[Strawbale] Strawbale dome - Aerial lime?

[Derek Roff]

I will start off with a comment on your soil and earthen plasters/renders (I tend to use the two terms interchangeably).  Excess silt is a problem with clay and earthen plasters.  As you mentioned, your test samples need to be larger, and probably thicker.  I would make some samples at 500mm x 500mm, and as thick as you intend your render to be.  The lack of cracking in your samples could be a sign that you have good soil for plasters, or it might mean you have low clay and excess silt.  After your samples have dried thoroughly, test for strength, hardness, and dusting.  

I'm sorry to offer largely negative remarks about other aspects of your project, but sometimes the best advice is "do something else".  I've lived in a geodesic dome.  I've built them, studied them, attended many meetings in domes of different sorts, and tried to patch countless leaks in them.  My view is that while domes are fascinating and great fun in the right context, they are not terribly human friendly nor eco-friendly structures.  But let's leave that for a moment, and go back to leaks.  Most domes have lots of leaks.  Leaks and strawbales don't go together well.  You say this is a temporary structure, but it may be much more temporary than you desire, if your bales rot after a few months.  

Creating and maintaining a monolithic, crack-free layer of plaster/render/stucco over a dome is close to impossible.  Small cracks in the render will allow water to reach the straw.  In the short term, the bales might soak up enough water weight to cause the dome to fail structurally.  Probably not, but it could happen.  More likely, the straw will rot in places near the cracks.  Windows and doors will also be a problem, since domes don't lend themselves to proper overhangs.  Water intrusion is always a concern around doors and windows, and all the more when the windows are not vertical.  In any case, each door or window will have to resist and shed all the water running along the skin of the dome above the door or window.  This is a flashing and detailing nightmare.  

Lime is a fascinating material.  I wish lime users would be more careful in their use of terminology.  As you have seen, the terminology is chaotic.  In addition, some practitioners make generalizations that remind me of religious dogma.  Here is what I believe:  There is a lot of variation in lime materials, and what may be true for one source or supplier may not be true for another.  Each source of limestone will produce a lime with somewhat different properties.  People in Britain love their high-calcium limes.  People in some other parts of the world love their calcium/magnesium limes, sometimes called dolomitic limes.  Some people love hydraulic limes.  Purity or impurity of the lime can make a big difference in its working properties.  Strawbale builder John Glassford of Australia has had great success with the type-S limes that he buys locally.  http://glassford.com.au/main/  Take a look at his website.  There is a lot of useful information there.  Other people hate the type-S lime that they have bought.  Try different products in your location, and see what works for you.  

Slaking your own quicklime will only produce a good product, if you use a good quicklime from a good limestone source, and very good slaking technique.  It is unlikely that people without experience can produce a good product.  The slaking process, which converts calcium oxide (quicklime) into calcium hydroxide (hydrated lime) produces lots of heat.  Quicklime reacts strongly with the skin, the lungs, and the eyes.  Heat plus caustic chemicals makes slaking quicklime a dangerous operation.  Even if you manage to protect yourself, you may not be able to protect the lime.  Slaking must be done in a way that allows the chemical reaction to proceed at the proper rate.  Too fast or too slow will decrease the quality of the lime product.  Most texts only talk about calcium oxide and calcium hydroxide.  If we were only dealing with those, life would be simpler.  Instead, we have the magnesium oxide to magnesium hydroxide reaction going on simultaneously, we likely have potassium hydroxide in the mix, which is even more caustic than quicklime, and we have a variety of impurities to deal with.  If your goal is to build a building, it probably makes sense to buy your lime, rather than spending months or years learning to slake lime effectively, and then aging your lime putty.  

The advantage of the type-S process is that this controlled, industrial-scale process can properly hydrate both the calcium and magnesium components of the lime in the proper way, without producing much over- or under-reacted particles.  If unreacted particles are present in your lime, they may continue to react over time in the applied render, causing spalling and pits in the surface.  There is still the question of the other chemical constituents and impurities in a particular supply of hydrated lime.  The best hope is that the type-S lime that is sold in your location will work well.  If not, perhaps other kinds of prepared lime are available, that will work well for you.  

Traditional lime experts like to soak their lime for 3-5 years, before use.  Prolonged soaking improves the lime, because continuing chemical reactions convert more and more of the unreacted chemicals into compound that contribute to the desired properties of the render.  Few of us are ready to wait three years before beginning to plaster.  John Glassford says that his type-S lime can be mixed and used the same day.  He has not seen much difference between fresh-mixed lime, and that which has soaked for several months.  Other limes benefit from soaking for as long as you can allow.  [Some people refer to soaking the hydrated lime as "slaking", but this just gives the word "slaking" two contradictory meanings.  Other people use the term "slaking" to include mixing dry hydrated lime, such as type-S, with water.  Since type-S lime is already hydrated, mixing it with water doesn't produce the dramatic chemical reaction of water and quicklime.  Calling the mixing of hydrated lime powder with water "slaking" gives us three contradictory meanings for the word.  I only use "slaking" to mean the mixing of quicklime with water, which causes the primary chemical reactions of converting oxides to hydroxides (mostly calcium oxide to calcium hydroxide, and magnesium oxide to magnesium hydroxide).]  Soaking your lime will never hurt (unless you let it freeze), and will usually help.  Dry, powdered lime in the bag can absorb water and carbon dioxide from the air as it sits in the bag.  It will react to these compounds, and then be less cooperative when you mix and apply the render.  Try to buy fresh lime, and don't store it in bags for a long period.  If you mix it with water, you avoid these risks, and for some limes, soaking will improve the working properties.  

I am doubtful of a number of remarks and suggestions on the Strawtec.com.au page that you linked.  Their text contrasts with the experts and advice that I follow.  For example, the use of beach sand can bring a couple of liabilities.  The introduction of salt will affect the chemistry of everything else in the lime (or earth, or cement) render.  In addition, beach sand tends to have many rounded particles of similar size.  Most plasterers prefer sharply pointed sand with varying grain sizes.  It sounds like the sand that you mentioned from your site is much more promising than beach sand.  

Combining clay and lime is a very unpredictable approach, because of the variations in both lime and clay.  These two materials are dissimilar in a number of ways, and lots of people have had problems with de-lamination between the layers, over time.  I wasn't clear on whether you were planning to combine layers of lime and clay, but the Strawtec site suggests it.  I advise against it, unless you are ready to do a lot of experimentation.  

Lime must be carefully protected from freezing and from drying out as it cures.  Keeping the surface moist for several weeks after application is necessary to produce a good, strong lime surface.  The longer, the better.  This is a challenge, and might be impossible on a dome.  The curing process essentially stops at below 40 degrees F/4.5 degrees C.  The lime render may not be damaged, so long as it stays above freezing, but it won't progress.  It sounds like temperatures, and the dryness, would be a problem in your location for curing lime render, during this season.  

The use of lime plaster/render on a temporary building seems at odds with your ecological goals.  When the building is no longer needed in its original location or function, the lime plastered/rendered strawbale or other material cannot be moved, reused or recycled in an effective way.  This makes the embodied energy, labor, and other costs much higher than they would be for a building with a longer lifespan.  

My suggestion would be to build a temporary structure from materials and methods that can be completely reused in the future.  Better still is to build a building that can remain where it is, while its use is revised over time, possibly with remodeling and rework.  Strawbales work very nicely for building quick, rectilinear buildings, although plastering anything tends to be time consuming.  

If you want my views on Earthships, try this link:  
http://www.facebook.com/groups/earthshipczechrepublic/permalink/10150575220879691/

Good luck, and best wishes,

Derek Roff


Wishful thinking is not a reliable building strategy, whether we are talking about a structure, a family, or a community.  

On Feb 9, 2012, at 5:43 AM, Christopher Fraser wrote:

> Hello,
> 
> We are currently starting up an open innovation / learning / sustainability experimentation site in Extramadura, Spain. We are loosely inspired by Open Source Ecology though our range of interests is wider than the Global Villiage construction Set. You can find some information about the project at http://openeland.org.
> 
> We've constructed a 6m metal frame dome, which we intend to cover in straw bale. This is an interim workshop / accomodation structure until we build more permanent buildings with proper foundations, planning permission etc.
> 
> I have no prior experience in strawbale construction but I've been doing a lot of reading on the web and as far as I can tell what seems most appropriate given our budget, requirements and resources on and around site is a clay render finished with lime.
> 
> The only lime I can find locally is powdered "aerial" hydrated lime ("Cal Aerea Hidratada CL-70-S"), which I believe is the same as fat lime. All the different types of lime are confusing, but I believe this is suitable, e.g. according to this:
> 
> http://www.limeindustries.com.au/index-3.html
> 
> I was planning on following this guidance, but have a queries about what they suggest:
> 
> http://www.strawtec.com.au/page.php?id=39
> 
> They don't recommend using hydrated lime, but slaked quicklime - by which I assume they mean liquid rather than powder form. Is the local hydrated lime going to be a big issue? It's only a interim structure and it's fairly dry here (seasonally winters are the wettest with an average of 150mm/month, though it's very dry this year).
> 
> One issue is the overnight temperatures are dropping below freezing at the moment. Should we be worrying about this? It may have lifted by next week, and it's a very dry cold at the moment.
> 
> One thing I'm not clear about is if I just mix the lime powder with sand and water and apply it to the straw? I know there are lots of different opinions about this but any thoughts about the aggregate mix ratio in this circumstance? (See below about the sand). I was assuming 2:1 for now. If there are any test suggestions I can do I'd be very interested to hear.
> 
> The lime comes in 18kg roughly 60cm x 30cm x 10cm sacks (an over estimate as the edges are rounded). If we were mixing 2:1 and covering around 60m2 to say 1cm do I just do the straight forward maths (something like 60 / (0.6 x 0.3 x 10 x 2) which is about 20 bags with a few spare)?
> 
> The site has some very sandy patches, which are at least 60% fine sand mixed with small rocky particles (around 1mm), quite few small rocks (5mm and up) and some silt. I would like to use this if possible. Does anyone have any advice on how to prepare this? I assume I'd want to wash all the silt out and screen with something a 3mm screen it to remove the small rocks. It's much finer than beach sand though, so should I think about getting some less fine builders sand too?
> 
> In terms of the earth for the clay, I dug away some top soil and made some test mud and it seemed to be fairly sticky and sun baked into a reasonably tough clay-like tile. My test piece was only about 50mm x 100mm x 15mm but it didn't seem to crack.  I'll try some bigger pieces but that much is looking good so far.
> 
> I did a soil separation test and it produced a slightly ambiguous result in that after the sand settled (not more than 15%) I was never able to make out a clear distinction between silt and clay - the fluid part was always opaque and the boundary indistinct. I'll try it again with less soil and some detergent but I'm assuming all this means the soil is reasonably cohesive and there's least some clay.
> 
> In terms of wall construction the plan was to dig a 10cm deep bale width trench around the edge of the dome, with a deeper, shallower drained-to-sky channel below that, fill the bottom with sand, hammer in some rebar for the initial row of bales, stack up rocks until they're about 5cm above ground level and then top with a plastic layer. I'll try and construct it so where the rebar punctures the plastic any trapped water will drain out. I know this is not ideal, but I'm hoping this will be sufficient to survive a few years. The ground is fairly dry and well drained.
> 
> So, any comments or suggestions on any of the above would be gratefully appreciated.
> 
> In terms of the project itself, we're very interested in sustainable, resilient, open source, permaculture type ideas so if you have a project you're interested in building, teaching about or experimenting with please get in touch. The plan is during the summer we'll be running a number of residential workshops on things like earthship and straw bale building (we have people interested in teaching the former, very interested in hearing from people who would like to do the latter). The goal is that everything is free or very low cost. The only real requirement for use of the land is that projects should be open and documented. We're also interested in developing action learning / demonstration permaculture growing spaces. Of course, in the more short term if you happen to be nearby and are interested in participating in building this or other structures please get in touch as well!
> 
> Cheers,
> 
> Christopher.


Derek Roff
derek at unm.edu




-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://amper.ped.muni.cz/pipermail/strawbale/attachments/20120210/b5217c0e/attachment-0001.html>