Mixing Lime Mortar

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

1. 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 2x4 cutoff.)

a) 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 notadequately mix lime mortar.

b) 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 15minutes of mixing, a small amount of water (about 8 fluid ounces at a time for a 15-gallon mixer load) may beadded.

c) Remember excess water leads to shrinkage; the more you add now, the harder you will have to work to

compress the mortar later.

d) 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.

e) When thoroughly mixed, lime mortar should be fairly dry to the appearance, but spreadable similar to creamcheese. Again “dry” mortar, “wet” substrate is the goal.

f) 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 themortar dropping off.

g) Fiber may be cut and teased into the fully mixed mortar in the last few minutes of mixing. Once mixed in, stopthe 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 fifteengallons 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 3/4”.

h) 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.

i) Plaster may be allowed to rest overnight in the mixer if it is tightly covered and mixed again before use the nextmorning. Un-fibered mortar can be mixed in advance and stored indefinitely in airtight containers with a 1” waterbarrier 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)

a.

First Coat (Harled Coat) = a “soupy” wet mix of mortar that can be thrown on the wall with considerable force toachieve 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.

b.

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 atrowel, tapped on the side of the mixer three times and inverted, the mortar maintains its bond to the trowel, evenwhen shaken.

c.

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 thesand used for backing coats to eliminate particles larger than 5/32”. Maintain a good range of aggregate sizes andnon-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)

a. Scratch or base coats

• Aggregates with a good bell-curve range of particle sizes and angularity with the largest aggregates topping outat 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 mortarpushed through and wrapping through and down over the back of lath.

b. Finish coat for Plaster

• Aggregates with a good bell-curve range of particle sizes and angularity with the largest aggregates topping out1/8”. 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 desiredplaster 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:

i. When plasterwork is firm, scour surface using only as much water as is necessary to moisten the surfaceand allow the float to work freely. Continue scouring until a dense, even, and close-grained surface isobtained. (Scouring with a cross-grained wood float leaves a dense open-grained finish; a plastic floatleaves a more polished homogeneous appearance.)

ii. Plasterwork may then be trowelled up using a steel trowel and a broad flat brush. Sprinkle water on thesurface, followed directly with the trowel.

Lime mortars, plasters and stuccoes should be applied to a consistent thickness, in the 3/8”-5/8” range asdescribed 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 thesurface where it can form a hard crust over a soft backing and deprive the interior of that coat of enough lime to bind thesand.

General Preparation and Use of Limewash from Lime Putty

a. 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.

b.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.

c. 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.

d.Pigments should be mixed with just enough warm water to fully “wet” the particles completely, before adding to limewash.

e. 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.

f.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.

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 suitable workforce training.