November 10, 2015
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Cement has been a prominent force in building and construction for the last 150 years. And since the first half of the last century, beyond its use in concrete production, it also started to dominate as a brick mortar, plaster, screed and grout.
The strength and ease-of-use of cement make it a perfect product for using in the construction of new brick buildings.
However, when it is used in buildings that have stood for 100s of years, it can lead to their destruction.
Its strength, hardness and impermeability – the qualities that make it so suited to binding new brickwork together – are the same qualities that give it a destructive force when applied to old buildings.
The thing with old buildings is that they are often made from stone, not brick. Stone is a lot softer and far more porous than modern brick. Old bricks dating back 70 years or more are also softer than new brick.
So if the old brick or stonework that makes up your house has been pointed with hard cement, suddenly your bricks have become the weak link in your walls and are destined for destruction.
How Does Mortar Work?
Before the advent of cement mortar, builders and craftsmen used to use the softer, more malleable lime mortar.
Historically, as well as holding a wall together, lime mortar shifted and moved with a building as it settled over the decades.
Also, during the normal wet and dry cycle of the seasons, the bricks and lime mortar would soak up the rain and then allow it to evaporate out through the mortar and the brickwork. As lime mortar is softer and more permeable than old brick and stone, the water would move from the stone into the lime mortar.
During the winter, when water froze in a wall and expanded, the expansion would be pushed out into the weak link: the lime mortar.
All of this action on the mortar meant that it took the brunt of the destructive forces due to movement in the building, water erosion and freeze-thaw action. Over the course of perhaps 15 to 30 years, the lime mortar will have retreated right back into the brickwork through natural damage to it. It is at this point that repointing of the brickwork is required.
The brick or stone wall itself will be in top condition, as it is the strongest point and has handed all the stress over to the mortar.
Enter Cement Mortar
And then cement mortar entered the scene. It’s not so bad when you’re using it to glue together new, hard brickwork, but use it to repoint stone or old brick at your peril.
The problem here is that suddenly the brickwork becomes the weak point and as the house moves, the brick cracks.
As water erosion takes place, it’s the old brickwork that acts as the conduit for the water, with tiny particles of brick migrating from the wall. Over a period of time this adds up to some serious damage.
During the freeze-thaw cycle, the hard cement pushes expanding ice into the brickwork, causing micro-fissures and cracks that can destroy the integrity of individual bricks.
Couple this with the fact that cement mortar can be waterproof and you end up with water pooling in your walls around the mortar, in the brickwork. The wall never truly dries out all that extra water in the brick means the freeze-thaw action really starts to attack the brickwork.
The very qualities that cement has, means that it will last decades longer than lime mortar. And that’s why it was originally so popular. It’s a solid solution that will stand the test of time. Unfortunately, if it’s not compatible with the brick, that can be its downfall too.
Many general tradesmen do not know the damage that cement mortar can do to old buildings.
Make Sure It’s Always Lime
So if you’re living in an old house, check your mortar right now! If it’s made from cement then you need to take action. It likely all needs to be raked out and replaced with lime-based mortar.
Cement mortar won’t destroy your walls overnight, but it will slowly crumble your bricks over a period of years.
Check the current condition of your brickwork and make the call as to when you decide to replace, but the sooner you do it the less brick repair will be needed.
If your existing mortar is retreating into the brick, make sure you have it repointed with lime mortar and your building will stay standing for many hundreds of years to come.
Here in our cottage, the previous owners have had the lovely stone walls repointed with cement. Raking out and replacing with lime mortar is definitely on our list of restoration and repair work. The architect that carried out the superstructure survey prior to us buying, said that we could leave the cement mortar in for up to 5 years, but there will be a lot more damage to the stone and further repairs will be required. Ideally it needs to be removed as soon as possible.
Another thing to bear in mind is that lime mortar takes a lot longer to dry out and harden than does cement mortar. It is also a lot more susceptible to weathering when it’s bedding in, so lime mortar should ideally be applied in early summer, to allow it maximum drying time before the bad weather sets in.
Types Of Lime Mortar
Making quick lime by heating limestone and chalk in kilns and using it to prepare mortars is an ancient process and can be found in the pyramids, Hadrian’s Wall and the Coliseum in Rome.
Historically, lime mortars either set by simply absorbing carbon dioxide from the air or have a hydraulic set, which can occur even in wet conditions. Both of these categories of lime mortar are still available today.
The type of lime that you need to use will depend on how much the building is exposed to bad weather conditions.
Lime Putty
The weakest lime mortars are made from lime putty. They are highly breathable and have huge flexibility. However, they are easily damaged by poor weather, particularly in the first few months after their application on the building.
Hydraulic Lime
Hydraulic lime mortars give much more resistance to bad weather and will last far longer than lime putty. They aren’t as breathable and flexible as lime putty, but are still far more yielding than cement.Any b
uilding that is just a single leaf of brick or stonework thick should be re-pointed or built with a hydraulic lime mortar.
In a future post I will go into the detail of how to go about re-pointing a wall.
Did You Find This Post Useful?
Have you had experience of working with lime mortars? Has your old building been re-pointed with cement mortar? Let me know how you’re getting on, as I’d love to hear from you. Please leave a comment below.
