortars typically range from 0.01 to 5 μm, with average pore sizes around 0.5 to 2 μm, depending on the NHL grade (NHL 2, NHL 3.5, NHL 5).

Capillary Action: Due to their smaller pore sizes, NHL mortars exhibit reduced capillary action compared to lime mortars. Water rises less rapidly through NHL mortars, helping to minimise the risk of moisture-related issues while still allowing for some degree of evaporation.

Vapour Diffusion: Despite the smaller pores, NHL mortars allow for more vapour diffusion compared to cement mortars, making them an excellent choice for situations where breathability is still required but increased strength is necessary. The balance of these characteristics makes NHL mortars particularly useful in both new construction and restoration projects.

Interaction with Water Droplets: The pore structure in NHL mortars can limit the passage of liquid water droplets more effectively than air lime mortars while still allowing water vapour to diffuse through the material. This balance helps to manage moisture effectively without compromising the strength of the mortar.

Moderate Flexibility: NHL mortars are more rigid than lime mortars but still retain some ‘flexibility’, which allows them to handle minor structural movements.

Cement Mortar: Strength with Significant Trade-offs

Cement mortars are composed of Portland cement, sand, and water, and they set and harden through a hydraulic reaction. Cement mortars are known for their strength, but their dense pore structure and lack of flexibility can lead to significant moisture management issues and long-term structural problems.

Pore Size in Cement Mortar: Cement mortars have very small, tightly packed pores, typically ranging from 0.01 to 0.1 μm. These small pores restrict the movement of both liquid water and water vapour.

Capillary Action: The small pore size in cement mortars results in rapid capillary action, meaning water rises quickly through the material. However, due to the limited pore size, evaporation is hindered, and moisture often becomes trapped within the wall.

Vapour Diffusion: Cement mortars have very low vapour permeability. The dense structure limits vapour diffusion, leading to moisture buildup inside the masonry. This lack of breathability can cause significant issues, such as internal dampness and decay.

Interaction with Water Droplets: The small pores in cement mortars are often too small for liquid water to pass through efficiently, leading to trapped moisture. This trapped water can cause internal dampness and eventually lead to deterioration of the mortar and masonry.

Rigidity and Cracking: Cement mortars are highly rigid and cannot accommodate the natural movements of buildings over time. This lack of flexibility often leads to cracking. Over time, these cracks can expand into larger fissures, or “fishers,” which allow water to penetrate deeper into the masonry, exacerbating moisture problems and potentially leading to structural instability.

Implications for Building Health and Conservation
The differences in pore size and moisture management between lime mortar, NHL, and cement have significant implications for both the long-term health and structural integrity of masonry buildings:

Moisture Management: Lime mortars, with their larger pore sizes, offer excellent vapour diffusion and slower capillary action, making them ideal for managing moisture in historic buildings. NHL mortars strike a balance, providing reduced capillary action while still allowing for substantial vapour diffusion. Cement mortars, with their small pores, often trap moisture, leading to long-term damp issues and potential damage.

Structural Integrity: Lime mortars’ ‘flexibility’ allows them to accommodate natural building movements without cracking, helping to preserve the structural integrity of masonry over time. NHL mortars provide some ‘flexibility’ and strength, making them suitable for various applications. Cement mortars, by contrast, are highly rigid and prone to cracking, which can develop into larger fissures over time, compromising the masonry’s stability.

Compatibility with Historic Masonry: Lime mortars are more compatible with traditional masonry, which relies on breathability and flexibility. NHL mortars provide a good balance for historic structures needing added strength without sacrificing too much permeability. Cement mortars, however, can disrupt the moisture balance in historic buildings, leading to trapped moisture, accelerated decay, and structural issues.

Understanding the role of pore size in masonry mortars is essential for managing moisture, ensuring structural integrity, and maintaining long-term building health. Lime mortars, with their larger pores and ‘flexibility’, offer excellent moisture management and are well-suited for historic structures. NHL mortars, with their reduced capillary action and improved vapour diffusion, provide a balance of strength and breathability. Cement mortars, while strong, often struggle with moisture management and are prone to cracking over time