![]() ![]() ![]() Mortar should always be softer than the brick it is paired with. ![]() When the selected mortar is harder than the brick it surrounds then the brick will become sacrificial and worn away rather than the mortar. Lime Mortar vs Portland Cementįor those restoring a historic building built before 1930 it’s important to select the right mortar to avoid spalling bricks. 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. 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. 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 first manufacturer of portland cement in America was David Saylor in the Lehigh Valley, Pennsylvania in 1871. 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. 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 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. Lime mortar and other natural cements were used almost exclusively in masonry structures prior to the introduction of portland cement in the 1870s. ![]() Lime mortar is essentially self-healing, getting harder each day and constantly pulling CO2 out of the atmosphere (the original “green” building product!). 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. 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. The heat burns off the carbon dioxide in the rock leaving calcium oxide, commonly called quicklime. The slaked lime used to make lime mortar is created by cooking limestone rocks at 1,650☏. It is essentially composed of only three ingredients (lime, sand, water) which are in abundant supply around the world. Lime mortar has been around since biblical times. Feel free to reference my previous post How To: Repoint Historic Mortar for the details of how the process works. 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. 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. 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. 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. ![]()
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