U.S. General Services Administration
Historic Preservation Technical Procedures
04100-03
PREPARING LIME MORTARS FOR REPOINTING MASONRY
PART 1---GENERAL
1.01 SUMMARY
A. This standard includes guidance on preparing lime mortars
for repointing masonry.
B. 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.
C. 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 the Regional Historic
Preservation Officer (RHPO).
1.02 REFERENCES
A. American Society for Testing and Materials (ASTM), 100
Barr Drive, West Conshohocken, PA 19428, (610) 832-9585
or FAX (610) 832-9555.
1.03 DELIVERY, STORAGE AND HANDLING
A. 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.
A. 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.
B. 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.
C. 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 the RHPO 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.
a. Crushed sand has sharp edges, which makes it
more "sticky" and difficult to work into the
joints.
b. Bar sand, on the other hand, has rounded edges
and flows easily during the mortar
application.
c. 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.
D. Clean, potable water: If the water must be transported
or stored in a container, the container must not impart
any chemicals to the water.
E. Stone dust finely ground from the same stone as that to
be repointed.
F. Additives: NO antifreeze compounds or other admixture
shall be used.
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
A. Surface temperature thermometer - can be either
mechanical (less expensive but must be calibrated often)
or digital electronic
B. Wooden mortar boxes
C. Hoe
D. Mesh screen
E. Hawks: Plywood or steel hawk (mortar board)
2.03 MIXES
A. 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.
a. 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.
b. 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.
a. 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.
b. 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.
c. 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.
B. 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
A. 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.
B. 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.
C. 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).
D. 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.
E. 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.
a. If the mortar falls off without shaking, it
has too much sand.
b. If more than one shake is required, the mortar
is too sticky or "plastic" and the lime
content must be decreased.
F. 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.
G. Use repointing mortar within approximately 1-2 hours of
final mixing. Retemper the mortar as necessary to
maintain workability.
NOTE: RETEMPERING 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).
H. For guidance on repointing, see 04520-02-R.
END OF SECTION