Exterior and interior plastering is underway on the carbon neutral Nauhaus prototype.
The interior surface of the hempcrete walls has a base coat of earthen plaster consisting of sub-soil harvested from the construction site and mixed with sand and water. The mix was chosen after testing sixteen different compositions, a process spearheaded by intern Shannon Levenson. Earth plaster serves the Nauhaus prototype mission because it requires almost no energy to make or transport, and therefore has very little carbon emissions associated with it. In addition, earth plastering is fairly easy to learn, requires few tools, and is instantly gratifying, both because it’s beautiful at any skill level and very similar to playing with mud pies, a therapeutic experience that many adults realize they have been neglecting for too long. Whatever the reason, the earth plastering process attracted volunteers and interns like flies to…well, compost.
The exterior wall surface has been covered with a base coat of lime-based plaster supplied by Lime Technology as part of the hempcrete wall system. Both interior and exterior plasters were applied directly to the hempcrete which proved to be an excellent plaster substrate. Fiberglass mesh, similar to mesh drywall tape, were embedded in plaster over any joints or cracks in the hempcrete. Together these plasters over hempcrete create a vapor permeable wall system, sometimes called a “breathable wall”. The idea is to create a wall that is open to taking on and giving off water vapor in response to humidity levels in the air inside or outside the building.
We believe vapor permeable walls will last much longer and help create better indoor air quality than cavity wall systems that dominate US residential construction. As any builder will tell you, it’s pretty much impossible to keep water out of walls. Permeable walls are designed with the idea that it’s okay if some water gets in as long as it can get out just as easily and won’t cause any damage in the process.
The Hemcrete installation continued today in the freezing weather, and is up to the second floor. Ian Snider from Mountain Works dropped by yesterday to discuss some of the sustainably harvested wood he will be supplying to the project. Ian’s company uses horses to remove the trees that they selectively cull as part of a forest stewardship process.
If you’re interested in volunteering for the Nauhaus Prototype Project, please contact Billy.
We were excited to have Ian Pritchett and Mario Machnicki from Lime Technology, makers of Hemcrete, come by to check out our building for the first time. We had some great discussions about Hemcrete, earthen blocks, construction details and more. The electrical work has been completed, and the walls are ready for the hemp installation.
About 20% of the Earth’s crust is limestone. It is a sedimentary rock composed of calcium, carbon, and oxygen, and magnesium. It is mainly formed from built up layers of decomposing bones and shells from small marine animals. Eventually those calcium rich deposits are compressed to form stone. Limestone is predominantly calcium carbonate (CaCO3), but there is normally some magnesium carbonate (MgCO3) which precipitates out of sea water:
Calcitic or high calcium limestone has < 5% magnesium carbonate.
Magnesium limestone is contains 5-40% magnesium carbonate.
Marble is the same stuff because it is the metamorphic rock that forms when limestone is subjected high temperature and pressure.
Lime used as a soil amendment (aglime) is usually limestone ground into small pieces rather than the chemical lime used in construction.
Heating limestone above 1500 F it gets rid of impurities and releases CO2 to the atmosphere to leave pure calcium and magnesium oxides. (if you heat it above 4300 F it glows, and before electricity they used to light theater stages with it – limelight). The reaction is called calcination:
CaCO3+ heat = CaO + CO2
During calcination, burning the fuel to heat the limestone is responsible for about 40% of the CO2 emissions. 60% of the CO2 released is from breaking the calcite down into CO2 and CaO or quicklime.100 lbs of calcium carbonate breaks down into 56 lbs of quicklime and 44 lbs of CO2.
Quicklime is pure lime or calcium oxide (CaO). Quicklime may also be called hot lime or lump lime. The main uses are:
combined with water to make mortar or plaster
combined with silica to make glass
absorbing phosphate during iron ore smelting to make steel (slag)
absorbing sulfur from power plant flue gasses (fly ash)
Hydrated Lime (Ca02H2)
When you combine quicklime with water you get calcium hydroxide (Ca02H2) or hydrated lime.
CaO + H20 = Ca02H2+ heat
The heat produced is significant. The water will boil violently for 2 to 5 minutes as the hydration reaction takes place.
56 lbs of quicklime is combined with 18 lbs of water to produce 74 lbs of hydrated lime. You can add it up using the molecular weights:
Calcium, Ca = 40
Magnesium, Mg = 24
Oxygen, O = 16
Hydrogen, H = 1
There are six forms of hydrated lime used in industry:
Type of Lime
Dry Hydrated Lime
70 – 55%
35 – 25%
Milk of Lime
20 – 1%
Construction lime is generally available in two forms: putty or hydrated. They both have the same Ca02H2 structure. The only difference between them is how the hydrogen is added. Putty has been soaked in water (slaked) and is sold in a mixture of roughly 50% lime and 50% water.
Prior to the invention of the hydration machine in 1930 all lime was sold as either quicklime or putty. Shipping water is expensive, so quicklime was the preferred method. That meant the user had to slake the lime. Quicklime is available in various sized chunks like gravel. Smaller pieces hydrate more quickly because it’s easier for the water to get to every molecule of CaO. However, eventually large pieces will break down into a fine powder due to the hydration reaction.
The longer the slaking time the better the stickiness, plasticity and water retention of the lime. A minimal modern slaking lasts several weeks, but two years was traditionally considered minimal. Five years was traditionally considered optimal. Lime that hasn’t been slaked long enough still has unreacted CaO in it. If it is used in plaster or mortar the hydration reaction will take place in the wall. As it pops it will blow little pits in the wall.
The hydration machine allowed for the slaking to take place at the lime plant. In a hydration machine, just enough water (steam) is added for the hydration reaction to take place, and hydrated lime is sold in dry powdered from, normally in 50 lb bags. Water is still added to it at the site, but because it is pre-hydrated, it only needs to slake for a short period.
Type N – Normal
In most of the world, Type N designates high calcium lime, but in the US it is probably dolomitic.
Calcium oxide combines with water easily at normal atmospheric pressure. However, magnesium oxide takes a long time or high pressure. Type N is partially hydrated because the magnesium hasn’t hydrated:
CaO-MgO + H2O = CaO2H2 -MgO + heat
The unreacted magnesium oxide is called grit. A dolomitic 60% Type N will have 40% grit. Type N requires a long slake time (>24 hours) to hydrate the magnesium, and if it is not fully hydrated before it’s used the magnesium hydration continues to occur over a long time. Magnesium hydration causes the lime to expand.
Type S – Special
Type S is more expensive than type N, but it is normally used in construction because it is hydrated in an autoclave at high pressure to fully hydrate the magnesium. It is also sifted to remove any remaining unhydrated grit. Air content is less than 7%.
CaO-MgO + 2H2O = CaO2H2 -MgO2H2 + heat
Type NA – Normal Air Entrained
Type N with air entraining agents to increase air content to 7-14%.
Type SA – Special Air Entrained
Type S with air entraining agents to increase air content to 7-14%. Air entrainment significantly reduces bond strength, and type SA should not be used for mortar.
Hydrated lime is also called air lime, calcium lime, high calcium lime, or non-hydraulic lime. More on that in a minute. Quicklime (unreacted calcium and magnesium oxides) is sometimes called hot lime because the hydration reaction is exothermic. Hydrated lime has already gone thru the reaction, so it doesn’t heat up when water is added like quicklime does.
Pure lime, putty or hydrated, cures by carbonation, meaning it sequesters CO2 out of the air. To do that the lime needs to be exposed to water and CO2 in the air:
CaO2H2 + CO2 = CaCO3 + H2O
If the mix is wet or the wall is too thick, air can’t get to it and there’s no CO2 to react. Carbonation is a slow process that takes many years depending on how well air can penetrate the lime. Sometimes air can’t get to the center of a wall until it’s demolished, and carbonation takes place then. Uncarbonated lime has been found in the center of walls 1000s of years old.
Lime is extremely caustic with a pH around 12 when wet (calcium hydroxide = 12.4, magnesium hydroxide = 10). As it drys and carbonates it becomes neutral.
The Lime Cycle
CO2 released during calcination gets reabsorbed during carbonation. However, since 40% of the CO2 released during calcination is from buring fossil fuels to create the heat, only 60% has a chance to be reabsorbed. The whole cycle is a little under 60% lime efficient.
Alite and Belite
Moving to the right on the chart below, higher and higher levels of impurities mixed with the lime make it more dense and brittle when it cures. The main impurity is clay, and clay is composed of mainly aluminum silicate. Lime + clay + moisture heated to over 1700 F forms calcium silicate. That either happens geologically (argillaceous limestone) or people do it in a kiln. If the firing temperature is below 2300 F it forms 2 CaO • SiO2 – belite (yellow in the chart). Above 2300 F it forms 3 CaO • SiO2 – alite (red in the chart).
Hydraulic lime (not hydrated) is pure lime mixed with belite. It cures by carbonation of the pure lime component and the hydraulic reaction of the belite component. The hydraulic reaction does not need air. It can take place under water. Belite’s hydraulic reaction generally takes a few months. Hydraulic lime can be in putty or hydrated (powdered) from. Clay content:
Naturally occurring argillaceous limestone typically has a content of 15-35% clay. Hydraulic lime is hydrated with just enough water to hydrate the quicklime, but not enough to cause the calcium silicate to set.
While still widely used around the world, hydraulic lime has fallen out of favor in the United States where it has been replaced by portland cement. In 2007, hydraulic lime was 15% of the total lime produced.
However, there is still a hydraulic lime standard: ASTM C 141 – 67; Standard Specification for Hydraulic Hydrated Lime for Structural Purposes
Cement is pure lime mixed with alite and belite, but modern portland cement (it’s called that because it’s similar to limestone from Isle of Portland in the UK) is mostly alite with a little belite and negligible free pure lime. It also cures by carbonation of the pure lime component and the hydraulic reaction of the belite and alite components. The more alite, the faster and harder it sets. The alite reaction typically happens within a month. With very high levels of alite, the hydraulic reaction can happen in a few seconds. Portland cement has gypsum added to slow the reaction down.
Pozzolanic Cement Additives
You can form additional belite or alite during curing by adding more silicates to a lime.
Belite tends to be formed by:
volcanic rock from Pozzuoli Italy
volcanic rock called Trass from western Germany
Alite tends to be formed by:
ground up blast furnace slag
Historically, the successful soft limes used for mortar and plaster were in the feebly hydraulic range on the chart. Because they didn’t have much belite, they cured slowly. They had a little hydraulic reaction followed by a long carbonation. They were also putties slaked on site so they had a nice smooth application and finish. The modern classification for limes totally ignores feebly hydraulic limes, but there is a new European standard that will bring back NHL 1.
Two things happened that made lime go out of favor. Portland cement started being commercially produced in 1870. Because it is mainly alites, it has a stronger compressive strength. It doesn’t carbonate, it’s set is all hydration, which is quicker, less sensitive to ambient temperatures, and it can take place underwater.
Then the hydration machine also changed the quality of lime. When lime is partially hydrated at the plant, it can be sold in powdered form and relive the user of the need to slake. However, as soon as it’s hydrated it starts to carbonate. The key to using hydrated lime is to get it fresh (less than a month old). The older it is the weaker it will be after it is applied.
Hydrated lime is typically added to cement mortar as a plasticizer, meaning it controls the mortar’s setting time and shrinkage by retaining water.
Modern mortar is so hard on old brick which is softer than today’s brick because the clay was fired at lower temperatures. Feebly hydraulic lime is softer than brick, so it was used as a sacrificial material. Any moisture in the wall went to the lime mortar and evaporated out of the wall. Eventually the lime needed to be replaced, but the brick was fine. Old brick fails quickly when repointed with modern cement mortar because the moisture in the wall has to leave through the face of the brick. The old brick becomes the sacrificial component. Salts (mainly from the cement) exiting through the brick are the cause of efflorescence. The salt also expands inside the brick and can knock the faces off of them.
Lime mortar is also self healing. It has high plasticity, so it tends to flex instead of crack. Small fractures to occur from a shifting building get filled in as CO2 is sequestered out of the air to turn the calcium hydroxide back into limestone. Modern cement mortars do not have the plasticity to flex nor do small cracks heal. Cement mortars crack allowing moisture into the wall.
Lime Plaster and Stucco
Plaster (or render in the UK) islime putty mixed with sand. Traditionally, plaster was made with hydraulic lime (belite). Stucco is cement (alite) and sand. Hydraulic lime is better because its more vapor permeable and it sheds water better. If the building settles stucco cracks and gives water a path in.
Ideally we want moderately hydraulic lime plaster – soft enough that it won’t crack and remains porous (vapor permeable), but hard enough to not need replaced as often as pure lime would.
Historically, fiber was added to the first plaster coats to help it avoid cracking. 100 years ago lime plaster reinforced with horse hair was common, but the hair was eventually replaced by asbestos and the lime was replaced by gypsum. This store in Ireland sells horse or goat hair, but they say is usually only needed for ceilings and “lath work”. In modern natural buildings, straw chopped into 2-3″ long pieces is used. The volume of fiber to lime is about 0.5:1. For every bucket of lime, mix in 1/2 bucket of fiber.
The best quality plaster comes from quicklime that’s slaked for years. Traditionally 5 years
Putty results in a smooth finish. Hydrated lime gives a gritty texture.
Applied like paint, limewash is a mixture of 15-20% high calcium hydrated lime in 80-85% water. Limewash is typically mixed in a large container (ie. 32 gallons) to insure consistency, and transferred to smaller (5 gallon) containers for application. Filtering the liquid thru a 30 mesh (0.6 mm) sieve when transferring to smaller containers insures that any remaining grit is removed. A typical limewash would be a 50 lb bag of Type S hydrated lime mixed with 30 gallons of water in a plastic trash can.
When applied it is translucent, but over time it becomes opaque. Unlike paint, unpigmented limewash remains partially translucent light able to enter the calcite crystals where it is refracted. The light is split into two rays, one fast, one slow. The intensity of the light is not affected, but the light emitted from millions of tiny calcite crystals results in a bright, vibrant surface with a subtle mottled internal texture.
Because of it’s high pH, compatible oxides are used as pigments. Light colors and pastels are typically used to retain the vibrant quality. Pigment concentrations over 5% tend to come out of suspension resulting in uneven coloring. The first few coats are usually unpigmented to maximize adhesion. Layers are built up inthin applications to avoid cracking and chalking. 3-5 base coats are typical with an additional 2 coats of pigment.
It is often recommended to shade limewash to avoid flash drying due to direct sunlight. However, experimenting on a project in the Mojave desert, Peter Mold discovered that cycles of quick drying and rewetting during application resulted in thorough carbonation and a very strong finish.