Preparing Lime Mortar For Repointing Masonry

 Disclaimer

Part 1 - General

1.01 Summary

1. This standard includes guidance on preparing lime mortars for repointing masonry.
2. Lime mortars are preferable to Portland cement mortars for repointing historic masonry:
   1. Lime mortars are more permeable by water. Water passing through lime mortar will dissolve a small portion of the lime and then will deposit it in hairline cracks as the water evaporates.
   2. Lime mortars expand slightly during setting, and resists shrinkage which causes cracking.
   3. Lime mortars are more durable than generally recognized.
3. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
   1. Safety Precautions
   2. Historic Structures Precautions
   3. Submittals
   4. Quality Assurance
   5. Delivery, Storage and Handling
   6. Project/Site Conditions
   7. Sequencing and Scheduling
   8. General Protection (Surface and Surrounding)

These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from GSA Historic Preservation.

1.02 References

1. American Society for Testing and Materials
   http://www.astm.com/

1.03 Delivery, storage and handling

1. Storage and Protection: Lime and cement must be protected from rainwater and ground moisture, as water vapor in the air can begin the setting process. Other materials also should be protected from contamination.

Part 2 - Products

2.01 Materials

NOTE: The use of standard specifications for materials, such as those developed by the ASTM, provides an easily referenced level of quality.

1. Lime: Should conform to ASTM C207, Type S, high plasticity, Hydrated Lime for Masonry Purposes.
   1. Lime which meets this standard will “work” well, resists drying during curing, and is sufficiently strong for the purpose of repointing.
   2. Lime expands as it hydrates, making high lime mortars more resistant to crack formation.
2. Cement: Should conform to ASTM C150, Type I, White. It should not have more than 0.60% alkali nor more than 0.15% water soluble alkali. Use gray Portland cement ONLY if a dark mortar is to be matched.
   1. Cement meeting this standard should increase the workability of the mortar, accelerate the setting time and slightly increase the strength of the mortar.
   2. The low alkali content will prevent efflorescence.
3. Sand: Free of impurities and conforming to ASTM C144.
   1. Sand color, size, and texture should match the original as closely as possible. Provide a sample of the sand for comparison to the original, and have it approved by GAS Historic Preservation before beginning repointing work.
   2. When possible, use bar sand or beach sand rather than crushed sand for the repointing mortar.

Note: Bar sand or beach sand should be washed to remove the salts before using.

1. Crushed sand has sharp edges, which makes it more “sticky” and difficult to work into the joints.
2. Bar sand, on the other hand, has rounded edges and flows easily during the mortar application.
3. The working characteristics of mortar made with crushed sand may be improved by adding a slight amount of Portland cement. The amount of cement should be determined by experimentation, but should not exceed 20% of the total lime/cement binder. 20% OR LESS OF CEMENT HAS MINIMAL EFFECT ON THE HARDNESS OF THE MORTAR. CEMENT CONTENT ABOVE 20% WILL MAKE THE MORTAR TOO HARD.

NOTE: Do not use anti-freeze compounds. These compounds are designed for use with cement mortars, and their effectiveness with high lime mortars is questionable. Furthermore, the compounds contain salts which can lead to serious problems in the masonry at a later time.

NOTE: Air entraining agents are not recommended. These agents are designed for use with cement rather than lime, and they result in decreased bonding of the mortar and the masonry. Air entraining is not necessary with high lime mortars because of the natural ability of these mortars to flex with temperature changes.

2.02 Equipment

1. Surface temperature thermometer - can be either mechanical (less expensive but must be calibrated often) or digital electronic
2. Wooden mortar boxes
3. Hoe
4. Mesh screen
5. Hawks: Plywood or steel hawk (mortar board)

2.03 Mixes

1. Some factors to consider when mixing lime mortar include durability, color and texture, and workability.
   1. Durability: Repointing mortar should be softer than the masonry units and the original mortar to reduce stresses at the edge of the masonry and, in the case of lime mortar, to reduce shrinkage which can cause cracks in the mortar.
      1. If the new mortar is harder than the masonry or the original mortar, it can cause serious stresses within the wall during thermal expansion and contraction, which can lead to deterioration of the masonry units rather than the mortar.
      2. If the mortar is softer, any deterioration which occurs will take place in the mortar, which is easier to replace than the units themselves.
   2. The repointing mortar should allow the passage of water, both liquid and vapor. If the mortar does not allow water to pass freely through it, the water can become trapped inside the wall, freeze and cause serious deterioration to the masonry.
   3. Color and texture: The repointing mortar should match the original mortar in color, texture and physical characteristics.
      1. Obtaining an accurate color match is best achieved by selecting an appropriate sand.
         1. Use sand which is similar to the original in color and gradation. Sand from more than one source may be required.
         2. For repointing of natural stones, use finely ground stone “dust” in the mortar to match the joints as closely as possible to the stone.

      2. If the original mortar was tinted, or if it is impossible to obtain a color match through the use of sand, it may be necessary to use a special mortar pigment.

         Caution: Pigments may react with other ingredients in the mortar to form efflorescence. They may also weather at a different rate than natural coloring and cause a color variation in the mortar.
         
         Note: If pigments must be used, pure mineral oxides should be used because they do not fade or leach out of the mortar. Amount of pigment should not exceed 2% of the mortar mix by weight.

      3. Many mortars used before the twentieth century have small lumps of incompletely burned or ground lime, or other impurities. To match the original appearance of the masonry, these impurities must be included in the new repointing mortar. Use identical materials, such as ground oyster shells (obtained at feed stores) or lumps of lime, to duplicate original lumps.
   4. Workability: The workability or plasticity of the mortar is a direct result of the selection of materials.
2. Mortar mix:
   1. Have the existing mortar completely analyzed to insure that the repointing mortar will not be less permeable/harder than the masonry units or the original mortar. It is better to have mortar that is more permeable than less.
   2. Measure all ingredients by cubic volume using a pre-established uniform measure, such as a small bucket, rather than a less uniform measure such as a shovel.

   3. For historic masonry set in lime mortar, use the following mortar mix:

      1 part portland cement 3 parts lime
      8-12 parts sand (To match existing mortar as closely as possible.)
      
      Note: The exact mix required will relate to the grain size and sharpness of the sand and will vary depending on the supply.
      
      -OR-
      
      For historic masonry set in standard mortar, use the following mortar mix (ASTM C270 Type “0”) as a starting point:
      
      1 part portland cement
      2 parts lime or lime putty
      6 to 9 parts sand and stone dust (To match existing mortar as closely as possible.)
      
      -OR-
      
      For Limestone (ASTM C270 Type “N”):
      
      1 part portland cement
      1 parts lime
      4-6 parts aggregate
      Enough water to form a workable consistency
      
      -OR-
      
      For Granite (ASTM C270 Type “S”):
      
      2 parts portland cement
      1 part lime
      7-9 parts aggregate
      Enough water to form a workable consistency
      
      Note: For deteriorated granite or granite walls indicating movement, use ASTM c270 type “n” as listed above for limestone.

   4. Mix a final “job-size” batch once the correct sand color, cement content, etc. have been determined through small tests to ensure the on-site mixing conditions will result in the same final product.

Part 3 - Execution

3.01 Erection, installation, application

1. Mix Hydrated Lime:
   1. Add dry bagged hydrated lime to water. Stir and hoe the mass to form a thick cream.
   2. Allow to stand at least 24 hours before use.
2. Prepare Roughage Premix (for later use):
   1. Accurately proportion the sand and lime using measuring boxes constructed to contain the exact volume of each ingredient required to make on batch.

   2. Mix sand and lime thoroughly for about ten minutes. Store in plastic-lined drums and seal until required.

      Note: This compound may be stored indefinitely if kept sealed from air and kept from freezing.

   3. When required for use, add and mix the correct portion of gauging cement as specified and use immediately. ACCURATE PORTIONING IS VERY IMPORTANT.
3. Add cements to lime and aggregate mixes immediately before the use of the mortar.
   1. Perform all batching with wooden boxes or plastic pails of known volume to ensure standardization and conformity of measurement; SHOVEL MEASUREMENT OF MATERIALS IS NOT PERMITTED.

   2. Use box sizes that are sufficient for producing a batch size equal to one mixer load.

      Note: Mix dry ingredients thoroughly before adding any water (approximately five minutes).

4. Add a small amount of water so that the mortar is just wet enough to hang on a trowel.

   Note: Excess water will cause shrinkage and too little water will retard carbonation. Record the amount of water added so that it may be used as a guide for future batches.

5. Mix mortars at least 10 minutes before using to improve workability and ensure thorough mixing.

   Note: Automatic mixers should have rubber blades. Clean mixing boards and mixing machines thoroughly after each use to prevent hardened lumps of mortar from contaminating the next batch of mortar.

   1. Repointing mortars may sit 1-2 hours after initial mixing and then may be remixed to a workable consistency. This is done to reduce shrinkage.
   2. Test the mix by holding a trowel with mortar on it upside down and shaking it once.
      1. If the mortar falls off without shaking, it has too much sand.
      2. If more than one shake is required, the mortar is too sticky or “plastic” and the lime content must be decreased.
6. Coloring mortars:

   1. Take samples of freshly-broken mortar from the original masonry pointing. Note color of aggregate for color-matching. Do not try to match the color of the binder.

      Note: use unweathered, unsoiled samples only.

   2. Prepare test patties of mortar approximating the inner color of the sample and set aside to dry for at least 72 hours. Drying time may be accelerated by placing the patty sample in an oven or over a hot-plate.
   3. Break the sample test patties and compare the inner portions to the original.
   4. See Section 2.03 above for additional information on coloring mortars.

7. Use repointing mortar within approximately 1-2 hours of final mixing. Retemper the mortar as necessary to maintain workability.

   Note: Re-tempering is permitted to maintain workability. Remixing is not permitted. Add water at the mortar-board using a spray bottle to replace only water lost through evaporation.

   Note: use all mortar within two hours of gauging; throw out left over mortar; do not re-temper or remix mortars after this time has elapsed.

   Note: this time limit may vary depending upon the outside temperature (longer on cooler days and shorter on warmer days).

8. For guidance on repointing, see 04520-02-R.

Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.

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We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.