Helpful Guidelines When Working with Lime Products

Understanding Water – suction and evaporation when working with lime – is crucial to the successful application of lime mortars, plasters and stuccoes and their durability. It is impossible to overstate the importance of working with as dry a mortar as possible while adequately dampening the substrate. This is the opposite of working with cement mortars. Lime sets or carbonates through absorption of CO2 in the air or dissolved in rainwater. Lime mortars do not have an hydraulic set from reaction with water like Portland cement. Cement-based finishes do not allow for adequate moisture movement (evaporation of both water vapor AND liquid water) necessary to keep porous walls dry.

Lime-sand only mortars are more plastic and better able to accommodate any settling or movement in the wall. This is unlike cements that, once set, do not adjust to changes around them. Lime stuccoes and limewashes are more breathable, and have better water shedding characteristics. Cement stucco is likely to crack under stress or movement, allowing water to infiltrate to the interior where it becomes trapped. Lime stucco adjusts to early movements because it doesn’t fully set immediately. The interior carbonates more slowly. Reseal any cracks that open as acidic rainwater enters those cracks. Drawing some of the remaining calcium hydroxide into the crack. Slightly acidic rainwater partially dissolves calcium carbonate along the edge of the crack temporarily creating calcium bicarbonate. It re-deposits it, toward the front of the crack as calcium carbonate again. This self-healing characteristic of lime is well described in the literature as “autogenous healing.”

It may seem that “stopping water” is a good idea. Many of our modern building materials act in conflict with the nature of water. Better to “think like water” and detail the building to keep water away with a “good hat and boots.” Design to direct water away from the walls with a good roof overhang and when building new, install non-absorbent foundations that extend up out of the ground. Consider that annual mulching will increase the grade near the building. Foundations should rise above the splash line of any yard or bedding materials.

The calcium proportion of  lime carbonates quickly. Only properly calcined and slaked, high-calcium lime, can be expected to rapidly set. Type S and other hydrated limes work well in conjunction with cement. They are not manufactured with the high surface area and high porosity that speed carbonation for a lime-sand only mortar.

Mortar Components

Sand. The quality of sand is of primary importance to achieving a high quality lime mortar. Sands’ job is to provide structural strength. Lime putty coats and binds the particles together.

  • Sand should be clean and free of silts or organic material. Wash sand containing silt until clear. (In cases where on-site sand is being used to match an original mortar, obtain sand by washing soil from the site and mix the mortar well in advance so that the lime has an opportunity to stabilize clays).
  • Sand should be as sharp or angular as possible. Sharper or more angular sands are preferable. Rounder sands are less structural because they do not lock together as well.
  • Sand should be of various size particles, including coarse grains and fine. A quick assessment of sieved sand would give a bell-curve graph. The majority of particles are in the mid-range size with the quantity of larger and smaller particles decreasing as you move to the particle size extremes. A good range of sizes allows the sand grains optimal packing, thereby reducing shrinkage and cracking. (See definitions below).

Void Space Ratio. Lime should coat the sand particles to bind everything together without excess lime pushing the particles apart and weakening the mortar. Use the void space test to determine the lime to sand ratio.  Different sands mean different lime:aggregate ratios.

  • Use alcohol (150 + proof ie: Everclear) for this quick test. Alcohol eliminates water that can skew readings. This test requires very dry sand for accuracy. Dry the sand in an oven at 200°F for half an hour.
  • Determine sand proportions by filling a beaker with clean and very dry sand to 100mL. Tap the container until the sand is densely compact, then slowly add pure alcohol until it wets the top of the sand.
  • Measure the amount of alcohol to assess the void space in the aggregate.
  • For example, if for 100 ml of sand, it took 30 ml of alcohol to wet the sand, you would have approximately a 1 lime to 3 sand ratio.

Particle Size Distribution of Aggregates.  Clean sand for building should be sharp or angular, not rounded, as these will pack together more tightly, providing  structural matrix.  Golf balls are somewhere between angular and round because of their multi-faceted surface, but spherical shape. A stack of golf balls would leave huge gaps between the balls in the same way that a sand comprised of only one particle size would not pack together tightly. Mortar strength increases with better packing. An appropriate building sand has a range of particle sizes from small to large, with the majority of particle sizes in the middle range. On a graph, this sand will have a bell curve shape. (Imagine golf balls, marbles, bb pellets and table salt mixed together, with the smaller aggregates filling voids between the larger ones.)

Clean Water. Chlorine, fluorine or high iron content in the water will produce unacceptable blemishes in the mortar.  Reduce the chlorine and fluoride in municipal water supplies by storing for a week.  Add an inexpensive charcoal filter to a hose to reduce/eliminate chlorine etc.

Safety Precautions When Working With Lime

Lime is extremely caustic when wet! With a pH of 12 (Lime becomes pH neutral when carbonated).  Wear protective goggles, gloves and clothing. Minimize bare skin.

Clean water should always be at arms length if lime gets in someone’s eyes or on their skin. Most importantly neutralize skin with a mild acid, such as vinegar or lemon juice to reduce skin reactions. Flush eyes with fresh water for several minutes and seek medical attention.

Working Conditions and Methods

  • Lime Putty cannot carbonate when in a sealed container with an inch of water on top to protect it from carbon dioxide. To store, consolidate mortar in the bucket and cover with water.
  • As with all lime products, the substrate must be damp/wet prior to application. Do not install on extremely windy days or in straight summer sunlight. If necessary, mist the installation over several hours, allowing it to dry slowly.
  • Lime will etch aluminum and can discolor copper under certain conditions, protect surrounding metalwork.
  • Do not apply lime mortars or limewash when risk of freezing is present during application or curing process (for up to a month after applying exterior stucco) because this may cause failures.
  • These guidelines are provided in good faith. They are not a substitute for a thorough workforce training.

Mixing Lime Mortar

Lime mortar or plaster must maintain a consistent proportional mixture of lime, sand (and fiber, if used).

  • Mix lime plaster thoroughly. (Mixing makes lime more plastic and workable, keep this in mind if the stucco on your hawk begins to get stiff). Mix on your hawk, it will instantly become more workable.
  • Use a vertical shaft mixer! (Avoid false economy: The cost of a mixer is well worth the labor costs and the thoroughness of mixing). Do not use rotating drum or barrel mixers; these do not adequately mix lime mortar.
  • Add sand and lime alternately to the mixer while it is running. Add water if the sand is extremely dry. (Lime gets more liquid and workable the longer it is mixed.) If the mixture is crumbly after 15 minutes of mixing, add a small amount of water (about 8 fluid ounces at a time for a 15-gallon mixer load).
  • Remember excess water leads to shrinkage; the more you add now, the harder you will have to work to compress the mortar later.
  • Mix for a minimum of 20 minutes. Do not worry about over-mixing. Watch the water content!
  • When thoroughly mixed, lime mortar should be fairly dry to the appearance, but spreadable similar to cream cheese. “Dry” mortar, “wet” substrate is the goal.
  • When ready to use, the mortar will create a strong bond to the trowel, hawk etc. Hold upside down, it shouldn’t fall off.
  •  Cut and tease fiber into mortar in the last few minutes of mixing. Stop the mixer so that the hair or fiber is not pulled out of the mortar again and wrapped around the mixer paddles.
  • Add hair or structural fiber at a rate of 1 gallon to 5 gallons of lime putty or 1 gallon to 15 gallons of Lime Mortar.
  • Mortar will have hairs or fiber sticking out at regular intervals of about ¾” from a trowel.
  • If the mortar is too wet, spread it on plywood, place at an incline to allow water to run off. Do not leave the mortar like this for more than an hour or carbonation may begin.
  • Plaster may be allowed to rest overnight in the mixer if it is tightly covered. Mix again before use the next morning.  Mix Un-fibered mortar in advance and store indefinitely in airtight containers with 1”  of water on top. Haired or fiber-ed mortar will not store as long due to the high alkalinity (six months maximum).

General Mortar Ratios by Coat for Exterior Applications (Stucco)

First Coat (Harled Coat) = a “soupy” wet mix of mortar that can be thrown on the wall with considerable force to achieve both a mechanical and chemical key.

  • – Ratio = coarse aggregate to lime ratio as derived from void-space test.
  • – Do not add fibrous materials to the harled coat.
  • – Harled coat will go on about 1/4” thick.

Intermediate Coats – second through third coats depending on specified final stucco thickness. Called “scratch coats” or “base coats” with the mortar referred to as “coarse stuff.”

  • Ratio = same as harled, coarse aggregate mixed to void-space proportions
  • Cut and tease hair into mortar in the final moments of mixing at rate of 1 gallon to 5 gallons of lime in batch.
  • Apply Intermediate coats at approximately 5/8” thickness.
  • Mix intermediate coat mortars as dry as possible. Lift on a trowel, and invert, if the mortar maintains its bond to the trowel, it’s ready.

Finish Coat for Stucco. For ideal durability, the finish coat should maintain the same coarseness as preceding coats. Otherwise only this final layer may incorporate finer sand. 

  •  Mix as dry as possible, do not incorporate hair or fiber
  •  Apply no less than 3/8” thickness.

General Interior Plaster Ratios

Scratch or Base Coats

  •  Mix dry and apply to dampened masonry, bales or lath.
  • Apply in 3/8” coats. If applying through lath, estimate almost double mortar usage to accommodate mortar pushed through and wrapping over the back of lath.

Finish Coat for Plaster

  • Mix dry, wetting as the finish is worked.
  •  If a more polished surface is desired with a finer aggregate, achieve this without admixtures, as follows:
  •  Once plaster work is firm, scour the surface using only as much water as is necessary to moisten the surface and allow the float to work freely. To obtain a close-grain, dense, even surface continue scouring.  (Scouring with a cross-grained wood float leaves a dense open-grained finish; a plastic float leaves a more polished homogeneous appearance.) Plaster work may then be troweled up using a steel trowel and a broad flat brush. Sprinkle water on the surface, followed directly with the trowel.

More Tips for Plaster & Stucco

  •  Apply Lime mortars, Plasters and Stuccoes to a consistent thickness, in the 3/8”-5/8” range (harled coat on masonry excluded).
  •  Do not keep working lime plaster or stucco once it is well adhered to the wall. This may cause lime to be drawn to the surface. It can form a hard crust over a soft backing and deprive the interior coat of enough lime to bind the sand.

General Preparation and Use of Limewash from Lime Putty

  • Mix Limewash by thinning lime putty to the consistency of whole milk.  If adding pigments, mix to the consistency of heavy cream.
  •  Whip Lime putty using a right angle drill with a masonry mixing paddle before adding water. Mix to a consistent creaminess.
  • Add water a half-gallon at a time. Adding too much water at once complicates mixing. Do not add pigments until limewash is thin.
  • Mix pigments with just enough warm water to fully “wet” the particles completely,
  • Wet the substrate.
  • Do not apply limewash on extremely windy days or in straight summer sunlight.
  • If necessary, water mist the limewash over several hours, allowing to dry slowly.
  • Brush, spray, or roll, wear personal protection and protect surrounding surfaces.

© Preservation Science 2007
Used with permission.