Lime Mortar Tips

Types of Lime Mortar

There are two types of lime mortar used for restoration work: non-hydraulic and hydraulic lime mortars.

For a more detailed discussion of the types of lime mortar, please read this article Matt Wolf recently wrote for Masonry Design magazine.

1. Hydraulic Lime

Hydraulic lime is chemically very similar to non-hydraulic lime but it contains impurities, called pozzollans, that allow it to set in two stages: the first set is when water is added, and the second occurs as the mortar is exposed to the atmosphere. The first set is important, since it allows the mortar to attain strength more quickly and to cure in wet conditions.

Hydraulic lime is available in two forms: Natural Hydraulic Lime (NHL) and Pozzollanic Hydraulic Lime (PHL).

Both are sold as dry bagged products. NHL mortar is produced using limestone that contains naturally occurring impurities (pozzollans) such as clay and other minerals which enable it to cure when water is present. The mortar is categorized by the level of naturally occurring impurities: Feebly Hydraulic (NHL 2.0), Moderately Hydraulic (NHL 3.5) and Eminently Hydraulic (NHL 5.0). The most popular NHL mortar in our climate for exterior work is NHL 3.5, whereas NHL 2.0 is generally used for interior plaster.

PHL also has hydraulic characteristics but it is a manufactured lime product and pozzollans are added in a controlled factory environment. PHL is available as either a Moderately Hydraulic (PHL 3.5) or Eminently Hydraulic (PHL 5.0). The most popular PHL for our Midwestern climate is PHL 5.0.

Both NHL and PHL mortars are flexible and breathable enough for historic restoration, and they are durable enough for the extreme conditions.

Natural Hydraulic Lime (NHL)
Natural Hydraulic Lime (NHL)
Pozzollanic Hydraulic Lime (PHL)
Pozzollanic Hydraulic Lime (PHL)

2. Non-Hydraulic Limes

In the past, masons would create non-hydraulic limes by slaking (adding water to) quicklime, or lump lime, to create a putty that would then be mixed with sand. This putty was often aged before it was used, and the resulting lime putty mortar is soft and very permeable, but it cures very slowly and doesn’t withstand extreme weather or significant exposure.

NOTE:“Lime putty” is often used as a generic term for lime mortar, but this is a misnomer; lime putty is in fact a specific type of non-hydraulic lime.

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Lime Putty
Lime Putty

Tips for Preparing Mortar Joints

  • Grind or chisel out mortar joints to a depth of 2-2.5 times the width of the joint or deeper until sound original mortar is located.
  • The minimum depth of thin joints is 3/4″.
  • Remove all mortar fins left by the grinder from the top and bottom bricks so pointing mortar obtains a direct bond to the brick.
  • Remove all dust and debris from joints to ensure sound bonding.
  • Perform the above steps without damaging the masonry units or building.
  • Pre-wet the wall with copious amounts of water. Setting up a lawn sprinkler on the masonry and letting it run for an hour or so is usually a very good way to ensure the wall is thoroughly wet. Simply misting the wall with a hand sprayer or Hudson sprayer is NOT sufficient.
  • Lime mortar is an extremely dry mix so you want to keep the walls from absorbing what little water is in the mix. If your walls are not thoroughly pre-wet your mortar will fail and/or the color will shift.
  • Protect the walls from high winds, direct sunlight and/or heat by tenting the area with dampened burlap attached at the wall on the top and draped over the scaffold. If the workers are comfortable and happy working then the lime mortar they are installing will be comfortable and happy.
  • Lime mortar does not require water to grow crystals, cure and get hard like cement based mortars. Limes get hard by absorbing carbon dioxide from the atmosphere. As such, you must add only just enough water to make the mortar workable.
  • Mortar can be mixed by hand or in a modern cement mixer. Mix the mortar for 5 minutes, allow to rest for three minutes and re-mix for another three minutes. USE A TIMER.
  • Add water slowly as the mixer is running to help control the amount added. It is very easy to add too much water.
  • The final consistency of the mortar should be that of brown sugar. To test for proper consistency you can do either of the following:
    • Grab a handful of mixed mortar and form it into a ball. Toss the ball into the air and let it land in your palm several times. The ball of mortar should just barely hold together without breaking apart but it should not leave very much (if any) residue on your skin.
    • Take a handful of mixed mortar and squeeze it in your palm. If the mortar readily oozes between your fingers you have mixed with too much water. If the mortar just starts to push between your fingers you have a good workable consistency.
  • Dry lime mixes are good for the integrity of the mortar itself and great for the contractor. The drier the mix the less mortar smears you will get on the building during the pointing process. Most lime mortar pointing projects can be completed with little or no washing afterwards which saves the contractor significant time and money.
  • Compact mortar into joints using back fillers. Never use grout bags or pointing guns which require too wet of a mix and segregate the paste from the aggregate.
  • Apply mortar in one lift.
  • Remember the more your compact the mortar the denser the joint will be thus reducing vapor permeability. Ultimately, do not over compact mortar joints; allow them to transmit water vapor and “breath”.
  • Have fun with different joint profiles. Historic mortar joints were rarely struck in a concave pattern. Lime joints look great in a “V” struck, weather struck or raked joint profile.
  • Remember lime mortar does not “cure” but rather carbonates over a long period of time. The longer you can damp “cure” lime mortar the more resilient your joints will be. Humidity and frequent misting deposit CO2 into the masonry that lime requires to get hard.
  • If at all possible protect your pointed walls with dampened burlap raised 1-2 inches away from the wall for a period of at least 3-5 days.
  • Keep the burlap damp by misting it with water periodically. Dampened burlap shades the wall keeping it cool and provides for a humid environment that lime mortar loves as it gains initial strength.
  • If you cannot drape burlap then gently mist the wall frequently to keep the wall damp.
  • A good rule of thumb for temperature is that if the mason is comfortable then the mortar is comfortable. In colder weather the temperature should be above 40 degrees F for a minimum of seven days, and mortar needs to be protected against wind and rain. In the summer mortar is best installed at temperatures of 85 degrees F and lower. If this is not possible then be sure to “follow the shadows” so that you are working in shadowed areas throughout the day. Damp burlap and regular misting are especially critical in hot temperatures to prevent flash curing. We do NOT recommend chemical admixtures of any kind.
  • We never recommend washing lime mortar joints with any type of acidic product.
  • If you need to remove mortar smears from masonry simply use a green scouring sponge and water. Most smears are easily removed with the method within 24-48 hours after pointing.
  • If you absolutely must clean lime pointing work with an acid use Vanatrol at a dilution of at least 12:1 with a dwell time of only a few seconds. This chemical should only be applied to a thoroughly pre-wetted wall.
  • We never recommend using film forming sealers on any masonry and rarely recommend using water repellants on lime mortar.
  • Cold walls like parapets or chimneys should be treated with water repellants.
  • Use only Siloxane based water repellents for treating lime based mortar.