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The Lime Mortar Guide

Lime mortar was developed by ancient Egyptians and used extensively in building the Roman Empire, it is steeped in rich history.

What is Lime Mortar?

It is a traditional building material that was used predominantly until the turn of the 20th century. In light of common misconceptions, it is just as important to state what a lime mortar isn't.

The emphasis is on the traditional element and the avoidance of modern ingredients which lime mortars pre-date. It was the adoption of modern cement that marked the decline of lime mortars in general construction.

Why Use Lime Mortar?

There are various reasons you might want to use a lime mortar.

  • Building Conservation
    Legislation stipulates the use of lime mortar for listed buildings and buildings in conservation areas.
  • Old Property
    Many heritage homeowners choose to repair using original material and preserve period details.
  • Prevent Damp
    Lime mortar is breathable, it can prevent and alleviate damp problems, particular in old buildings with solid walls.
  • Structural Movement
    Lime mortar is relatively flexible, accommodating a greater degree of movement.
  • Environmental
    The carbon footprint of lime products are much lower than cement which is responsible for up to 10% of global CO² emissions.

It is well accepted that lime mortars are superior to modern cement mortars offering many significant advantages.

Lime Mortar Composition

A lime mortar is made using traditional lime, sand and water. Traditional lime can be grouped into two broad categories; air lime (non-hydraulic limes, those that set when exposed to air) and water lime (hydraulic limes, those that set when immersed in water).

These traditional limes harden sufficiently on their own, they can be used to produce mortar. All were used historically as each have their own characteristics, advantages and disadvantages making them suitable for different applications.

Types of Lime Mortar

There are a few types of lime mortar available.

Hydraulic Lime Mortar (Water Lime, Hydraulic)

Made with Natural Hydraulic Lime (NHL). A good balance of strength and flexibility, available in a range of strengths to suit different applications. It sets in damp conditions and quickly reaches a strength preventing frost damage. At the same time, it remains softer and sympathetic to the masonry when fully cured. The material is available in powdered form, it feels similar to a modern cement mortar to mix and use, it is also the most forgiving of the inexperienced.

Non-Hydraulic Lime Mortar (Air Lime, Non-Hydraulic)

Made with non-hydraulic lime, commonly known as lime putty. This mortar is extremely soft and flexible, perfect for conservation of delicate masonry. It carbonates very slowly but this leaves it vulnerable to frost damage in cold conditions, it is not suited to damp conditions where it may not carbonate at all. The material is available as a wet, pre-mixed mortar with a shelf life that is practically indefinite if stored correctly.

Hot Mixed Lime Mortar (Air Lime, Non-Hydraulic)

Made with quicklime, a lot of heat is produced during mixing, hence the name. This sticky, lime rich mortar is a favourite with building conservationists as hot mixing was a popular technique. However, like non-hydraulic lime mortar it has a low resistance to frost. The material is available as a wet, pre-mixed mortar although it is typically served cold and matured.

Pozzolans

A pozzolan can be added to an "air lime" mortar to induce hydraulic setting characteristics. An initial hydraulic set drives early strength and can introduce some frost resistance in non-hydraulic and hot-mixed lime mortars. Pozzolans are natural materials that come in various forms, most commonly:

N.B. Although pozzolanic additives can address vulnerability concerns with non-hydraulic mortars; there is a compelling argument for using hydraulic lime mortar in the first place (providing it contains high quality Natural Hydraulic Lime).

The properties of Natural Hydraulic Lime (NHL) are documented and tested; manufactured under controlled factory conditions. In contrast, pozzolana is added to mortar on-site at the time of use (prior addition would cause it to harden too soon). This produces a mortar with unknown properties and potentially excessive strength.

Choosing a Lime Mortar

Mortar, by design, is a sacrificial element of the structure. As well as binding masonry units together, a lime mortar introduces a flexible, breathable layer that absorbs movement and expels moisture. This supports and protects the masonry, being much easier and cheaper to replace.

There are a few principles that are balanced to specify a type of lime mortar, there isn't always a single correct answer.

Compatibility

The mortar should be softer than the surrounding masonry or sympathetic as sometimes described. A hard mortar would abrade and damage the surface of the masonry.

This is the primary consideration. The key is in understanding the hardness of both the mortar and the masonry. The lime mortar compatibility table can help; it shows most building materials in relation to compatible mortars.

Durability

The mortar should also be as strong and durable as possible (while still softer than the masonry). This will reduce maintenance and prolong the life of the mortar, an important factor and a worthy pursuit.

This is often determined from the weather conditions the mortar will be exposed to. BS 8104:1992 is a code of practice to estimate exposure to wind-driven rain throughout the United Kingdom. This simple map of masonry exposure shows an overview and suitable lime mortars.

Planning and Conservation

It should go without saying, if a conservation officer is involved with the works - seek guidance and follow their advice. They have specialist experience and first-hand knowledge of the project, it's their decision that counts.

Best practice seeks to maximise compatibility. Like-for-like replacement is preferred wherever possible although traditional materials are now manufactured to modern standards and may differ from the originals - that's not to say the materials available today are inferior.

Lime Mortar Design

A lime mortar's functional and aesthetic properties are drawn from its ingredients. High-quality materials are therefore essential but the proportions also equally important, it is the correct mix ratio that ensures the whole is greater than the sum of its parts.

Standards

The Specification for masonry mortar, BS EN 998-2:2016 is a Eurpoean Standard that specifies requirements for masonry mortars for bedding, jointing and pointing. It applies to manufactured mortars but is naturally the go-to guide for anyone producing or specifying a mortar for general building.

Lime Binder

A traditional lime binder is required, it should comply with Building Lime, BS EN 459-1:2015. The principles outlined in the choosing a lime mortar section above can help ascertain the most suitable type of lime.

Aggregate

Fine aggregate, or sand, is available in a multitude of grades and colours and should comply with the BS EN 13139, Aggregates for mortar standard.

Well graded sands are preferred, the different-sized particles interlock to create a sound structure. The remaining voids are filled with the lime binder creating a porous micro-structure. The pores produce a poulticing effect and expel moisture from the masonry, a major benefit of lime mortar.

The specification requires clean, washed sands; dirty sands can contain excessive impurities such as clay, silt, loam or fines (particles that pass through a 0.063mm sieve). These impurities absorb more water thus increasing the likelihood of shrinkage or dynamic cracking after placement.

Mix Ratios

There is a myth that a lime mortar mix ratio can be prescribed, drawn from experience and industry wisdom. It simply doesn't work that way. The popular recommendations of 2:1 or 3:1 ratios for bricklaying or repointing may not work well, it all depends on the sand you're using and varies from quarry to quarry.

A mortar mix ratio is directly related to the percentage of voids in the sand, this is established by void testing. The void percentage determines the amount of lime binder that can be safely introduced, excessive binder (over-binding) can produce a heave effect and cause cracking, insufficient binder (under-binding) will impair the bond strength. Finally, with the optimum proportion of lime binder calculated, these percentages are converted to a mix ratio.

The British Standard BS 812-2:1995, Testing aggregates describes the methods to determine voids and other properties of the sand such as water absorption, particle density and bulk density.

How to Mix Lime Mortar

Lime mortars can be mixed by hand or machine depending on the scale of the work and type of lime.

Hydraulic Lime Mortar

A dry powdered material, it is mixed in the same way as a modern cement mortar. This is a straightforward process well known to many, a variety of equipment can be used:

  • Bucket and Gauging Trowel
  • Board and Shovel
  • Trug, Drill and Paddle or Whisk Attachment
  • Drum Type Cement Mixer

The complete method is described step-by-step in the guide to mixing hydraulic lime mortar.

Non-Hydraulic Lime Mortar

Known in the trade as "knocking up", the lime putty and sand are literally forced together, similar to a dough. Traditionally, this was done by hand; kneading, beating, ramming and chopping the materials together with a batten. Today, heavy duty equipment is highly recommended, various forced action mixers are used depending on the volume you're mixing:

  • Dual Paddle Power Mixer
  • Roller Pan Mixer
  • Pugmill

Note: A standard drum type cement mixer with a drop action is not suitable for mixing a lime putty.

Non-hydraulic lime mortar sets very slowly by carbonation, that means it keeps almost indefinitely providing it is wrapped and sealed airtight. Although it will eventually harden, it can be "knocked up" repeatedly to return it to a workable state.

The savings on effort and equipment make ready mixed lime mortar an attractive proposition for anything other than large works, especially given its long shelf-life.

Consistency

A mortar’s consistency is varied by adding more or less water.

The ideal consistency is something a tradesman learns instinctively from experience over time. It may vary on personal preference but typically depends on the work at hand.

Lime Mortar for Pointing

The ideal pointing mortar is similar in consistency to a snowball. This can be handled easily, compacted well into the joint and tends to cause less staining on the surface of the masonry. A pointing mortar typically requires the addition of 8% to 10% water.

Lime Mortar for Bedding

A bedding mortar should be spreadable and contains more water than a pointing mortar. You can make allowance for the density of the masonry, for example, a bedding mortar for porous bricks should contain more moisture than a mortar to bed large granite blocks. A bedding mortar typically requires between 12% and 14% water.

Don't Add Too Much Water

Adding too much water will cause separation of the lime from the aggregate, it can also affect the colour of the mortar. The idea is to add just enough water to make the mortar workable, the masonry itself can be dampened to reduce the absorption of the moisture if you’re working with porous masonry.

Working with Lime Mortar

There are several dos and don'ts particular to lime mortars.

Preparation

Dampen the masonry surface before placing the mortar. This will prevent the masonry from absorbing the moisture from the mortar too quickly. The degree to which the masonry is dampened will depend on the porosity of the masonry and the temperature. The mortar will “go off” (stiffen) much more quickly on absorbent surfaces such as sandstone and limestone, especially on a hot day. This will take much longer on denser materials such as granite and slate and on cooler days.

Placement

While bedding masonry or pointing, excess mortar is cut leaving joints flush and unfinished. Adequate drying time is required for the mortar to "go off" (stiffen) before finishing takes place.

Finishing

The mortar finish is purely functional, not decorative. It can be applied when the mortar has "gone off" (stiffened). The surface is stippled with a soft bristled brush in order to:

  • Compact the joint
  • Increase surface porosity
  • Maximise vapour permeability

The stippling action is important; brushing up, down or side-to-side would not have the desired effect.

A soft coconut brush is ideal for stippling and superior to a churn brush that is sometimes suggested. A churn brush is generally very stiff, it is more likely to over compact the joint and remove excess particles from a mortar that was recently placed.

Although counter-intuitive, joints should not be finished with tools such as trowels, jointers and pointing irons. These tools are fine for placement but smoothing with these tools increases surface tension and density thus reducing vapour permeability.

Suitable Conditions

Lime mortars are subject to the same temperature limitations as cement mortars and should not be applied in temperatures approach freezing.

Do not apply mortar externally in temperatures falling below 5°C. The curing process depends upon the temperature; this takes much longer in winter and the performance of the mortar may be reduced.

Starting late on a morning (10am) and finishing early afternoon (3pm) is a general best practice working through winter months. A rise and fall site thermometer will be required on site to monitor actual temperatures.

Protection

The work is at risk until the mortar has adequate time to cure. Although often overlooked, protection is critical in the meantime:

  • Rain may wash out the mortar before it has adequate time to set.
  • Cold temperatures may cause the mortar to freeze.
  • Sunlight and strong winds both accelerate the drying process.
    This leads to colour alterations (bleaching) and cracking (shrinkage).

The work should be protected with a breathable membrane such as hessian sheeting. Protection is recommended for the following periods depending on the ambient temperature:

Ambient Temperature Protection (days)
>= 5° C 56
>= 10° C 28
>= 15° C 14

Wet Curing (Cherishing)

Wet curing or "cherishing", as it is known in the trade, is a technique that provides the ideal curing conditions for lime mortar. The hessian sheeting (or other breathable membrane) is sprayed regularly with water, this simple procedure increases humidity and controls moisture loss.

Maintaining a stable, high level of humidity aids carbonation (for air lime) and hydration for (water lime) which are optimal at 60% and 90% relative humidity respectively. It also prevents shrinkage (cracking) that occurs from rapid drying.

The conditions on site will determine requirements but sheeting is typically sprayed with a fine mist sprayer, anywhere from once daily to every 4 hours. This is recommended for the first 3 weeks although the first 7 days is most critical.

Important: Sheeting is sprayed in temperatures above 10°C with no risk of frost. Spraying in lower temperatures increases risk of frost damage significantly.

Setting and Curing

Lime mortars harden progressively until fully cured. Regardless of the type of lime mortar you're working with, the process can be broken down as follows to establish some practical guidelines:

  • Initial Set
    The material stiffens or "goes off", it is no longer easy to manipulate.
  • Technical Set
    The material becomes hard enough to resist a fingernail when pressed.
  • Cured
    The material is fully cured when it reaches its maximum strength.

The time it takes to reach any one of these milestones depends on the type of lime, temperature and humidity. The table below provides a general guide, site trials will determine the time more accurately in localised conditions.

  Initial Set Technical Set (days) Cured (days)
Hydraulic Lime Mortar Overnight 30 90
Non-Hydraulic Lime Mortar Overnight 30 365
Hot-Mixed Lime Mortar Overnight 30 365

Estimates for non-hydraulic and hot-mixed lime mortar assume warm, dry weather and may be significantly longer.

Colour

The colour of mortar continues to lighten until it fully cures, the change can be quite dramatic and surprising. In good weather, you will usually see the greatest change after 10 days.

Heritage Resources

The websites of the following governing bodies offer a rich source of heritage information and advice:

  • Historic England
    Historic England publishes an extensive range of expert advice and guidance to help you care for and protect historic places.
  • Historic Environment Scotland
    Historic Environment Scotland is the lead public body established to investigate, care for and promote Scotland’s historic environment.
  • Cadw
    Cadw champions the appreciation, protection and conservation of the historic environment of Wales.
  • DoE Northern Ireland
    Historic Monuments; Historic Buildings; Historic Parks and Gardens; Maritime Heritage; Industrial Heritage; Defence Heritage and Archaeology.