The Effects of Freezing on Lime Mortar and Plaster

frosty morning in woods

Lime mortar and plaster can be severely impacted by weather and temperature changes. Most building materials should not be used in temperatures below 5°C. So what effect does cold winter weather have on lime building materials?

Lime mortar and plaster should not be used in conditions colder than 5°C. Generally, the lower the temperature, the slower the set time. It’s common to see lime mortars fail at higher temperatures but with strong winds. It’s important to consider the wind chill factor and the temperature. If temperatures are expected to drop below 5°C in a two-week period, work should not commence, or precautions should be taken.

The Importance of Temperature for Lime Building Materials

Lime mortars and plasters have been used for thousands of years. They have a rich history worldwide and remain a brilliant, versatile building material today.

Binders are the part of a mortar or plaster that allow it to set, harden and adhere to surfaces. The lime used to make lime mortars and plasters is a type of binder. Cement is another example of a binder. Before binders set, they are wet workable materials.

Because binders are wet when used, they can be affected by all weather conditions during the curing phase. Frost is a particular concern when building. It can cause the binders in mortar and plaster to fail. This often results in a weak, cracked, crumbly and flakey mortar or plaster. Sometimes, the material must be replaced with fresh non-frosted mortar or plaster.

Understanding Freezing and Thawing Cycles

A freeze-thaw cycle is a relatively simple process. Water freezes when temperatures drop below the freezing point, 0°C. When temperatures rise above 0°C, the frozen water will begin to melt (thawing). This is one freeze-thaw cycle. When weather conditions are just right, freeze-thaw cycles can happen daily. But why are freeze-thaw cycles an issue?

Unlike most other materials, water expands as it freezes. The amount water expands when frozen is usually around 9%. Whilst this doesn’t sound like much, it can wreak havoc on building materials.

Freshly used lime mortar or plaster is very susceptible to frost damage. If the materials haven’t fully carbonated or set, they will still contain a significant amount of moisture. As the temperature drops, this moisture expands. This expansion causes the material to break apart, significantly reducing the ability of the binder to harden.

Mortars or plasters that have been frosted will be significantly weaker than those that haven’t.

The Effects of Freezing on Lime Mortar and Plaster

Lime mortars and plasters that haven’t cured and have been exposed to frost will usually have some tell-tell signs. Usually, the material in question will form cracks and may have a flakey surface. In addition, the mortar or plaster could have completely failed, which would look blown out, or the plaster/render may have fallen off the wall or ceiling.

As the water expands and pushes apart the material from the inside, it can leave lots of small voids and cracks. When the thawing process starts, these new voids can fill up with water. The next freeze cycle will cause any waterlogged voids and cracks to expand further. Over time this process will continue, slowly breaking apart more and more of the lime and working itself deeper.

frost damage to bricks
An example of the damage frost can cause to building materials.

Tips to Prevent Frost Damage When Working With Lime in The Winter

It’s normally recommended that lime building work be suspended until you have a good 2-week period with no frost risk. I do understand that this isn’t always feasible. Here are some tips on working with lime in the winter months.

Cover the Lime with Hessian as Insulation

Hessian is an extremely common way of protecting lime year-round. In the winter, hessian can be hung in front of lime as a form of insulation. In the summer, damp hessian is hung in front of the lime to stop it from drying too quickly.

When using hessian or other sheeting, you mustn’t hang it directly on the mortar or plaster. You need to leave a large enough gap, so the sheet doesn’t brush against and damage the lime. Here’s a good hessian to protect lime in winter and summer projects.

Use a Stiffer Mix

Using a stiffer mix is a simple and cheap way to help avoid frost damage and improve set times. A stiffer mix will carbonate and gain harder faster than a softer one. Keep in mind that the material will probably be harder to use and, as a result, may slightly increase labour time.

Use a Hydraulic Lime to Speed up Set Times

Hydraulic limes set via a reaction with water. This means they set much faster than fat limes (lime putty). The more hydraulic the mortar, the faster and harder the set.

Remember, mortar should never be stronger than the masonry it’s used with. Whilst a faster and harder set may be good for working in frosty conditions, it may not be suitable for the masonry of the building. If you are looking into using hydraulic limes instead of fat lime, make sure you speak to an expert to ensure the right materials are being used.

Related article: What is Fat Lime?

Add a Pozzolan to The Mortar or Plaster

Pozzolans work effectively in the same way as hydraulic limes. They react with water to harden. Adding a pozzolan can increase curing times at the expense of breathability and softness. If you are considering using a pozzolan, make sure you speak to an expert, as it can be a fairly involved process.

How to Fix Frost Damage

Sometimes you just get caught out, and the lime ends up getting frosted. If this happens, and there is more frost on the way, insulate the lime as best you can with hessian, as detailed above. Also, do not remove the crusty layer of lime that will have formed. This layer can help insulate and protect lime deeper in the wall, making the repair job less intensive for you.

Once the frost risk has passed, you can use a mortar pick to rake off the crusty/flakey layer. Remove any lime that looks damaged in any way. Any lime which crumbles away should be removed too. Usually, the best course of action is to repoint frost-damaged lime, as it will likely lose most of its integrity.

If you have any questions about frost damage, please call your local lime specialist. Companies such as Limebase, Cornish Lime and Limestuff will be able to help you.