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t a k i n g   g r e e n   t o   t h e   e x t r e m e

 

LIME PLASTER

overview
how lime is made & applied
where to use lime
common questions
benefits & challenges
additional resources
related articles

OVERVIEW

Lime is a confusing term, because it can refer to various chemically different (but related) materials.  (Not to mention the citrus fruit!)   Lime has been used for thousands of years as a fabulous binder in mortars, plasters, and paints.  It wasn't until the post-World War II housing boom that quick-setting cement products eclipsed lime in construction.  Lime does cure more slowly than cement, but it holds many advantages as a workable, self-healing, breathable, nearly carbon neutral material, making it much more suited to natural building.  First, because it has a lower environmental impact.  (Cement production creates 1.25 pounds of CO2 for each pound of cement produced, whereas lime is nearly carbon neutral.)  Perhaps more poignantly, because lime is breathable - meaning it allows air-borne moisture to travel freely through - it does not trap moisture inside a wall system.  Without moisture buildup, biodegradable materials, such as wood or straw, are protected from decomposing.  I use three coats of lime plaster/stucco as the exterior finish on most of my strawbale projects.  I also use it in bathrooms if strawbale walls are within a shower area.

HOW LIME IS MADE

  1. Limestone, shells, or other material high is Calcium Carbonate is burned in a kiln, where the heat drives off Carbon Dioxide, leaving Calcium Oxide, also called Quicklime.  Quicklime is a dry powder that is highly reactive with water.

  2. Quicklime (Calcium Oxide) reacts with water in an extremely heat-producing reaction, a process called "slaking".  The result is Hydrated Lime, or Calcium Hydroxide, since hydrogen bonds to the Calcium Oxide molecule from the water.  This reaction can be quite dangerous, so most people purchase Hydrated Lime (Calcium Hydroxide) instead of Quicklime (Calcium Oxide).  Calcium Hydroxide can be a putty or a powder.

  3. Once Calcium Hydroxide is exposed to air (whether it's in powder or putty form), the lime reacts with Carbon Dioxide in the air and ends up where it started...as Calcium Carbonate.  So except for the energy of the kiln, the lime is carbon neutral.

Because lime plasters react with carbon dioxide from the air in order to harden, you can easily keep the lime in putty form indefinitely by storing it with an inch or so of water on top of it.  This effectively prevents the lime from getting into contact with air and thus prevents curing until you are ready to use it.

MIXING & APPLICATION

Lime putty increases plasticity and workability the longer it is mixed.  I recommend mixing in a mortar mixer (not a cement mixer!) for at least 20-30 minutes.  Alternate the addition of sand and putty so your ingredients mix thoroughly.  You can add a small amount of water if your mix is extremely thick.  The plaster should be stiff but should spread easily, like cream cheese.  Allowing the mixed lime plaster to sit overnight improves workability, but remember to remix the plaster again before using.

Note that lime is highly alkaline, and can severely burn your skin.  Unlike acid burns, you generally do not feel an alkali burn until the damage has been done.  So please use full protective gear whenever working with lime, including elbow-length rubber gloves, long sleeves, eye protection, etc.  If your clothes get lime putty or lime water on them, change so the lime is not in contact with your skin through your clothing.  I always keep a bucket of water & vinegar nearby to neutralize my tools, gloves, and hands throughout the day.

To prepare your walls for lime plaster, first shape your walls exactly how you would like them to look once plastered.  It is time-consuming to build up the lime plaster to fill in large voids (since it must be applied in thin coats).  Next, install expanded lath to cover any slick surfaces, such as wood.  Make sure your lath bridges across the wood and at least 6" into the straw so you don't get a crack right where the lath ends.  I do NOT recommend using lath over all of the strawbale, unless you live in a seismic region and your code require this.  Chicken wire and lath can impede the lime plaster from bonding fully to the straw.

When applying lime stucco over strawbale walls, I recommend 3 coats of lime directly on the strawbales.  The first coat can be up to 5/8" thick if it is applied to strawbale, otherwise each coat should be a maximum of 3/8" to 1/2" thick.  Any thicker and the lime cannot absorb carbon dioxide adequately for curing to fully take place.  Score each coat except the finish coat to allow for maximum surface area to key two coats together.  Allow at least 4 to 5 days between coats to give each ample time to cure.

I do NOT recommend lime plaster over clay plasters for exteriors in wet climates.  The clay substrate shrinks and swells depending on moisture content.  The lime cannot shrink and swell with the clay and so it will be more susceptible to cracking when used over clay plaster in a wet climate.  Lime can be used over solid clay walls, such as cob & adobe, because there is so much more clay present to absorb ambient air moisture without measurable swelling.

Be sure to wet your walls down well before applying each coat of lime plaster.  For the first coat, this means soaking the strawbales until they are damp and pliable.  For each subsequent coat, soak the wall down the day before you will plaster, again the morning of plastering, and throughout the day keep the wall damp as you work.

Do not rework plaster once it has stuck to the wall, this pulls lime to the surface and leaves what is underneath sandy with less binder.  You can work the surface to create a desired texture once the lime is green hard.  And you can buff or polish the lime as it is curing to create a very fine surface texture.

You want the lime to cure NOT dry out.  If it dries out before it has cured by reacting with carbon dioxide in the air, the resulting plaster will be weak and possibly crumbly.  So protect the plaster from wind and sun until it has cured, and it helps to dampen the wall daily as it is curing.  Do not apply exterior lime stucco if there is any risk of freezing, otherwise moisture in the plaster can freeze, expand, and cause critical failure of the plaster.  The temperature needs to be above 40 F for at least a week to keep the curing process going.

WHERE TO USE LIME

EXTERIOR STUCCO
Exterior stucco may be the most common use of lime in natural building, and specifically in strawbale construction.  Lime provides a durable finish for exterior applications (as well as wet areas, like shower enclosures).  Lime is particularly compatible with strawbale construction because it is a breathable finish, which means that it allows vapor in the air to move freely through a wall without getting trapped inside the wall.  If moisture becomes trapped inside a wall, it can build up to quantities high enough to the activate microbes that cause decomposition.  Breathable wall systems are particularly important when constructing with a biodegradable building material, such as straw or wood.

BATHROOMS & OTHER WET AREAS
I specify lime plasters instead of clay in bathrooms that have a bathtub or shower.  If you are not going to install a splash in the kitchen, I recommend lime plaster over strawbale walls there as well, instead of clay.  The lime lets you scrub the walls, and is generally more durable and can handle repeated exposure to moisture.  Clay can work in these applications as well, especially if additional coats of sealer are applied, but lime gives you a bit of added durability and washability in wet situations.

COB OVENS & HOT TUBS
You can protect exterior cob structures, such as cob oven or hot tubs, with three coats of lime plaster.  Apply the lime in warm weather so it has plenty of time to cure (carbonate) before any freezing temperature. And make sure the cob has dried completely before applying any sealer or lime plaster, otherwise you risk trapping moisture inside.  I like to fire the oven at least once before plastering.
LIME WASH
Lime wash is a naturally mold & mildew resistant paint that has been used for centuries.  Unlike many conventional paints, lime does not off-gas at all, even when wet.  Paints made with lime are semi-translucent, and can be easily pigmented and applied in layers to create beautiful, complex finishes.  When preparing walls for lime wash, be sure walls are completely free of any dust or debris.  If you are lime-washing over a clay plaster, I recommend dampening the clay before applying the lime wash.  By itself, lime wash is a brilliant white color, though it accepts pigments nicely.  Use pigments that work in an alkaline material, either specifically made for lime or those used in concrete, and dissolve the pigment in water separately (hot water helps), then sieve the pigment into your lime wash paint.

Actually making the lime wash is a simple process:

  1. Use well-slaked lime (as described below under "common questions")
  2. Beat the lime putty thoroughly
  3. Add clean (drinking quality) water or lime-water until the consistency becomes similar to skim milk
  4. Mix thoroughly with a paint mixer attachment on a power drill (or similar)
  5. Add any pre-dissolved & strained pigments
  6. Mix again thoroughly
  7. Brush the paint onto walls in thin coats; as many as 6 coats, as desired (note that lime wash may look nearly invisible when you first put it on)
MORTAR
Lime putty and sand create a strong and forgiving mortar.  Lime mortars stay workable longer than cement mortars, which is a plus if you are just learning!  (It's also a nicer color than cement gray...)  Because lime is a breathable material that allows moisture to pass through, it is a good compliment for cordwood construction.

COMMON QUESTIONS

WHAT KIND OF LIME TO USE?
I use fresh hydrated powdered lime and then soak it on site from the very beginning of construction (so ideally several months).  I have had most consistent results with vertical kiln products from Mississippi Lime.  The vertical kiln operates at a lower temperature and so there is less inert material in these products.  I ask for bags that are date stamped less than 6 months prior to purchase.  This ensures the lime is fresh.  If it has been in the bag for a long time, it gets exposed to CO2 in the air and begins to carbonate and become inert.  Powdered lime that has turned to calcium carbonate looks identical to calcium hydroxide (hydrated lime), but when you soak it, it will not get very thick and when you put it on the wall it will dust or crumble.
 

HOW LONG TO SLAKE BAGGED LIME?
I soak the fresh powdered hydrated lime at least 6 weeks, or until it has the consistency of thick sour cream.  The longer the better!  Traditional lime plasterers let their lime putty hydrate for a generation!  I recommend to begin soaking your lime when you begin your project so it has ample time to absorb water.  And don't forget to keep an inch or more of water on top of your putty so the lime can't react with CO2 in the air and become inert.

WHAT KIND OF SAND TO USE?
The sand can be either concrete sand (which is very course) or mason's sand (which is much finer).  The important variable is that the sand must be "angular", which means it has a lot of surface area to bond with the lime.  I use "toothy" or "angular" mason's sand for all three coats of lime plaster.  Note that the color of the sand will impact the final color of your finish coat of lime.  If you want very white plaster, experiment with white sand.
WHAT ARE THE PROPORTIONS OF SAND TO LIME PUTTY?
I use 3 parts sand to 1 part slaked lime putty for all three coats of lime plaster.  For a smoother finish, I use a flexible pool float to smooth the surface of the lime as desired.  Or you can buff or polish the lime as it is curing for a very smooth sheen.

 

CAN LIME BE PIGMENTED?
Yes!  This blue knee wall is plastered in pigmented lime, as is the yellow wall behind.  Any pigment that can be used in concrete will work with lime.  The pigments must be able to handle the alkalinity of the lime.  Mineral pigments generally are fine, plant-based pigments generally will not work (they change color and fade due to the alkalinity).  In any case, do several test patches to confirm how much pigment to add to achieve your desired color.

BENEFITS & CHALLENGES

BENEFITS of Lime Plasters

  • provides a breathable finish for strawbale wall systems

  • provides evenly distributed thermal mass, which improves performance of strawbale insulation

  • naturally mold and mildew resistant

  • nearly carbon neutral (as opposed to cement-based stucco which is a high CO2 contributor)

CHALLENGES with Lime Plasters

  • not an off-the-shelf product and not all hydrated lime is created equal; requires some knowledge to be sure you use the correct material

  • can be time consuming and physically demanding

  • difficult to find affordable skilled labor that is familiar with how to properly hydrate, mix, and apply lime plasters

  • high alkalinity poses safety considerations when handling lime

ADDITIONAL RESOURCES

WEBSITES

National Lime Association website
www.lime.org

Lime Plaster in Wikipedia
en.wikipedia.org/wiki/Lime_plaster

BOOKS

Building with Lime: A Practical Introduction by Stafford Holmes and Michael Wingate - THIS MY LIME BIBLE!

Using Natural Finishes: Lime- and earth-based plasters, renders & paints by Adam Weismann and Katy Bryce

RELATED ARTICLES

Clay Plaster

Natural Building Overview

Down to Earth Design
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