Techniques > Briefs > Repointing Mortar Joints > Execution of the Work >


• volumetric measurement
• mixing batches
• knocking up
• English Heritage: mortar pan mill
• French Academy in Rome, mortar pan mill
• modern paddle and drum type mixers
• color pigments

• National Lime Association
200 N. Glebe Road, Suite 800
Arlington, VA 22203

Using Lime Putty to Make Mortar

Mortar made with lime putty and sand, sometimes referred to as roughage or course (coarse???) stuff, should be measured by volume, and may require slightly different proportions from those used with hydrated lime. No additional water is usually needed to achieve a workable consistency because enough water is already contained in the putty.

Sand is proportioned first, followed by the lime putty, then mixed for five (5) minutes or until all the sand is thoroughly coated with the lime putty.

But mixing, in the familiar sense of turning over with a hoe, sometimes may not be sufficient if the best possible performance is to be obtained from a lime putty mortar. Although the old practice of chopping, beating and ramming the mortar has largely been forgotten, recent field work has confirmed that lime putty and sand rammed and beaten with a wooden mallet or ax handle, interspersed by chopping with a hoe, can significantly improve workability and performance. The intensity of this action increases the overall lime/sand contact and removes any surplus water by compacting the other ingredients. It may also be advantageous for larger projects to use a mortar pan mill for mixing. Mortar pan mills which have a long tradition in Europe produce a superior lime putty mortar not attainable with today's modern paddle and drum type mixers.

For larger repointing projects the lime putty and sand can be mixed together ahead of time and stored indefinitely, on or off site, which eliminates the need for piles of sand on the job site. This mixture, which resembles damp brown sugar, must be protected from the air in sealed containers with a wet piece of burlap over the top or sealed in a large plastic bag to prevent evaporation and premature carbonation. The lime putty and sand mixture can be recombined into a workable plastic state months later with no additional water.

If portland cement is specified in a lime putty and sand mortar — Type O (1:2:9) or Type K (1:3:11) — the portland cement should first be mixed into a slurry paste before adding it to the lime putty and sand. Not only will this ensure that the portland cement is evenly distributed throughout the mixture, but if dry portland cement is added to wet ingredients it tends to "ball up," jeopardizing dispersion. (Usually water must be added to the lime putty and sand anyway once the portland cement is introduced.)

Any color pigments should be added at this stage and mixed for a full five minutes. The mortar should be used within 30 minutes to 1-12 hours and it should not be retempered. Once portland cement has been added the mortar can no longer be stored.