Guidelines for Working with Natural Lime Mortar
Understanding water – suction and evaporation – 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 as with Portland cement. Cement-based finishes do not allow for adequate moisture movement (evaporation of both water vapor AND liquid water) necessary for to keep porous walls dry.
Lime-sand only mortars are more plastic and better accommodate any settling or movement in the wall; this is unlike cements that, once set, do not adjust to changes around them. This means that although lime stuccoes and limewashes are more breathable, they also have better water shedding characteristics. Cement stucco is likely to crack under stress or movement, allowing a route for water infiltration into the interior where it will be trapped. In contrast, lime stucco can better adjust to early movements in the building because it does not set fully all the way through immediately, but only as the interior more slowly carbonates. Any tiny cracks that open can be resealed as acidic rainwater enters those cracks either draws some of the remaining calcium hydroxide into the crack or as the slightly acidic rainwater partially dissolves calcium carbonate along the edge of the crack temporarily creating calcium bicarbonate and 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.”
Although it may seem that “stopping water” is a good idea, many of our modern building materials are designed to 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. Stone or concrete foundation are ideal. Also consider that annual mulching will increase the grade near the building. Foundations should rise above the splash line of any yard or bedding materials.
Only the calcium proportion of a lime carbonates quickly, so only properly calcined and slaked high-calcium lime can be expected to rapidly set and provide a very durable work in all climates. While S-type and other hydrated limes work well in conjunction with cement as they were designed to do, they are not manufactured with the high surface area and high porosity that speed carbonation for a lime-sand only mortar.
Sand. The quality of sand is of primary importance to achieving a high quality lime mortar. It is the job of the sand to provide structural strength, with the lime putty coating and cementing the particles together.
- Sand should be clean and free of silts or organic material. Sand containing silt should be washed until all silt or organic materials have been rinsed away. (In cases where on-site sand is being used to match an original mortar in an historic building and obtained by washing soil from the site to the appropriate degree, it is advisable to 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. Rounder sands are less structural because they do not lock together as well. Sharper or more angular sands are preferable.
- Sand should be comprised of various size particles, including coarse grains and fine. A quick assessment of sieved sand would give a bell-curve graph meaning that the majority of particles are in the mid-range of sizes 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 just coat the sand particles to bind everything together without having excess lime to push the particles apart and weaken the mortar. The following simple on-site test to determine the void space in a given sand should be used to find the sand-to-lime ratio for mortar. Different sands mean different lime:aggregate ratios.
- Ideally alcohol used for this quick on-site test should be done be 200-proof to eliminate water that can skew readings, but cheap 190-proof grain alcohol [e.g. Everclear] from your local liquor store will do the trick in a pinch. This test requires very dry sand for accuracy. If necessary, the sand can be dried in an oven at 200°F for half an hour or put in a microwave oven until it stops steaming up the interior, then cooled to ambient temperature.
- Ideal sand proportions are determined by filling a measured beaker with cleaned and very dry sand to 100mL. Tap the container until the sand is densely packed down, then slowly adding pure alcohol from another measured container until it just wets the top of the sand.
- Measure the amount of alcohol added to assess the void space in the aggregate.
- For example, if for 100ml of sand, it took 30ml of alcohol to wet the sand, you would have approximately a 1 lime to 3 sand ratio.[/unordered_list]
Particle Size Distribution of Aggregates. Sand used for building should ideally be clean and sharp or angular, not rounded, as these will pack together more tightly, providing a more structural matrix. For demonstration purposes, golf balls might be seen as somewhere between angular and rounded 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, so appropriate building sand has a range of particle sizes from small to large, with the majority of particle sizes in the middle range. If graphed, this sand is said to 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.)
Pigments. Not all pigments are stable in the highly alkaline limewash suspension. All natural iron oxide pigments and most earth pigments are alkaline stable. Your pigment supplier should be able to answer these questions.
Clean Water. Chlorine, fluorine or high iron content in the water will produce unacceptable blemishes in the mortar. Municipal water supplies with added chlorine and fluoride can normally be brought down to acceptable levels after a week of storage in sunlight in wide tubs.
Safety Precautions When Working With Lime
Lime is an extremely caustic material when wet, with a pH of 12. (Lime becomes pH neutral when carbonated). Protective goggles and gloves should be worn at all times. Additionally, protective clothing should be worn where risk of splatter onto bare skin is present.
Clean water should always be at arms length if lime gets in someone’s eyes or on their skin. Skin can be neutralized with a very mild acid such as vinegar or lemon juice. Repeatedly flush eyes with fresh water for several minutes and seek medical attention.
Working Conditions and Methods
- Lime Putty cannot carbonate as long as it is kept in a sealed container with at least an inch of water over top to protect it from any contact with carbon dioxide. As lime or mixed mortar is removed from containers, it should be reconsolidated into a tight pile at the end of the day that can be tightly secured and covered with water to stop carbonation.
- As with all lime products, the substrate should be thoroughly dampened before limewash is applied. Do not install on extremely windy days or in straight summer sunlight. Follow the sun around the building, don’t let it follow you, prematurely drying out the installation. If necessary, water mist the installation over several hours, allowing it to dry slowly.
- Lime will etch aluminum and can discolor copper under certain conditions, so 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).
- 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 and sand (and fiber, if used).
- Mix lime plaster thoroughly prior to use. (Mixing makes lime more plastic and workable, so keep this in mind if the stucco on your hawk begins to get stiff: if it is picked up and dropped on your hawk a few times, it will instantly become more workable, or pound it in a bucket or on your plaster stand a few times with a 2×4 cutoff.)
- Vertical shaft mixers should be used. (Avoid false economy: The cost of a new mixer will be well worth the labor saved 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. Unless sand is extremely dry, water is most likely not needed. (Lime gets more liquid and workable the longer it is mixed.) If the mixture is crumbly after 15 minutes of mixing, a small amount of water (about 8 fluid ounces at a time for a 15-gallon mixer load) may be added.
- 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 the mortar, although motor heat or mixing in direct sunlight may cause mortar to dry during prolonged mixing, so watch the water content. Cover the mixer with a small sheet of rubber membrane or something impermeable and weighty so it won’t blow off, in order to maintain water in the mix and limit CO2 uptake during mixing.
- When thoroughly mixed, lime mortar should be fairly dry to the appearance, but spreadable similar to cream cheese. Again “dry” mortar, “wet” substrate is the goal.
- The laborer should deliver mortar to each mechanic that if lifted by trowel and then the trowel tapped twice on the edge of the mixer will create a strong enough bond that the trowel can be held upside down without the mortar dropping off.
- Fiber may be cut and teased into the fully mixed mortar in the last few minutes of mixing. Once mixed in, stop the mixer so that the hair or fiber is not pulled out of the mortar again and wrapped around the mixer paddles.
- Hair or structural fiber. Hair to be added at a rate of about a gallon to five gallons of lime putty (or about fifteen gallons of mortar), cut and teased into a separate container
- before being teased into the mortar at the last minutes of mixing. Ideally, a trowelful of
- mortar will have hairs or fiber sticking out at regular intervals of about ¾”.
- If the mortar is made too wet, spread it out on a piece of plywood placed at an incline to allow water to run off and be soaked into the plywood as well as evaporate slowly. Do not leave the mortar like this unchecked for more than an hour or carbonation may begin.
- Plaster may be allowed to rest overnight in the mixer if it is tightly covered and mixed again before use the next morning. Un-fibered mortar can be mixed in advance and stored indefinitely in airtight containers with a 1” water barrier on top. Haired or fibered 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 be go on about 1/4” thick.
Intermediate Coats – second through third coats depending on specified final stucco thickness. Sometimes 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.
- Intermediate coats should be applied at approximately 5/8” thickness.
- Intermediate coat mortars will be mixed as dry as possible while achieving the desired consistency: lifted on a trowel, tapped on the side of the mixer three times and inverted, the mortar maintains its bond to the trowel, even when shaken.
Finish Coat for Stucco. For ideal durability, the finish coat would maintain same coarseness as preceding coats. Otherwise only this final layer may incorporate finer sand. Another options is to sieve the largest aggregate out of the sand used for backing coats to eliminate particles larger than 5/32”. Maintain a good range of aggregate sizes and non-rounded sand.
- Mix as dry as possible, do not incorporate hair or fiber
- Apply to no less than 3/8” thickness.
General Mortar Ratios by Coat for Interior Applications (Plaster)
Scratch or Base Coats
- Aggregates with a good bell-curve range of particle sizes and angularity with the largest aggregates topping out at 3/16”.
- Maintain aggregate:lime ratio determined by void-ratio test.
- Mixed dry and applied 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 through and down over the back of lath.
Finish Coat for Plaster
- Aggregates with a good bell-curve range of particle sizes and angularity with the largest aggregates topping out 1/8”. Before considering a significantly finer interior finish that may compromise durability, try a test panel with several coats of limewash on top to soften the appearance.
- Aggregate:lime ratio determined by void-ratio test may be altered slightly to increase lime to achieve desired plaster finish.
- Mixed dry, wetting as the finish is worked.
- If a more polished surface is desired with a finer aggregate, it can be achieved without admixtures, as follows:
- When plasterwork is firm, scour surface using only as much water as is necessary to moisten the surface and allow the float to work freely. Continue scouring until a dense, even, and close-grained surface is obtained. (Scouring with a cross-grained wood float leaves a dense open-grained finish; a plastic float leaves a more polished homogeneous appearance.) Plasterwork may then be trowelled 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
- Lime mortars, plasters and stuccoes should be applied to a consistent thickness, in the 3/8”-5/8” range as described above (harled coat on masonry excluded).
- Do not keep reworking lime plaster or stucco once it is well adhered to the wall. This may cause lime to be drawn to the surface where it can form a hard crust over a soft backing and deprive the interior of that coat of enough lime to bind the sand.
General Preparation and Use of Limewash from Lime Putty
- Limewash is mixed by thinning lime putty to the consistency of heavy cream if adding pigments or to whole milk consistency if leaving white. If applying multiple coats, the base coats should be applied unpigmented, building up the opacity quickly.
- Lime putty should be thoroughly whipped in a five gallon pail using a right angle drill with a masonry mixing paddle before adding water. Mix to a consistent creaminess. Cut a whole in the pail lid for the paddle spindle and secure the bucket lid tightly while mixing to keep limewash from splattering.
- Water should be added incrementally a half-gallon at a time. Adding too much water at once will complicate consistent mixing. Pigments should not be added until limewash is thinned.
- Pigments should be mixed with just enough warm water to fully “wet” the particles completely, before adding to limewash.
- As with all lime products, the substrate should be thoroughly dampened before limewash is applied. Do not apply limewash on extremely windy days or in straight summer sunlight. Follow the sun around the building, don’t let it follow you, prematurely drying out the limewash. If necessary, water mist the limewash over several hours, allowing it to dry slowly.
- Limewash can be brush-applied, sprayed or even rolled on, although each of these methods is increasingly messy and personal protection as well as protection for surrounding surfaces becomes a greater concern.
© Preservation Science 2007
Used with permission.