If you’ve got a masonry building built before the 1930s there is a good a chance you’ve got lime mortar rather than portland cement mortar, and if it was built before the 1880’s then it’s almost certain to be lime. But why does that matter?
The argument on whether to use lime mortar vs portland cement is actually a very important one and using the wrong mortar can cause irreparable damage to historic brick. In this post, I’ll explain the differences between the two, how to determine which you have, and even where to source matching mortar for your old house.
Once you know the difference between lime mortar and portland cement you can undertake the work of repointing or repairing damaged historic masonry feeling confident you’re using the right mix of materials and techniques. Feel free to reference my previous post How To: Repoint Historic Mortar for the details of how the process works.
History of Lime Mortar
Lime mortar has been around since biblical times. It is essentially composed of only three ingredients (lime, sand, water) which are in abundant supply around the world. The slaked lime used to make lime mortar is created by cooking limestone rocks at 1,650°F. The heat burns off the carbon dioxide in the rock leaving calcium oxide, commonly called quicklime.
The powdery quicklime would then be submerged in water for weeks or months to create a lime putty called “slaked” lime that would be mixed with sand (or other aggregates) and water to make lime mortar. Once the lime mortar is exposed to the air it pulls in carbon dioxide and releases water as it endeavors to return to its original state of limestone.
Lime mortar is essentially self-healing, getting harder each day and constantly pulling CO2 out of the atmosphere (the original “green” building product!). Lime mortar and other natural cements were used almost exclusively in masonry structures prior to the introduction of portland cement in the 1870s.
History of Portland Cement
Portland cement was Invented in 1824 by Joseph Aspdin by mixing calcined hard limestone with clay and mixing it down into a slurry before heating it a second time. It got its name because it had a similar color to a widely used stone in the Isle of Portland off the coast of England.
Portland cement was able to attain very quick setting times compared to lime, but its strengths were fairly limited compared to natural cements and it didn’t catch on for about 50 years. The first manufacturer of portland cement in America was David Saylor in the Lehigh Valley, Pennsylvania in 1871.
Portland cement came into rapid growth from 1871 to 1920 when its quick initial strength (though it had lower long term strength than natural cements) made it ideal in the rapid growth of America during the Industrial Revolution.
The thought was that stronger mortar is better (not always the case) and with that portland cement was king because of both its quick set time and high strength. It very quickly became a favored additive to residential and commercial lime mortars to attain a faster and higher compressive strength and eventually phased out the use of lime mortar almost entirely by the mid 20th-century.
Lime Mortar vs Portland Cement
For those restoring a historic building built before 1930 it’s important to select the right mortar to avoid spalling bricks. When the selected mortar is harder than the brick it surrounds then the brick will become sacrificial and worn away rather than the mortar. A sign of impending disaster.
Mortar should always be softer than the brick it is paired with.
The more portland cement is added to any mortar the harder it gets, and the harder it gets the greater potential you have to damage brick. In today’s home stores lime is largely missing from all mortars. The variety of strengths available today is mostly accomplished by other additives and air-entrenching in the mortar. You will find mortar available in the following types:
- Type M 2,500 psi
- Type S 1,800 psi
- Type N 750 psi
- Type O 350 psi
- *Type K 75 psi
*Type K is largely not available today since that is true lime mortar, but the other types are available at most locations or for order.
But why does this matter for old houses? Well, as mortars got harder through the years so did bricks. As kiln technology improved we could cook bricks hotter and more consistently than in previous years. A brick from the mid-1800s can be extremely soft compared to one from the mid-1900s and the appropriate mortar has to be selected to pair with the appropriate brick.
What Should You Choose?
If your house was built before 1880 then you likely have traditional lime mortar and should use only that. If your house was built after 1930 you likely have only portland cement mortar and can pick up the right mortar at your local Home Depot. That was easy! But what about the rest of us in the transition years between 1880 and 1930?
It’s not quite as easy for us, but there is a simple way to tell what type of mortar you should be using. Take your house key out and scrape it across the mortar joint in question. If the mortar scrapes away and you could dig it out without turning your key into a nub then you likely have lime mortar or at least a mortar with higher lime content than portland cement.
If the key leaves a mark but doesn’t do any damage then you’re in the portland cement club. Congratulations, you’ve just diagnosed your mortar in the least scientific, but most convenient way possible! If you are a more particular person (you know who you are Mr. Color-Coded Socks!) then you can send a sample of your mortar off to a lab like Limeworks.us for a Historic Mortar Analysis. And Limeworks can even make a batch of mortar to match your sample in color and strength exactly!
Take good care of your brick and stone by choosing the right mortar when you need to make repairs and your historic masonry will be protected for centuries to come, use the wrong mortar and in only a few years you may end up with disintegrating brick that is extremely difficult to replace.
As it always goes with old houses it’s about proper methods and materials. I’d encourage you to check out my friends at Limeworks. They are an invaluable resource selling lime mortars, cleaning products, masonry tools and everything you need to restore or repair historic masonry. Good luck and happy mortaring!