Choosing the right mortar is crucial to restore or maintain your historical property in London. Many homeowners with Victorian and Edwardian buildings face the dilemma of whether to use lime mortar or cement mortar for repointing and restoration.

This decision is especially important for the longevity and health of your building’s masonry. In this article, we will talk about the differences between lime mortar and cement mortar.

I will try to highlight the key factors that make lime mortar the ideal choice for historic London properties.

Lime Mortar Explained

Lime mortar is one of the oldest building materials, used for thousands of years in the construction of ancient Roman buildings and beyond. It is made from limestone, which is heated and mixed with water to create slaked lime.

This mixture is then combined with sand or aggregates to form mortar. There are different types of lime mortar, each with its own unique properties:

Types of Lime Mortar

NHL 2: A softer, more flexible lime mortar, often used for general purposes.
NHL 3.5: The most common and versatile type, used for general masonry work.
NHL 5: A stronger lime mortar, ideal for exposed areas where additional durability is required.
Non-hydraulic Lime (Putty Lime): This is the most flexible type, often used in restoration projects for its superior elasticity.

How Lime Mortar Works

Lime mortar sets through a process known as carbonation, where the lime absorbs carbon dioxide (CO2) from the air.

Unlike cement, which sets through a chemical reaction with water, lime mortar’s setting process is slower, taking several days or even weeks to harden. Over time, lime mortar continues to cure and harden as it absorbs more CO2, a process that can take months.

Key Properties of Lime Mortar

Breathability: Lime mortar allows moisture vapor to pass through the walls, preventing damp and mold buildup.
Flexibility: Its elasticity enables it to accommodate the natural movement of buildings, which is essential for old properties that settle and shift.
Self-healing: Lime mortar can “heal” small cracks by absorbing CO2 from the air, making it a durable and sustainable option for historic buildings.

Cement Mortar Explained

Cement mortar, specifically Portland cement, was invented in 1824 by Joseph Aspdin. This revolutionary material quickly became the standard for modern construction due to its strength and durability.

Cement mortar is made by mixing Portland cement with water and aggregate.

How Cement Mortar Works

Unlike lime mortar, cement mortar sets through hydration, a chemical reaction between cement and water. This process happens quickly, allowing the mortar to harden in just a few hours.

However, it continues to gain strength for up to 28 days, reaching its full compressive strength within this period.

Key Properties of Cement Mortar

Strength: Cement mortar provides high compressive strength, making it ideal for modern structural applications.
Speed: It sets rapidly, allowing for quicker construction and repair work.
Hardness: Once cured, cement mortar is rigid and inflexible, offering a solid, durable surface. However, it can crack under building movement.

Key Differences: Lime vs Cement Mortar

Category	Lime Mortar	Cement Mortar
Setting Process	Sets through carbonation (absorbing CO2 from the air)	Sets through hydration (chemical reaction with water)
Setting Time	Takes days to weeks to set, with months required for full cure	Sets in hours, with full strength reached in 28 days
Strength & Hardness	Softer, more flexible with lower compressive strength	High compressive strength, hard, and rigid
Breathability	Highly breathable, allowing moisture vapor to pass through	Non-breathable, traps moisture inside walls
Flexibility	Accommodates movement, elastic, and can “heal” small cracks	Rigid and brittle, prone to cracking under building movement
Durability	Self-healing and sacrificial, lasting for centuries	Very durable, but can cause damage to masonry if used incorrectly
Environmental Impact	Absorbs CO2, low carbon footprint	High carbon emissions, energy-intensive production
Cost	More expensive, labor-intensive to apply	Cheaper, faster, and readily available
Appearance	Natural texture and color variations, softer aesthetic	Uniform, smooth finish
Building Compatibility	Ideal for soft brick and historic buildings (pre-1920s)	Best for modern construction with hard bricks

When to Use Lime Mortar

Lime mortar should be used when restoring or maintaining any pre-1920s buildings in London, particularly those made of soft brick such as London stock brick.

This includes Victorian, Edwardian, and Georgian properties. If your property is listed or located in a conservation area, lime mortar is often required by law due to its compatibility with historic materials.

Lime mortar is perfect for buildings experiencing damp issues. Its breathability allows moisture to escape, preventing condensation and long-term damage.

When repointing historic properties, lime mortar’s flexibility and self-healing properties make it the ideal choice for maintaining the structural integrity of the original brickwork.

When to Use Cement Mortar

Cement mortar is suitable for modern construction or post-1920s buildings. If your property is made from hard brick or engineered brick, cement may be the better choice due to its strength and ability to withstand high load-bearing requirements.

Cement mortar is also ideal for areas such as foundations, chimneys, or external exposed areas where durability is a priority and breathability is not a concern.

Problems with Using the Wrong Mortar

Using the wrong mortar for your historic building can lead to serious damage. For example, using cement mortar on Victorian properties can result in moisture being trapped inside the walls.

This can cause damp problems, including mould, efflorescence, and spalling—a process where moisture freezes inside the brick and causes the surface to pop off. Over time, this can cause irreversible damage to the original masonry and be very costly to repair.

In addition to structural damage, using the wrong mortar can violate local conservation regulations, especially if the property is listed or in a conservation area. This could lead to fines or complications with selling the property.

Lime Mortar Repointing in London: Why It Matters

For London homeowners with Victorian and Edwardian properties, lime repointing is an essential part of building restoration.

Many historic properties were repointed with cement mortar during the 1960s-1980s, which has now caused significant issues such as spalling and damp problems.

Lime repointing is the best solution for restoring these properties, as it is compatible with the original materials and helps to preserve the structure’s breathability.

Using a professional lime repointing service ensures that the mortar applied matches the original composition and strength of the original material.

As heritage specialist, StoneMasonry London team understands the specific requirements for London’s listed buildings and conservation areas, and can help you achieve a period-appropriate restoration that maintains your property’s value and structural integrity.

Contact us for a free survey and expert advice on how lime repointing can protect your investment.

Conclusion

Choosing the right mortar is crucial for the long-term health of your property.

For Victorian, Edwardian, and other period properties in London, lime mortar is the clear choice, offering breathability, flexibility, and self-healingproperties that cement simply cannot provide.

If you’re unsure which mortar is right for your project, contact us today for a free property survey and expert advice