[GSBN] Plaster thoughts

Sat Mar 3 21:51:28 UTC 2012

Dear All

Must chime in here folks!

Why on earth (ha ha)  would you want to use things like cement and PVA when you can make perfectly good earthen plasters without these? As Mark S. says,  It's not better living through 'bigger hammer' chemistry.

I would be willing to speculate that the Africans had this sorted centuries ago  - animal dung has often been a good standby, and it sticks like the proverbial.......  Then there are various plant extracts?  

We have had great increases  in durability using cellulose fibres - totally mushed up and then drained old paper to you lot!   Too much increases shrinkage, but a goodly dollop sure helps a lot.

Using water that has had fermenting straw or other fibre in it also helps durability - don't ask me why. 

And then there is the question of using limewashes - it uses only very thin layers, i.e. very small amounts, of the higher tech stuff.

We all know the future problems of cement based material over natural earth creates.

cheers   

Graeme
Graeme North Architects
49 Matthew Road
RD1
Warkworth
tel/fax +64 (0)9 4259305

graeme at ecodesign.co.nz
www.ecodesign.co.nz

On 4/03/2012, at 6:09 AM, Bruce King wrote:

> 
> Marks, funny you should ask!
> 
> Tim Owen-Kennedy and I have been roped into working in Kibera (Nairobi) Kenya on the Human Needs Project (http://www.humanneedsproject.org/), one part of which involves renovating earthen huts in the shantytown that is Kibera.  The renovations mostly involve applying a nicer and more durable finish to the badly eroded wattle-and-daub walls.  You can see pics on the website;  their main thing is to build a community center that provides clean toilets, potable water, and solar-powered charging stations for mobile phones.
> 
> For the earthen walls, our client came to us already wedded to a cementitious system based on PVA fibers that bond to cement and give concrete/stucco huge flexibility and crack resistance.  Tim's and my job, which we're in the middle of right now, is to devise and test PVA-cement mixes that can bond to the earth & stay put.  We are going to test for delamination in a weather chamber that wildly fluctuates temperature and humidity.  We are acutely aware of the innumerable previous problems all over the world  associated with cement-over-earth, and know we need to have as much vapor permeability as possible,  So far we imagine using only large, course sand (no fines), minimal cement, the PVA fibers (they look like bleached, finely chopped straw), and possibly some lime.  As usual, we have very little time or money to play around.
> 
> Well report back to everyone as we progress over the next few months.
> 
> Bruce King
> bruce at ecobuildnetwork.org
> (415) 987-7271
> 
> 
> 
> On Mar 3, 2012, at 8:20 AM, huffnpuff at q.com wrote:
> 
>> Mark and All,
>>  
>> I am sure you will be aware of some of what I suggest, as it's common knowledge in the earth plaster world. Too much clay, too little sand/aggregate, the size of the sand/aggregate (too fine sand) can all cause excessive cracking. Removing clay, adding more sand/aggregate or upping the size of the sand/aggregate could help. I've found that the more/longer straw added can eliminate higher clay content cracking. I've used a 1:1:1 ratio of clay, long straw, masons sand for a very sticky, super strong, low/no cracking application to stick as a base over foam, lumber and plywood (placing some of this mix on the surface, tack mesh to subsurface, place more mix, for an added mechanical bond).
>>  
>> When you start adding lime and cement to a mix sounds like what we do in the US. If it doesn't move, get a bigger hammer. It's not better living through 'bigger hammer' chemistry. Often less is more. I think you'll find some of your answers to cement earth plaster ratios in a study by the U. of Penn as they attempted to do some repairs to the mortar on ancient ruins in the Mesa Verde and Chaco Canyon adobe structures.http://repository.upenn.edu/cgi/viewcontent.cgi?article=1044&context=hp_theses You'll find test results on "tensile strength, modulus of rupture, water vapor permeability, and water porosity (respectively) (and freeze/thaw) vs. volume-percent of Portland cement for amended mortars" . 
>>  
>> Also, you might check out Stabilized Compressed Earth Block sites (adobe block on steroids...it's what happens when engineer types make adobe block). My limited experience with SCEB's is they only use about 6-12% cement...lime may be substituted, but don't quote me. Check out the Auroville Earth Institute  www.earth-auroville.com for mixes using cement. Maybe others can chime in here with knowledge on substituting lime for cement. Of coarse, different areas have different clays that react in varying ways so experimenting with different mixes/ratios would be prudent.  The one advantage of using cement amended earth mixes is that it hardens much faster so the results of your experimenting is swifter.
>>  
>> For a shameless plug for the Conference in Sept. 2012 http://www.strawbaleconference.com/index.html the Auroville Institute will be represented at the conference by a presentation by Richard Presley. 
>>  
>> If the locals insist on using cement additives to the earthen plaster mix, visiting the information on these two sites should help with your educated guess as to ratio's. I did some extensive stone work this last summer and wanted to use an earthen mortar. I used a 9:1 mix of my earth plaster mix/cement. My mason buddy thought I was nuts. Eight months later, 4 feet of snow and lots of freeze/thaw in our Colorado winter climate and that mix seems to be holding up amazingly. There's little to no degradation of the mortar.
>>  
>> For those interested in pre-fab straw bale walls and those using cement stucco's these amended earthen mixes may be worth noting. 
>>  
>> Hope this helps Mark...Regards,
>> Mark (see you in September) Schueneman
>> Colorado Straw Bale Association
>> 303-444-6027 hm.
>> 303-591-9841 cell
>>  
>>  
>> 
>> From: "Mark Jensen" <markj at redfeather.org>
>> To: GSBN at sustainablesources.com
>> Sent: Saturday, March 3, 2012 3:44:05 AM
>> Subject: [GSBN] Plaster thoughts
>> 
>> Hello et al,
>> 
>> I am in Kenya right now working on two straw bale buildings, stacking for both Monday and Tuesday. I am struggling with plaster thoughts. I guess the last plaster they did was clay, sand, cow dung, chopped straw, lime, cement and water. Nobody knows the proportions but they had severe cracking, like a dried up lake bed.
>> 
>> Now they want to do 1 1/2 bags of lime, 1/2 bag cement, 4 wheelbarrows of sand, chopped straw and water. This seems like a recipe for disaster to me. They are on a very short budget. I suggested harvesting clay and just doing a earthen plaster, it's free other than some labor to harvest. 
>> 
>> Any thoughts on the recipe above?
>> 
>> Your input would be appreciated.
>> 
>> Asante Sana, Thank you very much,
>> 
>> Mark
>> 
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