In today’s buildings, we often think that the stronger the mortar, the better. This is, in fact, not the case at all. Contrary to popular belief, building mortars which are too strong can have very negative consequences on the building they are used on. So what happens if the mortar is too strong?
Mortars should never be harder than the masonry they are used with. Mortar that is too strong can cause the faces of brick, masonry and stone to ‘blow out’. If they don’t blow out, you may notice increased weathering. In addition, any movement of the building due to weather, poor foundations or other circumstances will cause stress on the weakest link in the wall. If the mortar is too hard, this will cause the masonry to take the brunt of this flexing. Over time this can cause the masonry, stone or brick to crack and begin falling apart.
Now that we understand some of the damage that excessively strong mortar can do let’s dive a bit deeper.
Strong Mortar Can Damage Masonry
Portland cement mortars are extremely strong. They can have compressive strengths of over 50N/mm2, compared to the hardest lime mortars, which can reach compressive strengths of 5N/mm2. This is a vast difference, and it’s what makes cement such a good building material for high-rise projects. However, these extremely hard mortars can cause severe damage to masonry, especially in older buildings.
Mortar should always be weaker than the surrounding masonry. If it isn’t, for instance, when cement is used, the masonry will be weaker. As the building flexes and moves with the weather and other environmental factors, the weakest link takes the strain. Traditionally, soft and flexible lime mortars were used, which helped protect the masonry by taking the strain of any movement.
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When extremely hard mortars are used, this role is reversed. The mortar stays rigid, and because it’s harder than the surrounding masonry, the masonry ends up taking this strain. Over time, this can cause masonry to crack and split, allowing water ingress, which further helps the deterioration through mechanisms like freeze-thaw cycles.
Old buildings (usually constructed before the 1930s) must be repaired using softer mortars than the masonry. Old buildings utilised far softer masonry and materials than today and can be severely damaged by modern cement. It’s always easier to repoint damaged mortar than to rebuild a wall because the masonry has been damaged by a mortar that is far too strong.
Stronger Mortars Aren’t Usually Breathable
Breathability can play a significant role in the longevity of a building. Hard and waterproof mortars made with cement can damage masonry in many ways. Using a non-breathable mortar to replace an old breathable one is a recipe for disaster. This is mainly an issue for old buildings and historical renovations, but if you own an old building, making repairs using non-breathable materials can cause significant damage.
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Old buildings were constructed with breathability in mind, and lime mortars played a massive role. Lime mortars act like a wick to suck up and evaporate away any moisture that gets stuck within the fabric of the wall. When this moisture-controlling process gets blocked by replacing the lime mortar with cement, the moisture will get trapped. As temperatures drop and the trapped water freezes, the faces of the masonry can get blown off by the frozen water.
This has been a common issue on old buildings that were “repaired” with cement since WW2. You may have noticed brick faces that are cratered inwards or that are entirely missing. The strong cement mortars often stick out to where the brick used to be, which looks awful. Thankfully, people are more aware of the damage strong mortars can do and tend to choose more appropriate lime mortars to make repairs.
Too Much Binder Can Cause Poor Workability And High Shrinkage
A binder in construction is a material that sets, hardens and adheres to other materials causing them to bind together. In mortars, binders are usually cement, lime putty or hydraulic lime. If the binder-to-aggregate ratio becomes too high, it will not just cause the mortar to become too strong. Especially when talking about cement, too much will make a strong unworkable mortar.
Mortars with poor workability are much more challenging to use. They will slow down the overall construction rate and will likely worsen the quality of work. In addition, cement mortars which are too strong can cause excessive shrinkage. In some situations, this shrinkage will cause the mortars to pull away from the masonry, breaking the bond and allowing water ingress. A standard mix ratio for cement building mortar is 3:1 or 4:1.
Lime mortars don’t suffer from these complications. They are much softer than strong cement mortars. Lime is also far more workable and ‘plastic’ when compared to cement. Even hydraulic limes, which are slightly less workable than putty-based mortars, will far outperform cement for workability. Cement mortars can have such poor workability that, in some cases, it’s necessary to add lime to help improve them.
How To Make Sure The Masonry Is Stronger Than The Mortar
When purchasing new building materials, compressive strength is normally given for cement, bricks and other masonry products. This allows you to ensure that the mortar is weaker than the masonry, helping to improve the longevity of the building. This isn’t the case if you’re dealing with an older building that requires repairs such as repointing.
If this is the case, the building was most likely constructed with a lime mortar. As a general guideline, using lime putty (fat lime) mortars is usually the best bet. These are the softest lime mortars and will help give the building maximum protection. Suppose putty-based mortars are unsuitable because the repair area is excessively wet or exposed. In that case, a hydraulic lime may be needed for its ability to set in wet conditions.
NHL 2 is great for internal repairs where putty-based mortars would not set (in very damp conditions). NHL 2 is slightly stronger than lime putty, so it’s the next best option. If the area is moderately exposed or wet, NHL 3.5 should be used. This is the most commonly used hydraulic lime, as it strikes a good balance between strength, set speed, flexibility and breathability. Finally, in extremely exposed conditions (think chimneys or bridges), NHL 5 can be used. This is the strongest lime mortar, reaching compressive strengths of 5N/mm2, which is still a far cry from cements 40 odd N/mm2!
Related article: What are the Differences Between Lime and Cement?
If you are unsure what mortar was used previously in your building, it’s a great idea to get it tested along with the masonry. Once you get the results back, you will be able to ensure a suitable strength mortar is used. Many lime companies will provide this service. They can even match old lime mortars to help preserve the aesthetic of your building.